Encyclopædia Britannica 1797/V

Encyclopædia Britannica; or, a Dictionary of Arts, Sciences, and Miscellaneous Literature, Constructed on a Plan, by which the different Sciences and Arts Are digested into the Form of Distinct Treatises or Systems, comprehending The History, Theory, and Practice, of each, according to the Latest Discoveries and Improvements, and full Explanations given of the various detached Parts of Knowledge… The third Edition… V, Edinburgh [Andrew Bell – Colin Macfarquhar] 1797.

pp. 105–107

Cobalt

COBALT, one of the semimetals, according to Cronstedt, of a whitish-grey colour, nearly resembling fine hardened steel, and of the specific gravity of 6.000; but according to others, of a bluish grey, or reddish white colour, and of the specific gravity of 7.700. It is as difficult of fusion as copper, or even gold; and when well purified, scarcely yields to iron itself in this respect. When slowly cooled, it crystallizes, forming on its surface small bundles of needles, or needle-formed prisms, laid on one another, and united into bundles; greatly resembling, according to Mongez, a mass of shaken basaltes. In order to succeed in this crystallization, it is sufficient to melt the cobalt in a crucible till it suffers a kind of ebullition; and, after having taken it from the fire, to incline the vessel while the surface of the semimetal is congealing. By this inclination the portion of metal still fused is poured out, and that which adheres to this kind of geode formed by the cooling of the surfaces of the cobalt is found covered with the crystals sought for. When melted with borax it affords a blue glass, which is the most obvious method of distinguishing its ores amongst all others. It cannot be calcined without considerable difficulty; and the calx, though black in appearance, is in reality of a deep blue. This calx melted with borax, or potash and siliceous sand, affords the blue glass called smalt, very much used in enamel painting and tinging of other glass, being the most fixed of all colours in the fire.

Cobalt, when calcined along with the calx of arsenic in a gentle heat, assumes a red colour. The same is naturally produced by way of efflorescence, and is then called the bloom or flowers of cobalt. When cobalt and arsenic are melted in a strong fire, they burn with a blue flame. It does not mix either with mercury by any means hitherto known, nor will it form any union with bismuth without the addition of some medium. It is easily soluble in spirit of nitre, and the solution either in this or any other acid is of a red colour; and it is observable that the colour of the acid solutions of this semimetal, instead of fading by dilution with water, becomes more vivid. It is precipitated of a pale red colour from its solutions by acid of sugar, which has the greatest attraction for it; though acid of sorrel likewise precipitates it.

Cronstedt, in speaking of this semimetal, makes mention of native cobalt; but other mineralogists assure us that it has never been found perfectly pure in the bowels of the earth. What passes for such, is said by Kirwan to be mineralized by arsenic. That called the grey cobalt ore comes nearest to the purity of the native semimetal, but always holds some quantity of arsenic and iron. It is found in Sweden, Saxony, Norway, and England, particularly at Mendip hills in Somersetshire, and in Cornwall, where Dr Lewis says it has lately been dug up in large quantities. Here it is sometimes found in conjunction with bismuth, and sometimes without it, resembling very much in appearance the Saxon ores from Schnuberg in Misnia, and produces the finest blue colours by proper management. An arsenicated grey cobalt ore has also been found at Chatelaudren in France.

This kind of ore is solid, heavy, and compact, sometimes dull and sometimes of a bright appearance, crystallized frequently in a tessular and sometimes in a dendritical form; being generally hard enough to strike fire with steel, when an arsenical smell is perceived. It grows black in the fire, is soluble with effervescence in the nitrous acid, from which it may be precipitated by the marine, and affords the Sympathetic INK mentioned under the article CHEMISTRY, n° 822. This and the blue colour communicated by it to glass are indeed the two characteristics by which the ores of cobalt are distinguished from other arsenical ores.

The most common ore of cobalt is that called the black or vitreous ore, and Kobalt Mulm or Schlaken Kobalt by the Germans. It is found in a loose powdery form, sometimes resembling lamp-black, sometimes of a grey colour, in which state it is called cobalt ochre; but when in scoriform half vitrified masses, it obtains the name of vitreous or glassy ore. When this kind of ore contains any sulphur or arsenic, they are only mechanically mixed with it. A small portion of copper, however, is sometimes found in it. It is frequently embodied in stones or sands of a black colour; sometimes it is contained in argillaceous earths of a blue or green colour. Talc, chalk, and gypsum, impregnated with it, are called by the same name; and by some spiegel cobalt.

3. Cobalt mineralized by the arsenical acid, is found either loose and pure, or mixed with chalk or gypsum, or indurated and crystallized in tetrahedral crystals. It is also found in a stalactitical form. It melts easily, and then becomes blue. It frequently invests other cobaltic ores; and is found sometimes in stone and sand. From the experiments of Bergman it appears that the arsenical acid, and not the calx of arsenic, enters into this combination; for cobalt is never red but when united to an acid. Flowers of cobalt, mineralized by arsenic without any silver, and intermixed with galena, have also been discovered in France.

The flowers or efflorescence of cobalt are often found of a red colour, like other earths, spread very thin on the cobalt ores; and is, when of a pale colour, erroneously called flowers of bismuth. A white cobalt earth or ochre is said to have been found, and examined by a celebrated mineralogist, who found it to resemble the cobalt flowers in every respect except the colour; and indeed it is possible that in these flowers the colour might by length of time, or some other accident, have lost their colour. The indurated flowers of cobalt are commonly crystallized in form of deep red semitransparent rays or radiations. It is found at Schnuberg in Saxony.

Cobalt, mineralized by sulphurated iron, is of a colour nearly resembling tin or silver. It is sometimes found in large masses, sometimes in grains crystallized of a dull white colour, and frequently has the appearance of mis-pickle. It has no mixture of arsenic. By calcination it becomes black and not red, which distinguishes it from the pyrites; and it contains so little sulphur, that none can be extracted from it. When dissolved in aqua regia the solution is yellow, but becomes green when boiling hot, which alternation, says Kirwan, is peculiar to marine cobalt. A coarse grained kind of this ore, found in Sweden, becomes limy in the fire, and sticks to the iron rods employed in stirring it while calcining. The flaggy kind contains a large quantity of iron, and affords a very beautiful colour as well as the former.

Cobalt mineralized by sulphur, arsenic, and iron, has a great resemblance to the harder kinds of grey cobalt ore, formerly mentioned; but it is never hard enough to strike fire with steel, and sometimes may even be scraped with a knife. The most shining kinds of this and the former species are called cobalt glantz.

The great consumption of cobalt is for the permanent blue colour which it communicates to glasses and enamels, either upon metals, porcelains, or earthenwares of any kind. It is the same blue prepared in a very cheap way by the Dutch, chiefly from the coarse glass or blue glass of cobalt, and called azur de Hollande by the French, and which is employed by laundresses. But although cobalt is applied to few or no other purposes, the quantities consumed in this way afford sufficient profit to those who have cobalt mines in their possession.

Ores of cobalt, as has already been said, are met with in many parts of Europe. The greatest quantities are found near Schnuberg [!] in the district of Misnia in Saxony ; also at St Andreasberg in the Upper Hartz, where large quantities have been met with for upwards of 30 years past. Formerly an iron ore only was found in this place ; but about the beginning of the 14th century, on sinking deeper, it was succeeded by a very rich ore of silver ; which also being in length of time exhausted, gave place to cobalt ores. Some pieces, however, are still found in these mines, that contain silver and gold.

The general method of preparing cobalt ores in the large way seems confined to Saxony alone; from whence all other parts of the world, even the East Indies, are constantly supplied. It is supposed that the Chinese, and more particularly the Japanese, had formerly mines of excellent cobalt, with which the fine blues of their ancient porcelains were painted; but it appears that these mines are now exhausted, and that the inferior blues of their present wares are painted with the Saxon zaffre imported to them by the Dutch. For the management of the ore in such a manner as to fit it for giving the desired colour, see the article ZAFFRE.

When cobalt is united to bismuth, by means of nickel, the compound is called speiss. This name is also given to a mixture of cobalt, nickel, bismuth, sulphur, and arsenic.

In Germany and Saxony, the word cobalt is applied to the damps, arsenical vapours, and their effects on the miners; which has induced the vulgar to apply it to an evil spirit whom they suppose to dwell in the mines.

