With matt glazes, as has been noted, a thick coat is necessary. If very porous, the shape may be soaked first in clean water to take some of the suction out of the biscuit. When glazed, it is essential that all dips and inequalities be removed. If thinly coated, the glaze will assume a glossy surface when fired.
Fig. 52
Whatever process is used, the lip and base of a pot should receive careful attention, the rims especially should be thickly and evenly coated with glaze. The thickness can be tested with the point of a knife and any scratches or bare spots should be filled in and rubbed flat as they are apt to show at the finish if the glaze is at all refractory. Never starve the glaze, for a thin coat imparts a poor cheap look to any pot that is seldom remedied with entire success.
When only one kind of glaze is used, the application is quickly learned. With hard and soft, lead and leadless glazes, both opaque and transparent, and possibly a combination of spraying, painting, and pouring, the difficulties are greatly increased. Practical experience will be the only safe guide. But speaking generally, a sprayed coat can be thicker than
a poured coat. Hard glazes give less trouble than soft if too liberally applied. Tin glazes will stand a lot of over-firing even when thin and the reverse holds good of matt glazes. Raw borax glazes require much more careful firing than raw lead glazes but often give better colour results and are less liable to sulphuring.
Fig. 53
When glazing with a transparent glaze over under-glaze painting a thin coat is advisable. If it be thick and run, the painting is spoiled; but if, after firing, it appears thin, another light spray may be tried. But whole chapters of writing will never settle these points. Repeated trials on odd pieces of biscuit will elucidate more than many books. In packing a glost or glaze kiln stilts and spurs instead of saddles must be used to prevent the pots from sticking to the floor or shelves. (Figs. 49 and 54.) For the same reason the pots must not touch each other. With glazes requiring a high temperature the space between pots of different colour should be considerable. An appreciable amount of glaze is liberated in the intense heat and a blue pot will often leave a distinct blue patch on any light pot placed too near. Soft
glazes liable to run or drip should be placed on separate bats well flinted beneath. Any crack in the muffle should be well stopped and lime may be used to neutralize the effects of any sulphur that may enter.