This comprises the pulverisation of the substances and the mixture of the powders so obtained. The crushing is effected dry or wet in various machines, mills, or cylinders, care being taken that there is no contact between the iron and the substances to be crushed. The grindstones, which are generally of hewn flint, are enclosed in an oak trough; they may be worked by hand (Fig. 704) or by steam (Fig. 702) or so made as to be worked in either way (Fig. 703). When the crushing is believed to be effected, the stopper is removed and the liquid mixture allowed to flow out. The crushing of the colouring substances which enter into the composition of coloured glazes is performed with grinding mills (Fig. 704).
The insoluble bodies in the water are subjected to direct crushing, but the soluble bodies (alkaline carbonates, boric acid, borax, etc.) must first be transformed into silicates or insoluble borates by fritting or vitrification.
Fritting is effected by heating the soluble substances in a crucible or on the floor of a kiln with other insoluble substances which enter into the glaze; under the influence of heat these different bodies cohere and form a mass, which is then pulverised. If the heating is pushed further, the substances melt into a glass,which is poured into water to disaggregate it. As vitrification requires more fuel than fritting, it is only used when indispensable for making glazes homogeneous; fritting, on the other hand, is preferable when the glazes contain chlorides or sulphides, on account of the substances which are formed, and which, in the fusion, would remain in the mass and produce explosions on contact with the water.
Fig. 701. Hand-worked Glaze Mill.
Fig. 702. Glaze Mill, worked by Steam.
Fig. 703. Glaze Mill (Laeis et Cie.).
Fig. 704. Grinding Mill.
In the form of sulphate, nitrate, or chloride, it enters into metallic lustres. The chloride of silver mixed with Cassian purple gives a violet
Antiraonic acid or its salt of potassium is employed for opaque glazes, the antimoniate of lead Naples yellow for the yellow ones.
Gives a gamut of yellows.
The oxide gives pure greens with
;; with alkaline glazes the tones are not harmonious. The chromates colour alkaline or lead glazes yellow, and the mixture of the two orange red.
Green in alkaline glazes.
Brownish black with excess of iron.
Green very little used
With not too much chromate of calcium, roses and reds are obtained.
Gives pink with chalk in an oxidising atmosphere.
The oxide has great colouring power, resists a high temperature, and gives always violet-blue which with addition of alumina becomes sky-blue.
Black or grey, brown with excess of iron.
Cupric oxide, CuO, with an alkaline glaze in an oxidising atmosphere gives turquoise blue, which becomes greener with boracic glazes and less intense with lead glazes. In a reducing atmosohere it forms with rple-red, which monies orange tinted or brown.
The copper gives hack to the green the bluish tone of the chromium in the alkn-line glaze.
Below 1000°, and in an oxidising atmosphere, ferric oxide gives a whole gamut from red to brown and violet. With alkaline glazes the colour is green or blue-green, with lead glazes it becomes yellow or brown. Yellow or red ochres are used.
Greenish black with excess of chromium.
Bluish black or grey with excess of cobalt.
Violet-black with excess of manganese.
The oxide gives a powerful reddish or bluish violet colouring with alkaline glazes, and violet-brown with lead glazes. It is much used for black in combination with other oxides.
Black, brown, or brown-yellow with excess of iron.
The oxide is not used alone on account of its uncertain tones: yellowish green with alkaline glazes, earthy green with boracic glazes, and green-brown with lead glazes. Combined with iron for browns.
The oxide is the base of the opaque enamels. The chloride enters into Cassian purple.
The oxide alone gives opaque white; it is combined with other substances for many colours.
An alkaline and chalk chromate gives pink.
In the metallic form it is much used for simple gilding or gilding fired under-glaze. Its other princi-pal use is its combination with chloride of tin to give Cassian purple.
In the form of stannous and stannic chloride it gives with chloride of gold
The oxide is pale yellow in the alkaline glazes, dark and orange yellow in lead glazes. In a reducing atmosphere it gives blacks.
Violet black with excess of manganese.
Frits are crushed either in the dry or the moist state according to the mode in which they are to be used.
As glazes are generally laid on in a state of paste, the mixture of their ingredients is generally effected in the presence of water.
The glazes thus obtained are colourless; but in order to colour them, it will be sufficient to incorporate in them a metallic oxide, but most frequently it is done by melting the whole together, then crushing and bringing it to the state required for the application. Coloured glazes are called enamels. The opaque enamels are of stanniferous base, and their principal constituent is calcine, a mixture of oxides of tin and lead in variable proportions obtained by calcining a mixture of lead (100 parts) and tin (15 to 3 5 parts) in a reverberatory furnace to a dull red heat. An average quantity of 120 kilog. of metals is treated, and yields 132 kilog. of calcine. This is then mixed with its own weight of Nevers sand, 2 per cent, of soda, 8 per cent, of salt, and 2 per cent, of minium, and the whole is melted. A high degree of heat is necessary to succeed in rendering the mass quite liquid, which is an indispensable condition. It is cooled, pounded, and crushed in presence of water to a state of fine division. Thus we get white enamel.
In order to obtain the other opaque enamels, oxides in a state of fine powder are added to the enamel. These are incorporated with it before it is fused, and consequently we get a more intimate mixture and can observe the colour better.
The fusibility of enamels varies with the nature of the pottery and the degree of firing to which it is subjected. The lower the temperature of firing is, the more varied are the enamels, and as it rises this variation diminishes, for it is not all colours which can resist a great heat, and only a small number of them remain fixed under the high temperatures of porcelain firing.