Glaze is a glass built up of 2 or more silicates. The normal felspathic glaze consists of sodic or potassic and aluminic silicates; salt glaze, of sodic and aluminic silicates; lead glazes are mixtures of plumbic with aluminic silicate, and small quantities of sodic and potassic silicates. There are also glazes containing zincic silicate and ferric silicate, and in some cases one silicate is replaced by a borate. Glazes may be rendered white and opaque by the addition of an infusible excess of stannic and arsenic oxides, and may be coloured by metallic oxides, in the same manner as glasses. Glazes are applied by dipping, and by volatilization. The difficulty in preparing a glaze is the regulation of the mixture, so that the contraction of the glaze after fusion shall not be unequal to that of the body to which it is applied. The decorative effect of "crackle - glaze" is obtained by an adjusted disagreement between the body and the glaze. If the disagreement be allowed to go too far, the glaze chips away from the body. The requirements of a glaze are - (1) agreement with body, (2) power of resisting solution and corrosion, (3) purity of colour, (4) power of developing applied colours.

The felspathic and sodic silicate glazes are the most durable; the plumbic silicate glaze is liable to gradual decay, which is indicated by the appearance of a beautiful iridescent film. "Smears" and "flows" are glazes applied by volatilization, in the former process, the saggers are washed inside with a mixture of one or more of the following substances: salt, red - lead or litharge, potassic nitrate, potassic carbonate, and china stone. The ware, generally fine stoneware, is exposed in these saggers, and receives a gloss by the deposition and combination of the volatilized mixture. The object of the use of "flows" is to soften or blur the outline of under - glaze painting or printing. Amnionic chloride, alum, and chalk, together with one or more of the materials used for "smears," are placed in small biscuit - cups in the saggers, together with the ware. The glass, which is formed by the deposition of the mixture on the ware, partially dissolves the colouring oxide, and softens the general effect. Plumbic silicate glazes are coloured by metallic oxides, and are used for colouring ware. In this way, imitation "crown" ware is often manufactured.

A mottled or marbled surface is sometimes produced on ware by instilling differently coloured glazes from a vessel containing several chambers communicating with a composite neck. Opaque glazes or enamels are used to conceal a coloured body. Majolica is generally made of common fire-clay or marl, and is coated with opaque white enamels, upon which, whilst still moist, coloured enamel decoration is applied. Limoges ware is a clay body decorated by the aid of coloured enamels. In Cloissonne ware, the outline is marked out by metallic threads soldered to a metallic body, and the interstices are filled up with enamels. Imitation Cloissonne' is produced by painting on a white clay body with coloured enamels, or by forming a raised outline by painting with a mixture of iron and copper dust, hardening the same by fire, and filling in with colour. Palissy ware has a white or coloured body, covered with transparent coloured glazes. Bristol ware is coloured by coloured felspathic glazes. A curious lustrous or glistening effect may be caused by applying coarsely - powdered mica to the surface of ware. Genuine lustre or iridescence is produced by the irregularity of a glazed surface, caused either by decay, or by the adhesion of an almost imperceptible metallic film.

Bismuth, gold, silver, copper, zinc, iron, and platinum are used for this purpose. The metallic salt is generally mixed with some strong reducing agent, and applied to the ware as a paint. The ware is then fired in a reducing atmosphere, the salt is reduced, and the metal is fixed upon the glaze of the ware by heat. For Brianchon's lustre, which is similar to that used at Bel leek and Worcester, a mixture is used of bismuth nitrate, rosin, and essence of lavender. If ferric or uranic nitrate be added to this mixture, the glaze of the ware will be tinted by the ferric or uranic oxide, and the effect of the lustre will be heightened. Instead of applying the reducing agent together with the metallic salt, a reducing vapour may be directed upon a pigment rich in copper, silver, or other metals, with similar results. The red lustre of Gubbio ware is due to the action of smoke upon cupreous oxide; it is usually applied to a coloured body. (Spons' Encyclopadia)

There are 3 sorts of glazes - viz. lead, salt, and felspar. Lead glaze can be formed in two ways. (1) The biscuit - ware is dipped into a tub containing a mixture of about 60 pints litharge, 10 clay, and 20 ground flint, diffused in water to a creamy consistence. When taken out, enough adheres to give a uniform glazing when re - heated. The pieces are then again packed up in baked clay cases resembling bandboxes, called saggers, with small bits of pottery between, and fired in a kiln. The glazing mixture fuses at a very moderate heat, and gives a uniform glossy coating. (2) The common clay vessels are painted over with red - lead. If the vessel thus painted is exposed to a moderate red heat, a glaze is produced. These red - glazed vessels are most dangerous for household use, since the glaze is dissolved off by acid. Salt glaze consists in throwing common salt into the kiln in which the vessels are heated. The salt is volatilized and decomposed by the joint agency of the silica of the ware and of the vapour of water always present. Hydrochloric acid and soda are produced, the latter forming a silicate, which fuses over the surface of the ware, and gives a thin but excellent glaze.

Felspar glaze may be either ground felspar or a mixture of gypsum, silica, and a little porcelain clay diffused through water. The piece is dipped for a moment into this mixture, and then withdrawn; the water sinks into its substance, and the powder remains evenly spread upon its surface; it is once more dried, and lastly, fixed at an exceedingly high temperature. The coarse stone - ware made at Bristol is glazed by the vapour of common salt, as is also the Staffordshire flint-ware. The last is the best. The yellow glaze is made by mixing together in water, till it becomes thick like cream, 112 lb. white lead, 24 lb. ground flint, and 6 lb. ground flint glass. Manufacturers in this differ. Nottingham black glaze: - 21 parts white lead (by weight), 5 powdered flint, and 3 manganese. Another method is to immerse dried pottery in sea - salt, and bake.