When the painting is finished, the colour is hardened on; that is, the gum and medium are fired off in the kiln, a dull-red heat being sufficient. This does anything but harden on, however, and the pot must be handled very carefully or the colour will rub off. The glaze should now be gently sprayed on, and then the final fire is given.
Another way is to apply a very thin spray of glaze before hardening on, just sufficient to fix the colour. The pot may then be dipped or poured without risk. In each case the oily medium must be quite dry before the hardening on takes place. After the glost fire the decoration is fixed and unalterable. Where possible, a hard transparent glaze is best for fine work. A soft glaze will always run if slightly over-fired, and the result is the obliteration of all brushwork.
Over-glaze decoration is applied in a very similar manner. Turpentine, fat oil, and lavender oil are used: the turps to run the colour, the fat oil to stiffen, and the oil of lavender to retard the drying.
The colour must be applied evenly and thinly, thick patches being likely to peel or crack. On hard glazes this process lends itself to elaborate effects. The hard and fast colours, the blues and greens, may be fired first, the delicate pinks and greys last. The whole effect may then be enriched with low-firing lustres. These, when bought in bottles, are ready to use and are applied directly with a fine brush, then fired at a dull-red heat. The pot should then be quite finished; quite frequently it is.
Pâte sur pâte or painting in relief colours is another process that has many attractive features. The colours have a clay carrier and are applied with a gum medium. Painted boldly with a certain amount of relief, this gives a rich enamel effect very suitable to simple figure decoration.
The full equipment of the painter will be as follows:
Colours, under- or over-glaze. Brushes, tracers, and shaders. A stick frame for holding the vase. Turpentine and lavender oil. A slab of ground glass. A muller for grinding. India ink and a colour slab. A palette knife of horn for very delicate colours. Some soft rags.
Before risking decent shapes in the fire, trials,—on
biscuit for under-glaze, on glaze for over-glaze,—should be made repeatedly. Graduated strips and stripes tartan fashion are the most useful and easily tabulated. To lay perfectly flat grounds some skill and practice are necessary. One method is to paint in the ornament or rather the space it will cover with thick molasses or black treacle. This is allowed to harden and the background colour applied with a soft dabber. It must be ground fine with fat oil and applied very evenly. Then the tile or vase is soaked in water, which causes the treacle to peel off. The oil is allowed to dry and the piece fired. This fixes the background, and the decoration itself is next applied.
Pierced work if skilfully done is most attractive. The pattern may be incised on the “original,” which is moulded, the design then showing in relief. This again shows as a slightly engraved pattern on each cast form or shape. Then with the aid of a fine-pointed knife or plaster tool the pattern is cut out and the edges are softened to take away any metallic look. The Chinese and Persians are said to have used rice seeds in some of their translucent effects. The seeds were embedded in the moist clay to form a delicate tracery. When fired, the grains disappeared, leaving holes which were completely filled with glaze.