What is known as biscuit is baked clay porous and without gloss—for example, a flower-pot.
Glossy pottery has a very thin layer of glaze upon it. The Samian ware of the Greeks furnishes examples of this finish. Glazed pottery is covered with a perceptible coating of glass.
Enamelled ware, or pottery with a mat-glaze, has a glazed coating made opaque with oxide of tin. This finish is used on some of the most beautiful art pottery. Glazes may be coloured with certain metallic oxides without losing their transparency.
One is often confused by hearing the terms enamel and glaze carelessly used. Enamel should be used to describe a vitreous coating that is opaque, and glaze a glossy, transparent surface—both may be coloured. The term mat-glaze is an exception to this rule. This means an opaque glaze with a glossy finish.
The materials of which glazes are composed are about the same as those that enter into the composition of clays with a few additions. There is in glazes, as in clays, the play of soft and hard materials, or the flux and frit. Flint, aluminum, or china clay forms the frit or hard, refractory portion; potash, soda, lead or borax the melting part or flux. The addition of clays gives durability.
In preparing the flux, it is melted like glass and then crushed to powder. The frit must first be melted, then dropped from the crucible into water before it can be powdered.
The making of glazes is not often done by amateurs, and unless one is planning to start pottery-making as a business, and requires large quantities of glazes, it is best to buy them from a reliable dealer. They come in the form of powder—soft glaze, hard glaze, and stannifère, which is a hard glaze with some oxide of tin added, to make it opaque. The soft glazes are used on ware which fires at a low heat, and is therefore not durable. A large proportion of hard glaze must be added to make the preparation applicable to a pottery which is fired at an intense heat, while if one wishes a mat-glaze the stannifère is also used.
The colouring materials may be bought of the same dealer. They are as follows: Antimony for yellow, cobalt for blue, copper for green, chrome for green, manganese for brown, and iron for brown. Red oxide of iron colours a purplish red, and carbonate of copper makes a gray green that is very beautiful. A ground glass or stone slab, on which to mix the glaze, and a palette knife, will be required; also a small quantity of gum-arabic and of gum tragacanth, a small teacup, and a measuring-glass. These, with two or three soft paint-brushes of various sizes—one that is an inch wide, one smaller, and another larger—will be enough of an outfit to start with.
The gum-arabic should be dissolved in water to the consistency of honey. It is used in the first coat of glaze to bind the glaze and prevent it from rubbing off. Quite a little of it may be prepared at a time and kept in a covered glass jar until needed. The gum tragacanth is bought in small quantities—five cents’ worth at a time. Cover this amount with one pint of water and let it stand over night. In the morning, strain it through a fine sieve and put it away in a glass jar till it is needed.
If possible, very large pieces of pottery should be fired before they are glazed—in the biscuit, as it is called. Small and medium-sized pieces may be glazed on the green clay—the term by which unbaked clay is known among potters. The piece should have dried thoroughly for several days, until it is light gray in colour, and what is known as bone dry. It is decorated, if decoration is required, and then glazed. All vessels that are used to hold or measure glaze should first be dipped in water to prevent waste from the glaze clinging to them. The hands should be washed thoroughly after working with glazes, as some of the materials used are poisonous.