Acid Substances. Silica (Si02) And Silicates

These are introduced in the form of white sand, used without any preparation, and of quartz and flint, which must be calcined and crushed.

These substances, in spite of their practically similar chemical composition, do not produce the same effect. Sand from one place would not take the place of sand from another; for instance, Fontainebleau sand is less fusible than that from Nevers, and flint is more fusible than Fontainebleau sand.

Alkaline silicates are only used in special cases. Among the silicates of alumina, those most used are the felspar and the non-ferruginous pegmatite; they enter into the composition of porcelain glazes and glazes for the faiences called flint-ware.

Kaolin and white clays enter also into the composition of glazes; they diminish fusibility and give an opaline transparency.

Boric Acid (Bo03h3) And Borax (Borate Of Sodium, Bo4o7na2, 10h2o)

The latter contains much water of crystallisation, and it must be calcined before use. The boracic glazes are hard and brilliant; at a high temperature they dissolve the metallic oxides and transform them into transparent enamels.

Basic Bodies. Alkalies And Alkaline Salts

The alkalies are more used in the form of carbonate of soda (CO3Na2 10H2O) or of potassium than in the free state. They dissolve the metallic oxides and give enamels a purity and transparency which no other fluxes obtain.

Among other alkaline salts: the chloride of sodium (NaCl) is frequently used in the preparation of the stanniferous enamels, which it purifies by dissolving all the impurities; applied to stoneware (salting), it gives it a brilliancy which does not make them sticky and which cannot be obtained by any other method. The nitrate of potash (KNO3) is decomposed at a dull red temperature setting free oxygen; this property is utilised for freeing frits from the organic impurities contained in them. It enters also into the composition of some enamels. In cryolite (double fluoride of sodium and aluminium) we have a means of introducing soda and alumina simultaneously into the glaze, for this body is decomposed at a red heat into these bases and into volatile fluoride of silicon. The glaze must then be silicious.

Alkaline-Earthy Derivatives

The carbonate of lime, CaC03 (chalk), and the sulphate of lime, CaS04, 2H20 (gypsum), form with silica, silicates which are not very fusible, but with borax they give glazes which are more fusible, and which are used for flintware and stoneware.

The fluoride of calcium, Fl2Ca (fluor spar), is sometimes used as a flux for pottery fired at a high temperature.

Derivatives Of Lead

The protoxide of crystallised lead, PbO (litharge), and its product of oxidisation, plumbate of lead (minium), are used in many glazes, being the more fusible because they are rich in lead.

The carbonate of lead, PbC03 (white lead), is also used for introducing lead into glazes.

As for the sulphide of lead, PbS (galena), it is in general use for the glazing of common pottery.

The oxide of bismuth (Bi203) acts like oxide of lead, but gives still more fusible silicates.

Derivatives Of Tin

Stannic oxide, Sn02 (tin putty), is the base of the opaque enamels, for it remains in suspension and does not dissolve in the vitreous substances; it is generally associated with oxide of lead. This mixture, called calcine, is prepared by calcining tin and lead together in a reverberatory furnace.

The oxide of zinc, ZnO, is sometimes used as an opaque body in silicious glazes; the same may be said of phospliate of lime, (P04)2Ca3.

2. Colouring Substances Entering Into The Composition Of Glazes

These are generally metallic oxides, sometimes salts, and more rarely the metals themselves.

The table on p. 390 shows the principal colours obtained, under different conditions, with the metallic derivatives either single or mixed.