Regulus of COBALT, a kind of semimetal prepared from cobalt, of a whitish colour inclining to red. See ZAFFRE, and CHEMISTRY, n° 1294, &c.

p. 150

Colour, in physics

COLOUR, in physics, a property inherent in light, by which, according to the various sizes of its parts, or from some other cause, it excites different vibrations in the optic nerve; which propagated to the sensorium, affect the mind with different sensations. See CHROMATICS and OPTICS.

pp. 150–152

Colour, in painting

COLOUR, in painting, is applied both to the drugs, and to the tints produced by those drugs variously mixed and applied.

The principal colours used by painters are red and white lead, or ceruss; yellow and red ochres; several kinds of earth, umbre, orpiment, lamp-black, burnt ivory, ivory, black lead, cinnabar or vermillion, gamboge, lacca, blue and green ashes, verdigris, bistre, bice, smalt, carmine, ultramarine: each of which, with their uses, &c. are to be found under their proper articles.

Of these colours some are used tempered with gum-water: some ground with oil; others only in fresco; and others for miniature.

Painters reduce all the colours they use under these two classes, of dark and light colours: dark colours are black, and all others that are obscure and earthy, as umbre, bistre, &c.

Under light colours are comprehended white, and all that approach nearest to it.

Painters also distinguish colours into simple and mineral.

Under simple colours they rank all those which are extracted from vegetables, and which will not bear the fire; as the yellow made of saffron, French berries, lacca, and other tinctures extracted from flowers, used by limners, illuminers, &c.

The mineral colours are those which being drawn from metals, &c. are able to bear the fire, and therefore used by enamellers. Changeable and permanent colours is another division, which, by some, is made of colours.

Changeable colours are such as depend on the situation of the objects with respect to the eye, as that of a pigeon’s neck, taffeties, &c. the first however being attentively viewed by the microscope, each fibre of the feathers appears composed of several little squares, alternately red and green, so that they are fixed colours.

Water COLOURS, are such as are used in painting with gum-water or size, without being mixed with oil.

Incapacity of distinguishing COLOURS. Of this extraordinary defect in vision, we have the following instances in the Philosophical Transactions for 1777. One of the persons lived at Maryport in Cumberland. The account was communicated by Mr Huddart to Dr Priestley, and is as follows. “His name was Harris, by trade a shoe-maker. I had often heard from others, that he could discern the form and magnitude of all objects very distinctly, but could not distinguish colours. This report having excited my curiosity, I conversed with him frequently on the subject. The account he gave was this: That he had reason to believe other persons saw something in objects which he could not see; that their language seemed to mark qualities with precision and confidence, which he could only guess at with hesitation, and frequently with error. His first suspicion of this arose when he was about four years old. Having by accident found in the street a child’s stocking, he carried it to a neighbouring house to inquire for the owner: he observed the people called it a red stocking, though he did not understand why they gave it that denomination, as he himself thought it completely described by being called a stocking. This circumstance, however, remained in his memory, and, together with subsequent observations, led him to the knowledge of his defect.

“He also observed, that when young, other children could discern cherries on a tree, by some pretended difference of colour, though he could only distinguish them from the leaves by the difference of their size and shape. He observed also, that by means of this difference of colour they could see the cherries at a greater distance than he could, though he could see other objects at as great a distance as they, that is, where the sight was not assisted by the colour. Large objects he could see as well as other persons; and even the smaller ones if they were not enveloped in other things, as in the case of cherries among the leaves.

“I believe he could never do more than guess the name of any colour; yet he could distinguish white from black, or black from any light or bright colour. Dove or straw colour he called white, and different colours he frequently called by the same name; yet he could discern a difference between them when placed together. In general, colours of an equal degree of brightness, however they might otherwise differ, he confounded together. Yet a striped ribbon he could distinguish from a plain one; but he could not tell what the colours were with any tolerable exactness. Dark colours, in general, he often mistook for black; but never imagined white to be a dark colour, nor dark to be a white colour.

He was an intelligent man, and very desirous of understanding the nature of light and colours; for which end he had attended a course of lectures in natural philosophy.

He had two brothers in the same circumstances as to sight; and two other brothers and sisters, who, as well as their parents, had nothing of this defect.

One of the first mentioned brothers, who is now living, I met with at Dublin, and wished to try his capacity to distinguish the colours in a prism; but not having one by me, I asked him, whether he had ever seen a rain-bow? he replied, He had often, and could distinguish the different colours; meaning only, that it was composed of different colours, for he could not tell what they were.

I then procured, and showed him a piece of ribbon: he immediately, and without any difficulty, pronounced it a striped, and not a plain, ribbon. He then attempted to name the different stripes: the several stripes of white he uniformly and without hesitation called white: the four black stripes he was deceived in; for three of them he thought brown, though they were exactly of the same shade with the other, which he properly called black. He spoke, however, with diffidence, as to all those stripes; and it must be owned, that the black was not very distinct: the light green he called yellow; but he was not very positive: he said, “I think this is what you call yellow.” The middle stripe, which had a light tinge of red, he called a sort of blue. But he was most of all deceived by the orange colour: of this he spoke very confidently, saying, “This is the colour of grass, this is green.” I also showed him a great variety of ribbons, the colour of which he sometimes named rightly, and sometimes as differently as possible from the true colour.

I asked him, whether he imagined it possible for all the various colours he saw to be mere difference of light and shade; and that all colours could be composed of these two mixtures only? With some hesitation he replied, No, he did imagine there was some other difference.

It is proper to add, that the experiment of the striped ribbon was made in the day-time, and in a good light.”

COLOURS for staining different kinds of Stones. See CHEMISTRY, n° 753.

COLOUR, in dyeing. See DYEING.

. . .

pp. 153–163

Colour-Making

COLOUR-Making, the art of preparing the different kinds of colours used in painting.

This art properly belongs to chemistry; and is one of the most curious, though least understood, parts of it. The principles on which colour-making depends are entirely different from those on which the theory of other parts of chemistry is founded; and the practical part being in the hands of those who find it their interest to conceal their methods as much as possible, it thence happens, that there is not only no distinct theory of this art, but scarce a single good receipt for making any one colour hath ever appeared.

1. Division of colours into opaque and transparent.

The first general division of colours is into opaque and transparent. By the first are meant such colours as, when laid over paper, wood, &c. cover them fully so as to efface any other painting or stain that might have been there before; the others are of such a nature as to leave the ground on which they are laid visible through them. Of the first kind are white-lead, red-lead, vermilion, &c.; of the latter kind are the colours used for illuminating maps, &c.

2. Oil and water colours.

Another division is into oil-colours and water-colours; by which is meant, such as are appropriated to painting in oil and in water. Most of those which are proper for painting in water, are also proper for being used in oil. There is, however, this remarkable difference betwixt colours when mixed with water and with oil, that such as are quite opaque in water will become perfectly transparent in oil. Thus, blue verditer, though exceedingly opaque in water, if ground with oil, seems totally to dissolve, and will become very transparent. The same thing happens to such colours as have for their basis the calx of tin, alabaster, or calcareous earth. The most perfectly opaque colours in oil are such as have lead, mercury, or iron, for their basis: to the latter, however, Prussian blue is an exception; for though the basis of that colour is iron, it proves quite transparent when ground with oil. In water-colours, those prepared from metals, Prussian blue alone excepted, are always opaque; from vegetables or animals, transparent. Coals, however, whether vegetable or animal, are opaque both in water and oil.

3. Simple and compound ones.

Colours again, may be considered as either simple or compound. The simple ones are such as require nothing to be superadded to them, in order to make a full strong colour, without regarding whether they are formed of many or few ingredients; and in this view, white-lead, red-lead, vermilion, calces of iron, &c. are simple colours. The compound ones are formed by the union of two or more colouring substances; as blue and yellow united together to form a green, red and yellow to form an orange, a white earth or calx with the red colour of cochineal or brazil to form a lake, &c.; and thus carmine, lake, rose-pink, Dutch-pink, English-pink, &c. are compound colours.

4. True and false colours.

The last and most important division of colours is into true and false. By the former are meant those which retain their colour under every possible variety of circumstances, without fading in the least: the others are such as do not; but either lose their colour altogether, or change to some other. What is chiefly apt to affect colours, is their being exposed to the sun in summer, and to the cold air in winter: but to this there is one exception, viz. white-lead; which, when ground with oil, retains its whiteness if exposed to the weather, but degenerates into a brownish or yellowish colour if close kept. In water this substance is very apt to lose its colour, whether exposed to the air or not. The great desideratum in colour-making is to produce the first kind of colours, viz. such as will not fade by exposure to the weather; and indeed it is to be regretted, that the most beautiful are in general the least permanent. It may, for the most part, however, be expected, that the more simple any colour is, the less liable will it be to change upon exposure to the air.

