They are applied in various ways : the pottery may be sprinkled with them, it may be immersed in a pulp formed of the glaze, or the latter may be applied by volatilisation.
In the first case, the pottery while still fresh may be coated with the glaze in a powdered state, which is generally an oxide of lead or of alquifoux. Silica and alumina are provided by the piece to be glazed. This process, which is the simplest and commonest, is only used for common pottery.
Immersion consists of the pottery in a dry state, either warmed or baked, being dipped into a pulp of the glaze neither too thick nor too clear to be absorbed by the paste. The composition should be such that the glaze will not become detached after mixture; hence it must have a certain plasticity.
Glazing by volatilisation is effected by throwing saline or metallic substances into an active kiln; these will attack the surface of the pottery and will form on it a thin layer of a transparent glass which constitutes the glaze. In other cases, instead of throwing the verifiable substances on the pottery when in an incandescent state, the interior of the cazettes containing the pieces is coated with the glazing material. When the heat becomes fairly strong, these substances (alkaline carbonates or borates, oxides of lead) become volatilised and act on the surface of the pieces.
Instead of plunging the piece into the glaze, the latter may be poured upon it, and this is to be preferred in the case of pastes which are not very porous and which would not, by immersion, absorb a sufficient layer of glaze. This process is called irrigation.
Sometimes the pottery is sprinkled with a brush dipped in the glaze; this is the sprinkling process, and used for coating pieces on certain parts in order to obtain special effects.
If we desire to cover a piece entirely by this process, we must use insufflation or pulverisation, the principle of which is well known.
A tube with a small orifice is dipped into the glaze pulp; another tube is fixed at right angles to the first, and introduces the air under pressure; the current of air causes an aspiration which draws the liquid into the other tube. For small objects an india-rubber pear will be enough, but for large surfaces we must have bellows which will transmit the air at a pressure of 1/3 to 1/4 of an atmosphere.
These are almost always stanniferous, the white enamel being the base of them. This enamel is prepared by melting together: 44 parts of calcine (oxide of tin and lead), 44 parts of sand, 2 parts of
Alicante soda (soda partially carbonated), 8 parts sea-salt, and 2 parts of minium (Deck). The mixture is roughly pounded, then finely crushed in presence of water until it has the required consistency. The coloured opaque enamels are obtained by adding different metallic oxides to the preceding mixture.
Formula according to Brongniart.
Naples yellow or oxide of antimony
Oxide of cobalt or azure
Battitures of copper
" " ...
Naples yellow ....
Peroxide of manganese .
Several methods are employed for decorating pottery with opaque enamels:
The method of decoration "au grand feu" on unfired enamel consists of coating the piece by immersion or irrigation with white enamel, which is allowed to dry, and then of placing on the friable raw enamel other opaque or coloured enamels by-means of a brush or by any other quick process, such as spraying. The whole is then fixed by baking.
Instead of working with raw enamel, we may decorate on fired enamel. In this case the firing which fixes the decoration is done at a somewhat low temperature; this allows of the use of a very rich gamut of colours, which should be easily verifiable.
When a clay which is naturally white or whitened by dip is decorated, the stanniferous enamels may be replaced by-transparent enamels, whose brighter colourings give different effects from those obtained with the opaque enamels. The presence in the paste of a sufficient quantity of limestone allows of either transparent or opaque enamels being used, the latter being especially reserved for light tints and white. The effects may be still more varied by giving dull tones to the enamels; for this purpose they are simply hardened, that is to say, they are rendered less fusible by the addition, for instance, of silica or alumina.
The alkaline transparent enamels are applied to all faience pastes. From the point of view of architectural decoration, they conduce to powerful effects and have the very interesting property of remaining as bright in artificial as in solar light, while under similar conditions lead enamels become dull. But the difficulty and delicacy of the preparation of alkaline enamels limit their use. Deck, the first ceramist who used them, gives the following fundamental formulae which, by varying the quantity of flux, may serve to make an infinite number of intermediate tints: -