The preparation of glazes and their composition would be relatively easy to carry out if it were not necessary to harmonise their physical properties, fusibility and dilatation, with those of the pastes which they have to cover. In fact, if this harmony does not exist, the glaze crazes or peels off.
The crazing or chipping is due to an excessive contraction of the glaze, causing penetration of the liquids which produce fissures when cooling takes place. If, on the other hand, it is the paste which contracts too much, there is peeling, that is to say, that in cooling the glaze becomes detached from the clay.
The dilatation of glazes and pastes is intimately connected with their composition; hence the first care of the ceramist is to become acquainted with the properties of the substances contained in the glazes and pastes. As regards the former, he must note that alkaline silicates are very fusible and contract much; that the silicates of lead, which are also very fusible, contract less; and that the other metallic silicates, that of copper excepted, undergo less contraction than alkaline silicates. The same may be said of the borates. The degree of fusibility of the silicates comes in the following order: lead, copper, manganese, cobalt, iron, uranium, chromium, nickel. If several enamels are applied to the same piece, they must melt at the same temperature, and it will be therefore necessary to diminish the fusibility of some and increase that of others by suitable additions of fluxes on set substances.
As for pastes, it must be observed that silica is very expansive, and that this property increases with the fineness of the grain, but diminishes when the grain is coarser or clay is added. Carbonate of lime and the alkaline sulphates alter the dilatation of pastes; they are therefore used to place them in harmony with the glaze.
The firing of the pieces to which glaze is applied may also lead to accidents, especially in the case of those which require a high temperature.
According to Deck, the following is the course to take in order to produce harmony between a paste and its glaze, the composition of both being known.
Composition of the Glaze.
To avoid chipping or peeling, we must:
Increase or Diminish.
Diminish or Increase.
It must not be forgotten that what prevents chipping tends to produce peeling, and vice versa.
Potash and soda
Composition of the Paste.
Rich white clay
Chalk . . .
Increase if the paste is to be more calcareous.
Increase or crush finer.
Diminish or do not crush so fine.
Increase if the paste is to be more alkaline.
Firing. Ordinary pottery
Biscuit not much fired.
Biscuit too much fired.
Regular firing necessary.
Biscuit not much fired.
A too strong firing causes fissures.
The question becomes still more complicated when coloured glazes are used, for it may happen that the most convenient composition for obtaining the desired colours is easily warped or is exposed to peeling. Only numerous experiments can help us to overcome these difficulties as they arise, and the scientific researches carried out with regard to this question have not hitherto been of much help to ceramists.