5. Instances of colours produced by exposure to the sun and air.

6. By the mixture of two vegetable juices.

The great difficulty of knowing à priori whether a colour will fade or not, is owing to our ignorance concerning the nature of colouring substances. With all our disadvantages, however, we may observe, that whatever change of colour is produced in any substance by exposure to the sun and air, that colour to which it changes will bid fair for being permanent, and therefore ought to be employed where it can be done. Of these changes the instances are but very rare. One is in the purple of the ancients, which assumed its colour by exposure to the sun, and consequently was exceedingly permanent. Another is in the solution of silver; which, being mixed with chalk, the precipitate turns to purplish black where it is exposed to the sun. A third is in solutions of indigo by alkaline substances, which constantly appear green till exposed to the air by spreading them very thin, upon which they become almost instantaneously blue, and continue so ever after. Sometimes, though still more rarely, a very remarkable change of colour happens, upon mixing two vegetable juices together. Almost the only instance of this we have on the authority of Mr George Forster, who informs us, that the inhabitants of Otaheite dye their cloth of a crimson colour, by mixing together the yellow juice of a small species of fig with the greenish juice of a kind of fern. But the most remarkable alterations of colour are effected by different metallic and saline solutions mixed with certain animal or vegetable substances; and with these the colour-maker will be principally conversant.

7. Effects of acids and alkalies on colours.

It is a common observation in chemistry, that acids mixed with blue vegetable juices turn them red, and alkalies green. It is equally certain, though not so generally known, that acids of all kinds generally tend to heighten red colours, so as to make them approach to the scarlet or true crimson; and alkalies to darken, or make them approach to blue or purple. Mixed with yellow colours, acids also universally tend to brighten the yellow; and alkalies to turn it to an orange, and make it become more dull. But though this is very generally the case, we are not to expect that all acids are equally powerful in this respect. The nitrous acid is found to heighten the most of any, and the marine acid the least of the mineral ones. The vegetable, as might be expected, are less powerful than the mineral acids. Thus, if with a tincture of cochineal, either in water or spirit of wine, is mixed the pure nitrous acid, it will change the colour to an exceeding high orange or flame colour, which it will impart to cloth. If the vitriolic acid is used, a full scarlet, inclining to crimson rather than orange, is produced. With marine acid a true crimson colour, bordering on purple, is the consequence. Alkalies, both fixed and volatile, change the colour to a purple, which is brighter with the volatile than the fixed alkalies.

8. Permanency of colours, by what determined.

Here it is obvious, that whatever colours are produced by the mixtures of different substances together, the permanency of these colours can only be in proportion to the ability of such mixtures to resist the weather. Thus, suppose a high scarlet or orange colour is produced by means of spirit of nitre, it is plain that, was such a colour exposed to the air, it could remain no longer than the spirit of nitre which produced it remained. In proportion, therefore, as the spirit of nitre was exhaled into the air, or otherwise destroyed, it behoved the colour to fade, and at last to be totally destroyed; and thus, in proportion to the destructibility of the substances by which colours are produced, will be the disposition of such colours to fade, or the contrary. In this respect alkalies are much more destructible than acids, and consequently less proper for the preparation of colours. With regard to acids, the nitrous seems most destructible, the vitriolic less so, and the marine the least of all. From the extreme fixity of the phosphorine acid and sedative salt, perhaps they might be of service in preserving colours.

9. Opaque or transparent colours, how formed.

10. Calx of tin, the most proper basis for the colours.

11. Precipitate of lead most proper for coarse ones.

As all colours, whether derived from the animal or vegetable kingdom, must be extracted either by pure water or some other liquid menstruum, they cannot be used for the purposes of painting till the colouring substance is united with some earthy or solid matter, capable of giving it a body, as the workmen call it; and according to the nature of this substance, the colour will be transparent or otherwise. This basis ought to be of the most fixed and durable nature; unalterable by the weather, by acids, or by alkalies. It ought also to be of a pure white colour, and easily reducible into an impalpable powder. For this reason all earthy substances should be avoided as being acted upon by acids; and therefore, if any of these were added to heighten the colour, they would not fail to be destroyed, and their effect totally lost. Precipitates of lead, bismuth, &c. though exceedingly fine and white, ought also to be avoided, as being apt to turn black by exposure. The only substance to be chosen in preference to all others, is calx of tin, prepared either by fire or the nitrous acid. This is so exceedingly refractory as not only to be unalterable by alkalies, acids, or the sun and weather, but even by the focus of a very large burning mirror. It is besides white as snow, and capable of being reduced to an extreme degree of fineness, insomuch that it is made use of for polishing metalline speculums. For these reasons, it is the most proper basis for all fine colours. For coarse ones, the white precipitate of lead, mentioned under the article CHEMISTRY, n° 703, will answer very well. It hath a very strong body, i. e. is very opaque, and will cover well; may be easily ground fine, and is much less apt to turn black than white lead; it is besides very cheap, and may be prepared at the small expence of 3d. per pound.

12. General method of preparing colours.

If what we have just now observed is attended to, the general method of extracting colours from any vegetable or animal substance, and fixing them on a proper basis, must be very easily understood. For this purpose, a quantity of calx of tin is to be procured in proportion to the quantity of colour desired. This must be well rubbed in a glass mortar, with a little of the substance designed for brightening the colour, as alum, cream of tartar, spirit of nitre, &c. after which it must be dried, and left for some time, that the union between the two substances may be as perfect as possible. If the colour is to be a very fine one, suppose from cochineal, the colouring matter must be extracted with spirit of wine without heat. When the spirit is sufficiently impregnated, it is to be poured by little and little upon the calx, rubbing it constantly, in order to distribute the colour equally through all parts of the calx. The spirit soon evaporates, and leaves the calx coloured with the cochineal. More of the tincture is then to be poured on, rubbing the mixture constantly as before; and thus, with proper management, may very beautiful colours, not inferior to the best carmine, be prepared at a moderate expence. If, instead of cochineal, we substitute brazil-wood, turmeric, logwood, &c. different kinds of red, yellow, and purple, will be produced. For the coarser colours, aqueous decoctions are to be used in a similar manner; only as these are much longer of evaporating than the spirit of wine, very little must be poured on at a time, and the colours ought to be made in large quantity, on account of the tediousness of the process.

13. Effects of different kinds of salts.

14. Solution of tin the most powerful.

Hitherto we have considered only the effects of pure and simple salts, viz. acids and alkalies, on different colours; but by combining the acids with alkalies, earths, or metals, these effects may be varied almost in infinitum, neither is there any rule yet laid down by which we can judge à priori of the changes of colour that will happen on the admixture of this or that particular salt with any colouring substance. In general, the perfect neutrals act weakly; the imperfect ones, especially those formed from metals, much more powerfully. Alum and sal ammoniac considerably heighten the colour of cochineal, brazil, turmeric, fustic, madder, logwood, &c. The same thing is done, though in a less degree, by common salt, Glauber’s salt, saltpetre, and many other neutrals. Solutions of iron in all the acids strike a black with every one of the above-mentioned substances; and likewise with sumach, galls, and other astringents. Solutions of lead, or saccharum saturni, universally debase red colours to a dull purple. Solution of copper changes the purple colour of logwood to a pretty good blue; and, in general, solutions of this metal are friendly to blue colours. The effects of solutions of gold, silver, and mercury, are not so well known; they seem to produce dark colours of no great beauty. The most powerful solution, however, with regard to a great number of colours, is that of tin, made in aqua-regia. Hence we may see the fallacy of Mr Delaval’s hypothesis concerning colours *, that the least refrangible ones are produced by the most dense metals: for tin, which hath the least density of any metal, hath yet, in a state of solution, the most extraordinary effects upon the least refrangible colours as well as those that are most so. The colour of cochineal is changed by it into the most beautiful scarlet; a similar change is made upon the colouring matter of gum-lac. Brazil-wood is made to yield a fine purplish crimson; logwood, a beautiful dark purple; turmeric, fustic, weld, and all yellow-colouring woods and flowers, are made to communicate colours far more beautiful than can be got from them by any other method. The blue colour of the flowers of violets, eye-bright, iris, &c. are heightened so as to equal, if not excel, the blue produced by a solution of copper in volatile alkali. In short, this solution seems to be of much more extensive use in colour-making, when properly applied, than any thing hitherto thought of. It is not, however, universally serviceable. The colour of madder it totally destroys, and likewise that of saff-flower, changing them both to a dull orange. It likewise spoils the colour of archil; and what is very remarkable, the fine red colour of tincture of roses made with oil of vitriol, is by solution of tin changed to a dirty green.

