Grinding the Glaze.—The grinding should continue about an hour. It should be said, however, that there is such a thing as grinding too fine. After the glaze has been properly ground a small quantity of gum tragacanth, dissolved in water, is to be added as a binder to prevent flaking and rubbing off in handling. It is also a good plan, though not always necessary, to strain the glaze, as soon as it is ground, through fine muslin. The tools and materials needed for this work may be itemized as follows: Lead carbonate, whiting, Canadian feldspar, Florida kaolin, French flint, white oxide of zinc, and various other oxides and colours noted in the text; earthenware bowls, 10 or 12 inches in diameter, for holding the glaze; large spoons, preferably aluminum; agate mortar and pestle, 8 inches in diameter.
Example of Matt Glaze and Bright Glaze Mixes.—Two mixes are given below calculated to fuse at cone 04. One has a "matt" or dull velvety surface, and the other has a "bright" or shiny surface. The matt will be found more desirable for general work because it harmonizes better with flowers and foliage. The figures given below refer to units of weight. Metric weights (grams) are most convenient to use, but any units will serve so long as the same one is used throughout. The matt glaze should be of the consistency of thick cream, the bright glaze somewhat thinner.
| MATT BASE | BRIGHT BASE | ||
| Lead carbonate | 154 | Lead carbonate | 142 |
| Whiting | 25 | Whiting | 20 |
| Canadian feldspar | 83 | Canadian feldspar | 84 |
| Florida kaolin | 51 | Florida kaolin | 8 |
| White oxide of zinc | 8 | ||
| French flint | 44 |
The bases itemized above are colourless and there must be added to them from 3 to 6 per cent. of colouring material that will stand heat. A list of materials with their colours when fired is given on [page 304], together with a few typical combinations of colours in quantity suitable to be added to the glaze bases given above. The following lists are by no means complete, and it is expected that the young decorator will make use of these colour mixtures simply as an introduction to quite extensive experimenting:
COLOUR MATERIALS | |
| Black oxide of cobalt | Blue |
| Black oxide of copper | Blue green |
| Green oxide of chromium | Yellow green |
| White oxide of antimony | Lemon yellow |
| Red oxide of iron | Yellowish brown |
| Green oxide of nickel | Dirty gray (for neutralizing) |
| White oxide of tin | Makes glaze opaque |
| Black oxide of manganese | Purplish brown |
| Yellow ochre | Yellow |
| Burnt umber | Brown |
COLOUR COMBINATIONS | |||
| Olive green | Blue-green | ||
| Iron oxide | 5 | Cobalt oxide | 1 |
| Cobalt oxide | .5 | Copper oxide | 7 |
| Yellow ochre | 4 | ||
| Dark brown | Dark gray-blue | ||
| Iron oxide | 3 | Nickel oxide | 5 |
| Nickel oxide | 2 | Yellow ochre | 3 |
| Manganese oxide | 5 | Cobalt oxide | 2 |
| Copper oxide | 1 | ||
| Light gray-blue | |||
| Tin oxide | 5 | ||
| Cobalt oxide | .5 | ||
| Copper oxide | 1.7 | ||
Applying the Glaze.—Before applying the glaze to the piece of pottery or bisque, the latter should be soaked in clear water for about five minutes or until the air is expelled from the pores. When this has been accomplished it should be removed from the water and all moisture should be wiped from its surface. The piece is then ready to be dipped into the glaze, or to have the glaze poured over it, if the size and shape of the piece make it more convenient to apply the glaze in this way. It is usually best to glaze the inside first, shaking out all the superfluous glaze before applying the glaze to the outside. During this process the piece must be held firmly but by as few points of contact as possible. These points of contact will generally need to be touched up before it is ready for firing.
It is sometimes necessary to glaze large pieces with a brush, putting on several coats in order to cover the surface with an even thickness. The matt glaze requires a greater thickness than the bright glaze in order to develop its characteristic velvety texture. The greater thickness may be easily secured, because it flows more slowly than the thinner bright glaze and is less likely to drip from the sides of the piece to which it is being applied; but, on the other hand, it is at a disadvantage in that it does not, in flowing slowly, correct inequalities of thickness so readily as the more mobile bright glaze does. A certain thickness, however, is essential; and if, after firing, it is found that the glaze was put on too thin, a second coat may be applied and the article again fired.
Firing the Glaze.—Before the glazed piece is fired the glaze should be dried and what runs down and collects about the bottom or foot should be scraped off. It should then be set in the kiln on a kind of pointed tripod of hard burned clay, called the "stilt," which prevents the glaze from sticking to the floor or shelves of the kiln. All glazed ware should be placed in the kiln with at least 1⁄2 an inch of space between the pieces to prevent them from sticking together during the fusing state of the glaze, when it is apt to bubble or "boil."