(b) Grind 1 part washed antimony with 2 parts red-lead to a stiff paste with water, and expose to red heat for 4 or 5 hours.
There are a great many modifications both in the ingredients and the processes, and a great variety of shades in consequence.
Taken as a whole, the Naples yellows are unsatisfactory pigments, very prone to deteriorate in impure air, and necessitating great care in their preparation to avoid contact with iron, which turns them green. They cover well, are fairly brilliant, and mix readily with water or oil, but their application is declining rapidly.
Ochres.—The large class of mineral pigments known collectively as ochres or sienna earths possess considerable importance, notably on account of their remarkable durability and their reasonable price. They all consist essentially of an earthy base coloured by oxide of iron or of manganese, or of both. Some authorities differentiate between ochres and siennas, and ascribe the latter name only to those earths which contain manganese, but this seems to be an arbitrary proceeding, because the term sienna, or more properly Siena, is derived solely from the name of the Italian province in which these minerals are worked. They are of widespread occurrence, both geographically and geologically, and the methods of mining and preparing them are not subject to much variation.
Consul Colnaghi, in his report on the mineral products of the province of Siena, says that Siena 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. The yellow earths and bole found on the western slopes of Monte Amiata 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 the third quality comes the terra guilla. 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 pezzo, giallo commune, and giallo impalpabile, the impalpable being worth more than the common yellow. The production of the Siena earths is estimated at about 600 tons per annum, of which amount about 50 tons are calcined, and the rest sold in the natural condition. The value of the trade is estimated at from £4000 to £6000.
The European trade in these earths is very large. Rouen exports some 5000 tons yearly, and Havre about 1500 tons.
Similar deposits occur in America, where they are known as “paint-beds,” and the earths are called “metallic paints.” A prominent example is the paint-bed at Lehigh Gap, Carbon County, Pennsylvania, which was originally opened as an ironstone mine. The mineral proved valueless metallurgically, but remarkably useful as a pigment, since it contains about 28 per cent. of hydraulic cement, which hastens the drying and causes the paint to set without any addition of artificial dryers, thereby making it eminently fitted for all outdoor application.
Along the outcrop of the paint, the beds are covered by a cap or overburden of clay, and by the decomposed lower portion of the Marcellus slate, which is 50 feet thick at the Rutherford shaft.