* See Chromatic, n° 8.

15. Directions for the choice of colouring materials.

16. Mr. Hellot’s improving the durability of brazil-wood.

The most important consideration in colour making is to make choice of such materials as produce the most durable colours; and if these can be procured, an ordinary colour from them is to be preferred to a bright one from those which fade sooner. In what the difference consists between the colours that fade and those which do not, is not known with any degree of certainty. From some appearances it would seem, that those substances which are most remarkable for keeping their colour, contain a viscous glutinous matter, so combined with a resinous one as to be soluble both in water and spirit of wine. The most durable red colour is prepared from gum-lac. This is very strongly resinous, though at the same time so far glutinous, that the colouring-matter can be extracted from it by water. Next to gum-lac are madder roots and cochineal. The madder is an exceedingly penetrating substance, insomuch that, when given to animals along with their food, it tinges their bones of a deep red colour. Its colouring-matter is soluble both in water and spirit of wine. Along with the pure red, however, there is in madder a kind of viscous astringent substance, of a dark brown colour, which seems to give the durability to the whole. The colouring-matter of cochineal, though soluble both in water and spirit of wine, is very tenacious and mucilaginous, in which it bears some resemblance to the purpura of the ancients, which kept its colour exceedingly well. Where the colours are fugitive, the tinging substance seems to be too resinous or too mucilaginous. Thus the colours of brazil, turmeric, &c. are very resinous, especially the latter; insomuch that the colouring-matter of turmeric can scarcely be extracted by water. Both these are perishable, though beautiful colours; and much more are the red, purple, and blue flowers, commonly to be met with. These seem to be entirely mucilaginous without the least quantity of resinous matter. The yellow flowers are different, and in general keep their colour pretty well. Whether it would be possible, by adding occasionally a proper quantity of gum or resin, to make the fugitive colours more durable, hath not yet been tried, but seems to have some probability. What tends a little to confirm this, is a process given by Mr Hellot for imparting durability to the colour of brazil-wood. It consists only in letting decoctions of the wood stand for some time in wooden casks till they grow stale and ropy. Pieces of woollen cloth now dyed in the liquor acquired a colour so durable, that they were not in the least altered by exposure to the air during four months in the winter season. Whether this change in the durability of the colour was effected by the ropiness following the fermentation, or by some other cause, or whether the experiment can be at all depended upon, must be referred to future observation.

17. Preparation of different colours.

Having thus collected all that can as yet be depended upon for establishing a general theory of colour-making, we shall now proceed to give an account of the different pigments generally to be met with in the colour-shops.

18. Lamp black.

1. Black. These are lamp-black, ivory-black, blue-black, and Indian-ink. The first is the finest of what are called the soot-blacks, and is more used than any other. Its preparation is described in the Swedish Transactions for the year 1754, as a process dependent on the making of common resin: the impure resinous juice collected from incisions made in pine and fir trees, is boiled down with a little water, and strained whilst hot through a bag: the dregs and pieces of bark left in the strainer are burnt in a low oven, from which the smoke is conveyed through a long passage into a square chamber, having an opening on the top on which is a large sack made of thin woollen stuff: the soot, or lamp-black, concretes partly in the chamber, from whence it is swept out once in two or three days, and partly in the sack, which is now and then gently struck upon, both for shaking down the soot, and for clearing the interstices betwixt the threads, so as to procure a sufficient draught of air through it. In this manner lamp-black is prepared at the turpentine houses in England, from the dregs and refuse of the resinous matters which are there manufactured.

19. Dr. Lewis’s observations.

On this subject Dr Lewis hath some curious observations. “The soot (says he) arising in common chimneys, from the more oily or resinous woods, as the fir and pine, is observed to contain more dissoluble matter than that from the other woods: and this dissoluble matter appears, in the former, to be more of an oily or resinous nature than in the latter; spirit of wine extracting it most powerfully from the one, and water from the other. The oiliness and solubility of the soot seeming therefore to depend on those of the subject it is made from, it has been thought that lamp-black must possess these qualities in a greater degree than any kind of common soot. Nevertheless, on examining several parcels of lamp-black, procured from different shops, I could not find that it gave any tincture at all, either to spirit or to water.

Suspecting some mistake or sophistication, or that the lamp-black had been burnt or charred, as it is to fit it for some particular uses, I prepared myself some soot from linseed oil, by hanging a large copper pan over the flame of a lamp to receive its smoke. In this manner the more curious artists prepare lamp-black for the nicer purposes; and from this collection of it from the flame of a lamp, the pigment probably received its name. The soot so prepared gave no tincture either to water or to spirits, any more than the common lamp-black of the shops. I tried different kinds of oily and resinous bodies with the same event; even the soots obtained from fish-oils and tallow did not appear to differ from those of the vegetable-oils and resins. They were all of a finer colour than the lamp-black commonly sold.

Some soot was collected in like manner from fir and other woods, by burning small pieces of them slowly under a copper-pan. All the soots were of a deeper black colour than those obtained from the same kinds of woods in a common chimney; and very little, if at all, inferior to those of the oils: they gave only a just discernible tincture to water and spirit, while the soots of the chimney imparted a strong deep one to both. The soot of mineral bitumens, in this close way of burning, appears to be of the same qualities with those of woods, oils, and resins: in some parts of Germany, great quantities of good lamp-black are prepared from a kind of pit-coal.

It appears, therefore, that the differences of soots do not depend altogether on the qualities of the subjects, but in a great measure on the manner in which the subject is burnt, or the soot caught. The soots produced in common chimneys, from different kinds of wood, resinous and not resinous, dry and green, do not differ near so much from one another, as those which are produced from one kind of wood in a common chimney, and in the confined way of burning above mentioned.”

20. Ivory-black

Ivory-black is prepared from ivory or bones burnt in a close vessel. This, when finely ground, forms a more beautiful and deeper colour than lamp-black; but in the common methods of manufacturing, it is so much adulterated with charcoal dust, and so grossly levigated, as to be unfit for use. An opaque deep black for water-colours, is made by grinding ivory-black with gum-water, or with the liquor which settles from the whites of eggs after they have been suffered to stand a little. Some use gum-water and the whites of eggs together, and report, that a small addition of the latter makes the mixture flow more freely from the pencil, and improves its gloss. It may be observed, however, that though ivory-black makes the deepest colour in water as well as in oil-painting, yet it is not on this account always to be preferred to other black pigments. A deep jet-black colour is seldom wanted in painting; and in the lighter shades, whether obtained by diluting the black with white bodies, or by applying it thin on a white ground, the particular beauty of the ivory black is in a great measure lost.

21. Blue-black.

Blue-black is said to be prepared from the burnt stalks and tendrils of the vine. These, however, the colour-makers seldom give themselves the trouble of procuring, but substitute in its place a mixture of ivory-black and the common blue used for clothes.

22. Indian-ink.

Indian-ink is an excellent black for water-colours. It hath been discovered by Dr Lewis to consist of a mixture of lamp-black and common glue. Ivory-black, or charcoal, he found to answer equally well, provided they were levigated to a sufficient degree of fineness, which indeed requires no small trouble.

23. White colours.

2. White. The white colours commonly to be met with are, white-flake, white-lead, calcined hartshorn, pearl-white, Spanish-white, egg-shell white, and magistery of bismuth. The flake-white and white-lead are properly the same. The preparation of the former is kept a secret; the method of preparing the latter is described under CHEMISTRY, n° 875. These are the only whites that can be used in oil, all the rest being transparent unless they are laid on with water. Calcined hartshorn is the most useful of the earthy whites, as being the least alkaline. Spanish-white is only finely prepared chalk. Pearl-white is made from oyster-shells; and egg-shell white from the shells of eggs. All these, by their attraction for acids, must necessarily destroy such colours as have any acid or metallic salt in their composition. The magistery of bismuth is apt to turn black, as are also flake-white and white-lead, when used in water. The white precipitate of lead recommended under CHEMISTRY, n° 703, is greatly superior as a water-colour to all these; being perfectly free of any alkaline quality, and not at all apt to lose its own colour, or to injure that of other substances.

