PART VI: ALKALINE GLAZES

The glory of the Persian and Egyptian blue is too alluring for potters to withstand. Though the pursuit of this glory leads one into all kinds of disasters and failures, the avenues of research that it opens add unending fascination to the study. Even one beautiful glowing pot out of twenty or more efforts is a stimulating achievement though it should not be thought that this is the usual proportion.

It is a continual source of astonishment that with a slight variation of glaze formula a positive green will swim into a vibrating blue. The addition or substitution of one substance or another in the glaze mix may be the key to an unexpected transformation and may give the potter a new palette of color.

The clay body has a very positive effect on alkaline glaze both in its composition and its color. This is especially true under a transparent glaze where the effect is considerable since the color of the glaze would be modified by the red or buff clay showing through.

If, therefore, the object of the potter is to obtain a brilliant "Persian" blue, a white clay body must be composed or a white _engobe_ applied over the buff or red clay to hide the color.

The Persians and Egyptians used a coarse, sandy body high in silica and covered the roughness of the clay with a fine white _engobe_ on which they painted their decorations in various colors. The whole was finally covered with the transparent alkaline glaze.

While the effect of colored clay under opaque glaze is less pronounced, it still makes sufficient difference to be considered.

The word _engobe_ is French and refers to a thin coating of clay, also called a slip, laid over a colored body to change the color or over a coarse body to give a finer texture.

The _engobe_ is usually composed of china clay, flint, and feldspar much as a white earthenware body is constituted but with a larger content of flint. Ball clay may also be used but the color is not so white.

The mixture of porcelain given on page forty will make an _engobe_ suitable for many clay bodies. If it should crack on drying more flint should be added.

An _engobe_ must, of course, be put upon the unburned or green clay ware and this should be leather hard, not dry. The body with the _engobe_ may be burned before glazing or the glaze may be put upon the unburned ware and the whole subjected to one fire only.

The ingredients in alkaline glazes are soda-ash, whiting, feldspar, flint and oxide of tin. The following is an example of a fritted glaze:

Na_{2}O .60 } K_{2}O .10 } Al_{2}O_{3} .10 SiO_{2} 1.30 CaO .30 }

Soda Ash 64 Whiting 30 Feldspar 56 Flint 42

The entire batch is fritted and ground in a ball mill with the usual amount of water for fritt grinding, adding a tablespoonful of gum tragacanth mucilage to the batch after it is sieved. The glaze should be the consistency of heavy cream when used.

It is also possible to use an alkaline glaze in the raw or unfritted state. This necessitates grinding by hand in a mortar, but great care must be taken to mix the dry ingredients thoroughly before adding water and to stir the glaze constantly while pouring in the water, otherwise the soda-ash will cake and harden and be very difficult to break up. A batch of glaze can be ground by hand in fifteen or twenty minutes if done vigorously. It is then put through a 120-mesh sieve. The consistency is of importance. If too much water has been added and the glaze has become thin, it cannot be used successfully and should be discarded. Unfritted alkaline glaze does not keep well when moist but the ingredients can be ground dry and kept ready to be moistened as needed.

The following is an example of an unfritted alkaline glaze:

Na_{2}O .59 } CaO .21 } Al_{2}O_{3} .20 SiO_{2} 1.6 K_{2}O .20 }

Soda Ash 62 Whiting 21 Feldspar 111 Flint 24

For color add the following oxides to a batch.

1. Egyptian blue, opaque--from 5 to 8 grams of black oxide of copper--16 grams of oxide of tin.

2. Persian blue, opaque--from 8 to 10 grams of black oxide of copper--16 grams of oxide of tin.

3. Sapphire blue--1 gram black oxide of cobalt.

4. Aubergine--9 grams black oxide of manganese.

The clear glaze without any coloring oxide can be used over any of the colored glazes. This is sometimes necessary when the colored glaze contains such a large proportion of coloring oxide as to show black on the surface.

The application of alkaline glaze is very important. Any of the three methods of pouring, dipping, and brushing can be employed. Brushing seems to give the best results but the glaze must be put on thick, in two or three coats, to give quality.

The firing is interesting and important because of the varied effects it develops from the same formula. The range of temperature is great, varying from cone .05 to 1, developing the alkaline glaze according to the result desired. If the biscuit is soft fired the color will be more intense; if hard fired, the color will be much lighter in value with a high sheen on the surface. An unfritted alkaline glaze burned to .05 develops a soft matt finish.

Where the color of a transparent Persian blue comes out olive green, too little glaze has been used on the piece or the buff of the clay has modified the color. Bubbles mean undeveloped glaze or sulphur in the clay or fuel. Black scum shows an excess of copper in the batch, or reduction in the fire. Sand paper surface proves too low firing or too thin a glaze.

If one desires to reproduce the underglaze Persian decoration the black outlines may be drawn with a black underglaze color mixed with clay. A little mucilage must be added to secure smooth working. The turquoise blue is copper oxide, the dark blue cobalt, and the purple manganese. The oxides must be diluted with white clay and used rather thin. The Rhodian red is a finely ground red burning clay mixed with a little flint. This red must be laid on quite thickly. It will probably be found necessary to fire the painted decoration to about cone .03 before glazing. The glaze may be either quite clear or slightly tinted. Another effect may be produced by using the black outline alone under a peacock blue or turquoise glaze.

A great many modifications and additions to this subject will suggest themselves to the potter as he works, and a continual study of the masterpieces of the Persians in the museums will prove the greatest inspiration.