Colnaghi 1883

[Dominic Ellis] Colnaghi, Report by Consul-General Colnaghi on the Trade and Commerce of the Province of Siena, Reports from Her Majesty’s Consuls on the Manufactures, Commerce, &c., of their Consular Districts VI, London 1883, pp. 1322–1358.

pp. 1343–1344

Siena Earths (“Terra di Siena,” or “Terra Bolare e Gialla”). – These earths, known also under the names of ochre, bole, umber, &c., are considered by some mineralogists to be ferruginous clays, by others minerals of iron. They are chiefly found in large quantities in the communes of Castel del Piano and Arcidosso (Monte Amiata).

† These communes, both on the western slopes of Monte Amiata, are situated in the province of Grosseto. I have included bole among the products of the province of Siena on account of the name, and because the company working the deposits has its headquarters in the city of Siena.

The yellow earths and bole found on this mountain are true lacustrine deposits, found amid the trachytic rocks, of which it is principally composed. They lie under, and are entirely covered by, the vegetable soil. Varying in compactness and colour, they are termed yellow earths when of a clear ochreous tint, and “terra bolare,” or bole, when of a dark chestnut colour. Each deposit consists, for the greater part, of yellow earth, beneath which bole is found in strata or small veins. The mineral being very friable its excavation is easy, and is generally conducted in open pits. The different qualities are separated during the process; the bole, which has the higher commercial value, being the more carefully treated.

After the first separation the bole is further classed into first, second, third, and intermediate qualities – “boletta,” “fascia,” “cerchione,” &c. Its most important characteristic is termed, in commercial language, “punto di colore,” or tint. The value of the bole rises as its tint deepens. Thus bole of the third quality is lighter than that of the second, and the second than that of the first. After number three comes the “terra gialla.”

The yellow earths, after excavation, are exposed to the open air for about a year, by the pit side, without classification. The bole, on the contrary, is placed in well-ventilated storehouses to dry for about six months. This diversity of treatment is owing to the fact that exposure to the elements brightens the colour of the yellow earths and raises their value, while it would damage the bole, by turning its darker tint first into an orange yellow, and, if continued, into an ordinary yellow earth. It also loses in compactness and crumbles up under exposure.

In addition to the “punto di colore,” the size of the pieces influences the commercial value of the bole, which increases with their volume. Thus the classification is “Bolo pezzo,” “Bolo grapolino,” and “Bolo polvere.” The yellow earths are classed as “Giallo in pezzi,” “Giallo comune,” and “Giallo impalpabile,” but in these cases the impalpable is worth more than the common yellow.

The production of the Siena earths may be calculated at about 600 tons per annum, of which sum 50 tons are calcined and the rest sold in their natural condition. The value of the trade may be estimated from 100,000 to 150,000 lire. A company (limited), for the working and sale of the yellow earths and bole of Monte Amiata, was formed at Siena in 1872, with a capital of 100,000 lire.