Stoneware is pottery made of hard paste, impervious and opaque, white or coloured, covered with a glaze or not. This definition, which at first sight seems clear, is difficult of application to intermediate products.
Imperviousness and hardness, distinguish stoneware from simple or made-up terra-cottas, but these are relative qualities; opacity distinguishes stoneware from porcelains, which are transparent, but where does transparency begin and opacity end?
Here again practice is better than rigorous classification. A man in the trade will not confuse a stoneware with a porcelain, and he will know with certainty whether a certain paste should be called stoneware or not.
However that may be, stoneware is obtained by the firing of clays, which contain naturally, or have added to them, substances called fluxes; the property of these is to effect in the mass a degree of softening sufficient to weld the molecules together and thus cause imperviousness, but not pronounced enough to cause loss of shape in the pieces. Clays which fire direct into stoneware are called natural stoneware clays.
These contain a certain quantity of alkalies and lime, and sometimes of oxide of iron, which act the part of fluxes. The proportion of them should be such that the paste is neither too fusible nor too infusible; the composition of these clays must then vary between narrow limits which average -
68 to 75 per cent.
20 to 25 „
Lime and magnesium...
10 to 20 ,,
3 to 5 "
Oxide of iron...
These clays are prepared either by adding a refractory clay to a fusible one, or the reverse, i.e. by adding fluxes to a refractory clay.
In the first case, we must not think that all fusible clays will, when mixed with a refractory clay, give good stoneware. The properties of these clays must be such, that the softening takes place without injury to shape. In this respect, clays containing alkalies (2 per cent.) in the form of silicates or felspar, are preferable to those which are simply calcareous. As to the composition of the mixture, experience alone can guide us in each particular case.
The addition of fluxes to refractory clays also requires the aid of experience in estimating proportions. The substances used as fluxes are felspar and pegmatite for stoneware of good quality, and marls, or better still, blast-furnace slag (silicate of lime) for cheaper stonewares. The clays used are - according to the products required - kaolin and white or coloured refractory clays.
This depends upon the composition of the paste, and varies from white to a more or less brownish yellow. White stonewares require pastes absolutely devoid of iron. A small quantity of this metal gives yellow, a larger quantity brown; and these colours pass into a more or less bluish grey if firing takes place in a reducing atmosphere. Real red tints are more difficult to obtain than with ordinary terracottas; a large proportion of oxide of iron is required, very little alkali, no lime, and a neutral final atmosphere, neither oxidising nor reducing.
According to the nature of their pastes, stoneware quarries, or those so called, are divided into -
I. Of slag base.
2. Of fusible clay.
B. Plain or incrusted stoneware quarries .
I. Of special clay firing to stoneware.
2. Of felspar base.