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.