24. Red colours.

3. Red. The red colours used in painting are of two sorts; viz. those which incline to the purple, and such as are of a full scarlet and tend rather to the orange. The first are carmine, lake, rose-pink, red-ochre, and Venetian-red. The second are vermilion, red-lead, scarlet-ochre, common Indian-red, Spanish-brown, and terra di Sienna, burnt.

We have already (n° 12.) laid down some general rules for the preparation of carmine and lake. Particular receipts have been delivered with the greatest confidence for making these fine colours; but all of them must necessarily prove ineffectual, because an earthy basis is recommended for striking the colour upon: from the principles of chemistry, however, we are certain, that if aquafortis, or solution of tin, is made use of for brightening a colour made with any earthy basis, it must infallibly be destroyed by that basis, by reason of its alkaline quality. Carmine is the brightest and most beautiful red colour known at present; the best comes from France. Lake differs from it in being capable of mixture with oil; which carmine is not, unless with great difficulty. The former is also much more inclined to purple than carmine. This last quality, however, is reckoned a defect; and accordingly, the more that lake approaches to the scarlet or true crimson, the more it is valued. On dropping solution of tin into an aqueous tincture of brazil-wood, a beautiful precipitate falls, of a purplish crimson colour. This may be very well substituted in place of the dearer lakes on many occasions.

Rose-pink is a very beautiful colour, inclining more to the purple than scarlet. It seems to be made of chalk, coloured with a decoction of brazil-wood, heightened by an alkaline salt; for which reason it is exceedingly perishable, and but little esteemed. The colour might be made much more durable as well as better, by employing for a basis the white precipitate of lead above mentioned, and brightening it with solution of tin.

Red ochre and Venetian red differ in nothing from the colcothar of vitriol well calcined. The calces of iron may be made to appear either purplish, or inclining to the scarlet, according to the manner in which the calcination is performed. If the matter is perfectly deprived of its phlogiston, and subjected to an intense fire, it always turns out red: but the mixture of a small quantity of inflammable matter gives it a purplish cast. Hence various paints are kept in the shops under different names, which yet differ from each other only in the slight circumstance above mentioned: and such are the scarlet-ochre, Spanish-brown, and terra di Sienna burnt. It is remarkable, that the calces of iron never show their colour till they become cold. Colcothar of vitriol, while hot, always appears of a very dark dusky purple.

Of the preparation of vermilion and red lead, an account is given under the article CHEMISTRY, no 1213, 1404. These are very durable colours; the first is the best red used in oil painting, but does not answer well in water; the other is rather an orange; and, like other preparations of lead, is in some cases apt to turn black.

25. Orange colours.

4. Orange. The only true orange-coloured paints are red orpiment and orange lake. The first is a sublimate formed of arsenic and sulphur: the other may be prepared from turmeric infused in spirit of wine, having its colour struck upon calx of tin, and brightened by a solution of that metal. All the shades of orange, however, may be extemporaneously prepared by mixing red and yellow colours together, in due proportions.

26. Yellow colours.

5. Yellow. The yellow paints most commonly in use are, king’s-yellow, Naples-yellow, Dutch-pink, English-pink, masticot, common orpiment, yellow-ochre, terra di Sienna unburnt, and Turbith-mineral.

King’s-yellow is evidently an arsenical preparation. Its colour is exceedingly beautiful, but apt to fade; on which account, and its great price, it is seldom used.

Naples-yellow was for a long time thought to be a preparation of arsenic, but is now discovered to have lead for its basis. It is therefore apt to turn black and lose its colour, which makes it the less valuable. It is nevertheless used in preference to king’s-yellow, on account of its inferiority in price. This colour is particularly liable to be spoiled by iron when moist, and therefore should never be touched by that metal unless previously ground in oil.

Dutch-pink is said to be prepared by striking the colour of yellow berries upon finely levigated chalk. But of this there is great reason to doubt; the basis of Dutch-pink seems much more hard and gritty than chalk, and its colour more durable than those struck upon that earth usually are. Very good yellows may be prepared with the white precipitate of lead, formerly mentioned, by using either yellow berries, fustic, or any other substance capable of yielding that colour. English pink is paler than the Dutch, and keeps its colour greatly worse.

Masticot is prepared by calcining white-lead till it assumes a yellowish colour. It is not apt to change, but the colour is so dull that it is seldom used either in oil or water.

Common orpiment is a pretty bright greenish-yellow, prepared by subliming arsenic with sulphur. Its nauseous smell, which is greatly increased by grinding in oil, makes it very disagreeable; nor does it keep its colour for any length of time. That kind of orpiment least inclined to green is to be preferred for the purposes of painting.

Yellow-ochre and terra di Sienna, are ferruginous earths, capable of becoming red by calcination. Green vitriol precipitated by lime may be advantageously substituted to either of them. See CHEMISTRY, n° 699.

Turbith mineral is but little used in painting, though its fine yellow colour seems greatly to recommend it. This preparation is in all probability very durable; and should seem therefore worthy of a preference either to king’s or Naples yellow. The method of preparing it is described under CHEMISTRY, n° 705.

Gamboge is a paint that can only be used in water, and is the most common yellow made use of for colouring maps, &c. but for this it is not very proper, being neither quite transparent, nor very durable.

27. Green colours.

6. Green. The only simple green colour that hath a tolerable degree of brightness is verdigrease, or preparations of it. This, however, though a very beautiful colour, is far from being durable. It is improved in colour, though not in durability, by dissolution and crystallization in distilled vinegar; in which state it is called distilled verdigrease. A more durable water-colour is made by dissolving the verdigrease in cream of tartar, or rather the pure tartarous acid; but in oil this is found to be equally fugitive with the verdigrease itself. For an account of these preparations, see CHEMISTRY, n° 894.

Compound greens are either made of Prussian or some other blue, mixed with yellow; but in whatever way these colours can be compounded, the beauty of the green produced is greatly inferior to distilled, or even common, verdigrease. The tartarous solution of verdigrease, mixed with a little gamboge, is the best transparent green water-colour we have had an opportunity of trying; and a mixture of Prussian-blue and turbith-mineral is probably the best opaque one.

Sap-green is a simple colour, but exceedingly inferior to distilled verdigrease, or even to the tartarous solution of verdigrease with gamboge. It is prepared from the juice of unripe buckthorn berries evaporated to the consistence of a gum. Its green colour is greatly inclined to yellow. A kind of compound green has been sometimes used, called Prussian-green, which consists only of Prussian blue and yellow-ochre. It has no beauty, nor is it durable. It is prepared as Prussian-blue, only not pouring on any spirit of salt to dissolve the ochreous sediment which falls at the same time.

Another green sometimes used is called terra verte. This is a native earth, probably impregnated with copper. It is of a bluish green colour, much of that taint called sea-green. It is gritty, and therefore must be well levigated before it is used. Its colour is durable, but not very bright.

28. Blue colours.

7. Blue. The blue colours are ultramarine, Prussian-blue, verditer, smalt, bice, and indigo. Of these the ultramarine is the finest, but its great price hinders its being much used. It is a preparation from lapis lazuli; is an exceeding bright colour, and never fades with whatever substance it is mixed. It is now, however, in a great measure superseded by Prussian blue, to the disadvantage of painting in general; as Prussian blue, though very beautiful, is far from being durable. For an account of its preparations see the article ULTRAMARINE.

The process for making Prussian blue is described, and its nature fully considered, under CHEMISTRY, n° 1163: so that it is sufficient here to observe, that Prussian blue is to be accounted of the best quality when it is deep, bright, and not inclined to purple. It ought to be tried by mixture with white lead, as the brightness of the colour will appear much more when diluted than when concentrated in the lumps of the blue itself.

The preparation of blue verditers is kept a secret, and the best chemists have been puzzled to find out the method. The colour is exceedingly bright, and has a considerable tinge of green. A method of preparing a colour equally beautiful, and agreeing in all respects with what is sold in the shops, except that of effervescing with acids, we have found to be as follows: Dissolve copper in strong caustic alkali, until the liquid has assumed a very deep blue colour; and the deeper this colour is, the finer will your verditer be. When the menstruum has dissolved as much of the metal as it can take up, it is to be poured out into a broad and well glazed earthen pan, held over a very gentle fire; and from the moment it is put on the liquor is to be continually agitated with a wooden spatula, so that the liquor may be heated as equally as possible. The whole secret consists in properly regulating the degree of heat; for if it exceeds the due proportion ever so little, the verditer will turn out of a dirty green. The proper degree is about 90° of Fahrenheit’s thermometer. In this gentle heat the alkali slowly evaporates; and in proportion to its doing so the verditer falls to the bottom. After it is once formed, freed from the alkaline liquor, and dried, it can bear the affusion of boiling water without the least injury. Dr Priestley, in his sixth volume, takes notice, that solution of copper in volatile alkali affords a blue precipitate by heat, but without taking notice of the requisites for its success. In making this preparation, it is necessary to dissolve copper in its metallic state; for the solution of any calx will not yield a blue but a green colour. This colour is durable in water, but dissolves in oil, and has then all the inconveniences of verdigrease above mentioned.

