These quarries are made in France, Belgium, Germany, and Spaifi. They are made of a mixture of rich clays and felspar, to which, for the coloured portions, are added metallic oxides. The factories in the North of France obtain their primary materials from Luxembourg, the banks of the Rhine, Belgium, and England; those of the centre from a less distance, on account of the cost of transport - part of their primary materials comes from Diou in the Allier (H. Gay), hence there is a difference of appearance between their products and those of the North. The ingredients of the pastes must be more or less pure according to the type of quarry which is to be manufactured, and their quantities are a matter of experience only possessed by those who carry on this class of manufacture.
The best pastes are always the result of a mixture of several plastic clays thinned with a cement formed of powdered terracotta. White or only slightly coloured products require fairly pure pastes containing little or no oxide of iron.
The felspar acts as a flux in consequence of the alkalies contained in it (p. 5), and the quantity of it necessary in the paste depends upon the refractory qualities of the clay and upon the temperature at which firing is to take place.
We may take as an example of a paste giving a good fine white artificial stoneware -
This stoneware bears a temperature of nearly 1500° C. The felspar may be replaced by the sands which are the fesidue of the washing of kaolins.
To produce the coloured patterns which adorn artificial stoneware quarries, metallic oxides are added to the mixture of pastes.
Black is obtained by the addition of oxides of iron and manganese (5 to 6 per cent, of each);
Blue with .5 to 5 per cent, according to the intensity required, of the oxide of cobalt;
Green is produced by the oxide of chromium (.5 to 1 per cent), bluish greens by a mixture of oxides of cobalt and chromium;
Reds and yellows with yellow and red ochres (anhydrous and hydrated oxides of iron).
The intermediate tints are produced by mixing the above oxides in various proportions.
This is performed with powdered dry clays. The primary substances are dried in large ovens heated to about 500 C. by one of the usual methods - hot air, steam, waste heat of the kilns, etc.
The clay may also be spread in a layer 6 or 8 inches thick upon stone or metal slabs heated underneath by waste heat. When large masses of material have to be dried, hot-air kilns are used. (See p. 55).