No immutable law can be laid down for the exact thickness of the coat of glaze. All glazes vary. One sixteenth inch may be enough for one and far too thin for another. From one twentieth inch for thin transparent glaze to one eighth inch for matts is a fair average. It is well to err on the thick side to avoid an impoverished look. Trials on odd pieces of biscuit fired in horizontal and vertical positions will best settle the point.
In all cases the inside is first half filled with glaze which is rolled quickly round and out. The deposit is then tested with a knife. The glaze for the inside should be slightly thinner in composition than for the outside, as it is inclined to pool in the bottom if too thick. The outside and the neck are then scraped free of all splashes.
Fig. 50
To glaze the outside of the pot the methods employed are painting, spraying, dipping, and pouring. In painting, two or three coats applied with a flat soft brush may be necessary. In spraying, the glaze is thinned considerably and blown through a vaporizer by means of a foot pump. (Fig. 51.) The shape, its mouth closed with a small bowl or saucer, is placed on a whirler in a draught box and slowly revolved. The draught draws the waste spray away from the operator, who should wear a respirator. This method is excellent where any gradation is required. It is easily learned. Unfortunately, it requires expensive apparatus to render it safe with lead glazes. Unless it is done on a big scale and the waste glaze retained, it is also wasteful.
In dipping, the pot is plunged right under in a tub of glaze which is kept well stirred to prevent the heavy constituents settling. (Figs. 52 and 53.) This requires considerable dexterity. In a school, glaze is seldom mixed in sufficient quantities to permit of this being acquired.
Perhaps the most satisfactory method is pouring. (Fig. 50.) The shape is reversed and stood upon a big stilt or two strips of wood over a bowl or tub. Then the glaze is poured from a jug round the edge of the base, until every part is covered. With a little practice a quite even coat can be thus applied.
Owing to the inversion of the shape it is slightly thicker at the shoulder and neck, an excellent point in glazing. When dry, the base is scraped and wiped clean with a sponge and the lip touched up with a brush.
Fig. 51