Smalt is glass-coloured with zaffre, a preparation from cobalt.* It is commonly so grossly powdered that it cannot be used in painting, and its texture is so hard that it cannot easily be levigated. Its colour is exceedingly bright and durable; so that when finely levigated it is used instead of ultramarine. The most proper materials for levigating this substance seem to be the plates of M. Reaumur’s porcelain recommended by Dr Lewis. See CHEMISTRY, n° 592, 599. For the preparation and qualities of bice, see the articles ARMENUS Lapis and BICE.

* See Zaffre and Smalt.

Indigo is but little used in painting either in oil or water, on account of the dulness of the colour. It requires no other preparation than being washed over. Its goodness is known by the darkness and brightness of the colour. See INDIGO.

29. Purple colours.

29. Purple. The only simple colour of this kind used at present is colcothar of vitriol. A beautiful purple lake may be prepared from logwood by means of solution of tin; but this method of preparing colours is very little known as yet.

30. Brown colours.

31. Dr. Lewis’s opinion concerning bistre.

30. Brown. The brown colours are, bistre, brown-ochre, Cologne-earth, umbre, and brown-pink. Under the article BISTRE is given a process for making that colour, by infusing soot in water, pouring off the tincture, and then evaporating it to an extract; but Dr Lewis is of opinion, with Mr Landois in the French Encyclopédie, that the soot is either boiled in water, or ground with a little liquid of some kind into a smooth paste; it is then diluted with more water, and after standing for about half an hour till the grosser substance of the soot has settled, the liquor is poured off into another vessel, and set by for two or three days, that the finer parts may fall to the bottom, and this fine matter is the bistre. This is a very useful colour in water, being exceedingly fine, durable, and not apt to spoil any other colours with which it is mixed. The brown pink is said to consist of chalk tinged with the colouring matter of fustic, heightened by fixed alkaline salts. It is therefore very perishable, and is seldom used. The other browns are a kind of ochreous earths; for a description of which see their proper articles.

32. Attempts to make lakes of all colours.

Having now considered most of the colouring substances usually to be met with in the shops, we shall next take notice of some attempts that have been made to produce all the different colours from vegetables, after the manner of lakes; which, though the methods hitherto tried have for the most part failed of success, may perhaps some time or other be found applicable to valuable purposes.

33. Black from astringents.

From infusions of astringent vegetables mixed with green vitriol, is produced a deep black liquor of very extensive use in dyeing †. The substances which produce the deepest blacks are galls and logwood. When a decoction or infusion of the galls is dropped into a solution of the vitriol largely diluted with water, the first drops produce bluish or purplish red clouds, which soon mingling with the liquor, turn it uniformly of their own colour. It seems to be on the quality of the water that this difference in the colour depends. With distilled water, or the common spring-waters, the mixture is always blue. If we previously dissolve in the water the most minute quantity of any alkaline salt, too small to be discovered by any of the common means by which waters are usually tried, or if the water is in the least putrid, the colour of the mixture proves purple or reddish. Rain-water, caught as it falls from the clouds in an open field in clean glass-vessels, gives a blue; but such as is collected from the tops of the houses, grows purple with the mixture of vitriol and galls: from whence it may be presumed, that this last has contracted a putrid tendency, or received an alkaline impregnation, though so slight as not to be sensible on other ways of trial.

† See Dyeing.

Both the purple and blue liquors, on adding more of the astringent infusion, deepen to a black, more or less intense according to the nature of dilution: if the mixture proves of a deep opaque blackness, it again becomes bluish or purplish when further diluted. If suffered to stand in this diluted state for two or three days, the colouring matter settles to the bottom in form of a fine black mud, which by slightly shaking the vessel, is diffused again through the liquor, and tinges it of its former colour. When the mixture is of a full blackness, this separation does not happen, or in a far less degree; for though a part of the black matter precipitates in standing, yet so much remains dissolved, that the liquor continues black. This suspension of the colouring substance, in the black liquid, may be attributed in part to the gummy matter of the astringent infusion increasing the consistence of the watery fluid; for the separation is retarded in the diluted mixture by a small addition of gum Arabic. If the mixture either in its black or diluted state is poured into a filter, the liquor passes through coloured; only a part of the black matter remaining on the filter. The filtered liquor on standing for some time becomes turbid and full of fine black flakes: being freed from these by a second filtration, it again puts on the same appearance; and thus repeatedly till all the colouring parts are separated, and the liquor has become colourless.

Dr Lewis, from whose Philosophical Commerce of Arts this account is taken, further informs us, that this colouring matter, when separated from the liquor and dried, appeared of a deep black, which did not seem to have suffered any change from the air by exposure for upwards of four months. Made red-hot, it glowed and burnt, but did not flame, and became a rusty brown powder, which was readily attracted by a magnetic bar; though in its black state the magnet had no action upon it. The vitriolic acid, diluted with water and digested on the black powder, dissolved the greatest part of it, leaving only a very small quantity of whitish matter. Solution of pure fixed alkaline salt dissolved very little of it: the liquor received a reddish brown colour, and the powder became blackish brown. This residuum was attracted by the magnet after being red-hot, though not before: the alkaline tincture, passed through a filter, and mixed with a solution of green vitriol, struck a deep brownish black colour, nearly the same with that which results from mixing with the vitriolic solution, an alkaline tincture of galls.

34. Black from combination of other colours.

It hath also been attempted to produce black from a combination of other colours; as green may be produced from a mixture of blue and yellow. Mr le Blon, in his Harmony of Colours, gives a method of forming black, by mixing together the three colours called primitive, viz. blue, red, and yellow; and, Mr Castel, in his Optique des Couleurs, published in 1740, says that this compound black has an advantage, in painting, above the simple ones, of answering better for the darkening of other colours. Thus, if blue, by the addition of black, is to be darkened into the colour called blue-black, the simple blacks, according to him, if used in sufficient quantity to produce the requisite deepness, conceal the blue, while the compound blacks leave it distinguishable. Le Blon does not mention the proportions of the three colours necessary for producing black. Castel directs 15 parts of blue, five of red, and three of yellow; but takes notice, that these proportions are rather speculatively than practically just, and that the eye only can be the true judge; our colours being all very imperfect, and our pigments or other bodies of one denomination of colour being very unequal in their degree of intensity. He observes, that the pigments should all be of the deepest and darkest kind: and that, instead of taking one pigment for each colour, it is better to take as many as can be got; for the greater discord there is of heterogeneous and discordant drugs, the more true and beautiful, he says, will the black be, and the more capable of uniting with all other colours, without suppressing them, and even without making them tawney.

Dr Lewis acquaints us, that by mixing different blue, red, and yellow colours, he has not been able to produce a perfect black; but has often obtained from them very dark colours, such as may be called brown-blacks, or grey-blacks; such as we commonly see in the dark parts of paintings, and such as the charcoal and soot blacks appear when diluted a little. The ingredients being each of a dark deep colour is a very necessary condition; for bright blues, bright reds, and bright yellows, mixed in such proportions that neither colour prevailed, produced only a grey. In effect, all compositions of this kind, physically considered, can be no other than greys, or some of the intermediate teints between whiteness and darkness; and these greys will be so much the lighter or darker as the component colours of themselves are bright or dark.

With regard to the extraction of the colouring matter from the different kinds of vegetables commonly to be met with of all colours, this would certainly be a very valuable acquisition, could the colours so procured be made durable. On this subject nothing hath yet appeared more satisfactory than what is delivered by Dr Lewis in his notes on Neumann’s chemistry. His observations are curious, but promise very little success to any who shall attempt to fix these vegetable colours.

