These exist nearly always in clay, sometimes as mere traces, sometimes in quantities of as much as 2 or 3 per cent.
Washing does not remove alkalies from the clay, and it is probable that these substances come from the felspar or mica parts scattered in the clay in excessively fine particles which remain, like the clay itself, in suspension in the water.
At a moderate temperature the alkalies have no effect, but at the temperature of china-kilns they act as fluxes; hence ordinary brick paste, fired at a high temperature, should only contain a small proportion of alkalies, otherwise they would be too fusible. But it is very different with stoneware and porcelain pastes. Alkalies (felspar, pegmatite, etc.) are added to the former in order to weld together the particles of the substance, and to give it that peculiar appearance characteristic of stoneware, and to the latter in order to obtain the characteristic transparency of porcelain.
The presence of this substance increases the fusibility of clays, and consequently diminishes the value of the refractory earths. But in those products which are fired at a less high temperature, if the lime does not exist in too large a proportion, it forms silicates which do not effervesce, and which play a part in the transparency of certain chinas. In the hard French china we find 3 to 6 per cent. of lime, a quantity which is increased to 10 or 14 per cent. in soft porcelain.
Side by side with this chemical action, lime plays an important physical part in fa*šences. Its presence is indispensable for the adhesion of the stanniferous glaze with which fa*šence pastes are coated. Moreover, in fa*šences intended for stoves, it prevents cracks and increases their resistance to fracture. The proportion of lime in faience pastes varies from 14 to 22 per cent.
The lime is always introduced in the form of more or less pure limestone. In the case of common pottery the quantity of limestone contained in the clay may rise as high as 10 or 12 per cent. without their workability being much affected, but it is inconvenient to use more calcareous clays alone, because the products obtained split up under atmospheric influences. They are therefore used as antiplastics in combination with rich clays.
Like lime and potash, traces of this occur in almost all clays, but those which contain it in appreciable quantity are rarely employed for pottery.
White clays contain none; these are the most uncommon (kaolins, refractory clays), the rest owe their yellow colour to the hydrated ferric oxide, and their red colour to the same oxide anhydrous. In firing it acts in two ways: it colours the pastes, and makes them fusible according to its quantity. Thus, common ferruginous stoneware contains from 5 to 8 per cent. of it. Above that proportion, the firing of the pastes becomes a delicate operation which must be stopped at the proper moment, otherwise the pieces will be deformed. The colour of pottery is not always proportioned to the quantity of iron contained in it; it depends upon the substances introduced into the paste, and especially on the state in which the iron is, a state which varies with the temperature and the atmosphere in which the firing took place.
Thus the same china paste containing oxide of iron sometimes bakes white and transparent, sometimes yellow and opaque, according to the atmosphere of the kiln. If the atmosphere is a reducing one, especially at the moment when the glaze begins to melt, the iron remains as an almost colourless silicate; if the atmosphere is an oxidising one, the iron may free itself in the form of ferric oxide; the quantity of oxide of iron being the same, the more silicious the paste is, the less is the coloration.
The influence of temperature is evidenced by this fact, that porcelains which are red when slightly heated, are transformed at a high temperature into transparent products, the oxide of iron becoming ferrous silicate.
In common faiences, charged with oxide of iron, a too great coloration of the biscuit, a coloration which would afterwards have to be hidden by a thick coat of opaque enamel, is avoided by firing in a reducing atmosphere (smoked).
The colour of the biscuit wares disappears superficially when they receive a glaze containing borax, because this latter dissolves the oxide. This is the case with English flint-ware, which is slightly coloured as biscuit, but becomes white on the application of the glaze.
In firing bricks, differences of colour are sometimes noticed depending upon their position in the kiln; these differences are caused by the varied oxidising and reducing power of the atmosphere due to the state of the fire.
This occurs in many clays, sometimes in the form of lumps or grains, sometimes scattered through the mass in an impalpable state; it is probable that the colour of certain plastic clays is caused by its presence.
In firing, the sulphuret of iron is decomposed into sulphurous acid which escapes, and into an oxide which, by attacking the surrounding parts, forms hollows in the products. Several clays in the neighbourhood of Vaugirard contain such large quantities of sulphuret of iron that their use has had to be forbidden on account of the volumes of sulphurous acid given out by the brickworks.
Sometimes there are found in these clays beautiful crystals of iron sulphate caused by the oxidisation of the sulphuret. This chemical decomposition is sometimes so pronounced that in the Soisson district these clays are worked to extract from them the sulphate of iron, and the double sulphate of aluminium and potassium (alum). The latter is produced by the sulphuric acid in excess formed in the oxidisation of the sulphuret attacking the clay.
These are of different kinds, and their quantity is very variable. Although non-existent or occurring in very small amounts in most clays, they are sufficient in others to colour them grey, brown, or black; and then in the firing these substances produce charcoal, or leave hollows within the products.
Finally, certain clays contain considerable quantities of matter resembling coal, and produce black pottery, which is rendered very refractory by the infusibility of the charcoal. Such are plumbago crucibles.