35. Dr. Lewis’s experiments on vegetable colours.

“Among the infinite variety of colours (says he), which glow in the flowers of plants, there are very few which have any durability, or whose fugitive beauty can be arrested by art, so as to be applied to any valuable purposes. The only permanent ones are the yellow, the red, the blue; and all the intermediate shades of purple, crimson, violet, &c. are extremely perishable. Many of these flowers lose their colours on being barely dried; especially if they are dried slowly, as has been usually directed, in a shady, and not warm place. The colours of all of them perish on keeping even in the closet vessels. The more hastily they are dried, and the more perfectly they are secured from the air, the longer they retain their beauty. The colouring matter extracted and applied on other bodies is still more perishable: oftentimes it is changed or destroyed in the hands of the operator.

The colour of many blue flowers is extracted by infusion in water; but there are some from which water gains only a reddish, or purplish blue. Of those that have been tried there is not one which gives any blue tincture to spirituous liquors: some give no colour at all, and some a reddish one. The juice pressed out from the fresh flowers is for the most part blue. The blue juices and infusions are changed red by all acids. The marine acid seems to strike the most florid red. The flowers themselves, macerated in acid liquors, impart also a deep red tincture. Alkalies, both fixed and volatile, and lime-water, change them to a green. Those infusions of the juices which have nothing of the native colour of the flowers, suffer the same changes from the addition of acid and alkaline liquors: even when the flowers have been kept till their colour is lost, infusions made from them acquire still a red colour from the one, and a green from the other, though in a less degree than when the flowers were fresh.

The red colour produced by acids is scarcely more durable than the original blue: applied upon other bodies and exposed to the air, it gradually degenerates into a faintish purple, and at length disappears, leaving hardly any stain behind. The green produced by alkalies changes to a yellow, which does not fade so soon. The green, by lime-water, is more permanent and more beautiful: green lakes, prepared from these flowers by lime-water, have been used as pigments by the the painter. The flowers of cyanus have been greatly recommended, as affording elegant and durable blue pigments; but I have never been able to extract from them any blue colour at all. They retain their colour indeed, when hastily dried, longer than some other blue flowers: but they communicate nothing of it to any kind of menstruum. Infusions of them in watery, spirituous, and oily liquors, are all of them more or less of a reddish cast, without any tendency to blue. Alum, which is said to heighten and preserve their blue colour, changes it, like that of other blue flowers, to a purplish red; acids to a deep red; alkalies and lime-water to a green; solution of tin added to the watery infusion, turns it of a fine crimson; on standing, a beautiful red fæcula subsides, but it loses all its colour by the time it is dry. The watery infusion, inspissated to the consistence of an extract, appears of a dark reddish brown: an extract made with rectified spirit is of a purplish colour. The colour of both extracts spread thin and exposed to the air quickly fades. The flowers employed in these experiments were those of the common blue-bottle of the cornfields.

Red flowers readily communicate their own red colour to watery menstrua: among those that have been tried, there is not one exception. Those of a full red colour give to rectified spirit also a deep red tincture, brighter, though somewhat paler, than the watery infusion: but the lighter red flowers, and those which have a tendency to purplish, impart very little colour to spirit, and seem to partake more of the nature of the blue flowers than of the pure red. Infusions of red flowers are supposed to be heightened by acids, and turned green by alkalies, like those of the blue; but this is far from being universal. Among those I have examined, the rose-colours and purplish reds were changed nearly in the same manner as the blues; but the full deep reds were not. The deep infusion of red poppies is changed by alkalies, not to a green, but to a dusky purple.

The colours of yellow flowers, whether pale or deep, are in general durable. Many of them are as much so, perhaps, as any of the native colours of vegetables. The colour is extracted both by water and by spirit. The watery infusions are the deepest. Neither alkalies nor acids alter the species of the colour, though both of them vary its shade; acids rendering it paler, and alkalies deeper: alum likewise considerably heightens it, though not so much as alkalies. An infusion of the flowers, made in alkaline ley, precipitated by alum, gives a durable yellow lake. In some of the deep reddish yellow, or orange-coloured flowers, the yellow matter seems to be of the same kind with that of the pure yellow flowers, but the red to be of a different kind from the pure red ones; watery menstrua take up only the yellow, and leave the red, which may afterwards be extracted by rectified spirit of wine, or by water acuated by fixed alkaline salt. Such particularly are the saffron-coloured flowers of carthamus. These, after the yellow matter has been extracted by water, are said to give a red tincture to ley; from which, on standing at rest for some time, a deep bright red fecula subsides; called from one of the names of the plant which produces it, safflower; and from the countries whence it is commonly brought to us, Spanish-red, and China-lake. This pigment impregnates spirit of wine with a beautiful red tincture, but communicates no colour to water. I have endeavoured to separate, by the same treatment, the red matter of some of the other reddish yellow flowers, as those of garden marigold, but without success. Plain water extracted a yellow colour, and alkaline ley extracted afterwards only a paler yellow: though the digestions were continued till the flowers had lost their colour, the tinctures were no other than yellow, and not so deep as those obtained from the pure yellow flowers. The little yellow flosculi, which in some kinds of flowers are collected into a compact round disc, as in the daisy and corn-marigold, agree, so far as they have been examined, with the expanded yellow petala. Their colour is affected in the same manner by acids, by alkalies, and by alum; and equally extracted by water and by spirit. But the yellow farina, or fine dust, lodged on the tips of the stamina of flowers, appears to be of a different kind. It gives a fine bright yellow to spirit, and a duller yellow to water; the undissolved part proving in both cases of a pale yellowish white. Both the watery and spirituous tinctures were heightened by alkaline liquors, turned red by acids, and again to a deep yellow on adding more of the alkali: I know no other vegetable yellow that is turned red by acids.

White flowers are by no means destitute of colouring matter. Alkaline lixivia extract from some of them a green tincture, and change their colourless expressed juices to the same colour; but I have not observed that they are turned red by acids. The flowers of the common wild convolvulus or bind-weed, which in all their parts are white, give a deep yellow or orange tincture to plain water; which, like the tinctures of flowers that are naturally of that colour, is rendered paler by acids, heightened a little by alum, and more considerably by alkaline salts. The vapours of the volatile vitriolic acid, or of burning sulphur, which whiten or destroy the colour of the coloured flowers, make no change in the white.

36. Colours from fruits.

The red juices of fruits, as currants, mulberries, elder-berries, morello, and black cherries, &c. gently inspissated to dryness, dissolve again almost totally in water, and appear nearly of the same red colour as at first. Rectified spirit extracts the tinging particles, leaving a considerable portion of mucilaginous matter undissolved; and hence the spirituous tincture proves of a brighter colour than the watery. The red solutions, and the juices themselves, are sometimes made dull, and sometimes more florid, by acids, and generally turned purplish by alkalies. The colours of these juices are for the most part perishable. They resist, indeed, the power of fermentation, and continue almost unchanged, after the liquor has been converted into wine; but when the juice is spread thin upon other bodies, exsiccated, and exposed to the air, the colour quickly alters and decays: the bright lively red changes the soonest: the dark dull red stain from the juice of the black cherry, is of considerable durability. The fruit of the American opuntia or prickly pear, the plant upon which the cochineal insect is produced, is perhaps an exception: This bright red fruit, according to Labat, gives a beautiful red dye. Some experiments, however, made upon the juice of that fruit, as brought into England, did not promise to be of any great advantage: but the particulars I cannot now recollect.

The ripe berries of buckthorn stain paper of a green colour. From these is prepared the substance called sap-green, a pigment sufficiently durable, readily soluble in water, but not miscible with oil. The berries dried while green, and macerated in alum water, are said to yield a yellow pigment; and when they have grown over ripe so as to fall off spontaneously, a purple one. It is said that the berry of the heliotropium tricoccum, which grows wild about Montpelier, stains paper of a green colour, and that this green turns presently to a blue: that the common blue paper receives its colour from this juice: and that the red rags called turnsol, employed for colouring wines and other liquors, are tinctured by the same juice turned red by acids. According to M. Nissole of the French academy of sciences (as quoted by Savary in his Dictionaire de Commerce), the colouring juice is obtained not from the berries, but from tops of the plant gathered in August, ground in mills, and then committed to the press. The juice is exposed to the sun about an hour, the rags dipt in it, dried in the sun, moistened by the vapour which arises during the slaking of quicklime with urine, then dried again in the sun, and dipped again in the juice. The Dutch and others are said to prepare turnsol rags, and turnsol in the mass, from different ingredients, among which archil is a principal one.

In some plants, peony for instance, the seeds at a certain point of maturity are covered with a fine shining red membrane. The pellicles of the seeds of a certain American tree afford the red masses brought into Europe under the names of annotto, orlean, and raucous *. Mr Pott, in the Berlin Memoirs for the year 1752, mentions a very extraordinary property of this concrete. ‘With the vitriolic acid it produces a blue colour, of extreme beauty; but with this capital defect, that all salts and liquors, and even common water, destroy it.’ The specimen of annotto, which I examined, was not sensibly acted upon by spirit of vitriol; it received no change in its own colour, and communicated none to the liquor. Nor did any visible change ensue upon dropping the acid into tinctures of annotto made in water, or in spirit.

* See Annotto.

37. Colours from leaves.

The green colour of the leaves of plants is extracted by rectified spirit of wine and by oils. The spirituous tinctures are generally of a fine deep green, even when the leaves themselves are dull-coloured, or yellowish, or hoary. The colour, however, seldom abides long even in the liquor; much less when the tinging matter is separated in a solid form, and exposed with a large surface to the air. The editor of the Wirtemberg Pharmacopeia observes, that the leaves of acanthus, brankursine, or bear’s-breach, give a more durable green tincture to spirit than those of any other herb. Alkalies heighten the colour both of the tinctures and green juices; acids weaken, destroy, or change it to a brownish: lime-water improves both the colour and durability: by means of lime, not inelegant green lakes are procurable from the leaves of acanthus, lily of the valley, and several other plants. There are very few herbs which communicate any share of their green colour to water; perhaps none that give a green of any considerable deepness. It is said, however, that the leaves of some plants give a green dye to woollen, without the addition of any other colouring matter; particularly those of the wild chervil, or cow-weed, the common ragwort, and devil’s-bit. The leaves of many kinds of herbs and trees give a yellow dye to wool or woollen cloth that has been previously boiled with a solution of alum and tartar. Weld, in particular, affords a fine yellow, and is commonly made use of for this purpose by the dyers, and cultivated in large quantity in some parts of England. There is no colour for which we have such plenty of materials as for yellow. Mr Hellot observes, that all leaves, barks, and roots, which on being chewed discover a slight astringency, as the leaves of the almond, peach, and pear-trees, ash-bark (especially that taken off after the first rising of the sap in the spring), the roots of wild patience, &c. yield durable yellows, more or less beautiful according to the length of time that the boiling is continued, and the proportions of alum and tartar in the preparatory liquor: that a large quantity of alum makes these yellows approach to the elegant yellow of weld: that if the tartar is made to prevail, it inclines them to an orange: that if the roots, barks, or leaves, be too long boiled, the yellow proves tarnished, and acquires shades of brown.” See the article DYEING.

38. Production of indigo accounted for.

The most capital preparations from the leaves of plants, are those of indigo and weld; which are both very much used in dyeing, though the first only in painting.* Both the indigo and woad plants give out their colour, by proper management, to water, and in form of a blue fecula or lake. Mr Hellot suspects that a like blue fecula is procurable from many other vegetables. Blue and yellow blended together, compose a green. He supposes the natural greens in vegetables to be compounded in like manner of these two colours; and that the blue is oftentimes the most permanent, so as to remain entire after the putrefaction or destruction of the yellow. The theory is specious, and perhaps just: we know of no other that accounts in any degree for the production of the indigo and woad blue. Dr Lewis, however, informs us, that he never was able to produce the least appearance of either blue or yellow from any of the plants he tried by treating them in the manner used for the preparation of indigo.

* See Indigo and Wood.

39. Colours from mosses.

There are sundry mosses, which in their natural state, like the indigo and woad plants, promise nothing of the elegant colours that can be extracted from them by art. The most remarkable of these is archil; for the preparation of which, and the colours that may be produced from it, see the article. Linnæus suspects that there are several other more common mosses from which valuable colours might be extracted: a quantity of sea-moss, having rotted in heaps on the shore, he observed the liquor in the heaps to be as red as blood; the sea-water, the sun, and the putrefaction, having brought out the colour. Mr Kalm, in an appendix to Linnæus’s paper, in 1745, mentions two forts of mosses actually employed in Sweden for dyeing woollen red: one is the Lichenoides coralliforme apicibus coccineis of Ray’s Synopsis; the other the Lichenoides tartareum, farinaceum, scutel-larum umbone fusco, of Dillenius. This last is a white substance like meal clotted together, found on the sides and tops of hills. It is shaved off from the rocks after rain, purified from the stony matters intermixed among it by washing with water, then dried in the sun, ground in mills, and again washed and dried: it is then put into a vessel with urine, and set by for a month: a little of this tincture added to boiling water makes the dyeing colour. In the same Transactions for the year 1754, there is an account of another moss which, prepared with urine, gives a beautiful and durable red or violet dye to wool and silk. This is the lichen foliaceous umbilicatus subtus lacunensis, Linn. flor. Suec. It grows upon rocks, and is readily distinguishable from others of that class, by looking as if burnt or parched, consisting of leaves as thin as paper, convex all over on the upper side, with corresponding cavities underneath, adhering firmly to the stones by a little root under the leaves, and coming asunder, when dry, as soon as touched. It is gathered after rain, as it then holds best together, and parts easiest from the stone. In France, a crustaceous moss, growing upon rocks in Auvergne, is prepared with lime and urine, and employed by the dyers as a succedaneum for the Canary archil, to which it is said to be very little inferior. Mr Hellot relates, that he has met with several other mosses, which, on being prepared in the same manner, acquire the same colour. The most expeditious way, he says, of trying whether a moss will yield an archil or not, is to moisten a little of it with a mixture of equal parts of spirit of sal ammoniac and strong lime-water, and add a small proportion of crude sal ammoniac. The glass is then to be tied over with a piece of bladder, and set by for three or four days. If the moss is of the proper kind, the little liquor which runs from it upon inclining the vessel, will appear of a deep crimson colour; and this afterwards evaporating, the plant itself acquires the same colour. Dr Lewis informs us, that he has tried a good number of the common mosses, many both of the crustaceous and foliaceous kind, and not a few of the fungi; as also the herbs chamomile and milfoil, which yield a blue essential oil; and thyme, whose oil becomes blue by digestion with volatile spirits; but never met with any that yielded a colour like archil. Most of them gave a yellow or reddish brown tincture. A few gave a deep red colour to the liquor: but, when diluted, it showed a yellowish cast, and when applied on cloth it gave only a yellowish red.

40. Some blue flowers may probably yield permanent colours.

41. Colours from roots.

To these observations we shall only add, that though, in general, the blue colours of flowers are exceedingly perishable, there seem to be at least two exceptions to this rule; for the blue flowers of iris, or flower-de-luce, and those of columbine, when treated with solution of tin, yielded a colour tolerably permanent. Indeed, when experiments are made with a view to extract the colour from any part of a vegetable, it will always be proper to try whether it can bear a mixture with this solution. If the colour is not destroyed by it, there is a very great probability that the solution will, by proper management, preserve, and give a durability to it, which could scarce be obtained by any other method. It must, however, be observed, that solution of tin cannot bear to be mixed with. These are principally sugar of lead and cream of tartar, as well as all the calcareous earths and alkaline salts. With alum it may be mixed very safely, and is in many cases the better for it. The roots of plants, however, seem to promise more durability of colour than the upper parts. We have seen a blue colour of considerable durability and brightness prepared from the roots of common radishes by expressing the juice, combining it with tobacco-pipe clay, and brightening it with a little alum. The root of the red beet is also said to yield a durable colour of a beautiful red, inclining to scarlet; but this we cannot affirm from our own experience.

42. Colours for maps.

With regard to liquid colours for maps, &c. we apprehend there can be very little difficulty of preparing all the possible varieties of them, if what we have above laid down is attended to. The only colour with which there can be any difficulty is blue; but the common solutions of indigo in alkalies or acids may be made to answer this purpose, though, on account of their strongly saline quality, they are not very proper. A very curious method of procuring a beautiful transparent blue colour is by extracting the colouring matter from Prussian blue, by means of a caustic alkali. This when laid upon paper appears of a dirty brown colour; but if washed over with a weak solution of green vitriol, is instantly changed to a most beautiful blue. This seems to afford a method of procuring blue transparent colours of greater beauty than they are usually met with. See specimens of transparent colours prepared according to the above rules, on the Chart subjoined to HISTORY.

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Colouring.

COLOURING, among painters, the manner of applying and conducting the colour of a picture; or the mixtures of light and shadows, formed by the various colours employed in painting. See PAINTING.