|requiring a second firing:|
The cold tin-opacified glazed vessel emerges from the kiln with a hard white glazed surface, perhaps with parts of the design picked out in coloured pigments. If the potter is making a lustre vessel, the next stage in the decorative process now begins. The lustre pigment consists of a mixture of compounds of silver and copper finely ground together and mixed with an earthy medium, and diluted in grape juice or vinegar. This is painted onto the surface of the fired vessel according to the required design. The vessel is then put into a reducing kiln and fired, according to Abul-Qasim, at very low temperatures (around 600-700°C) with a light smoke for seventy-two hours. Quantities of iron and sulphur in the pigment help reduction.
Carbon monoxide in the reducing atmosphere extracts oxygen from the silver and copper oxides, which are thereby deposited as a thin layer of metal on the surface of the glaze and bonded with it. When the vessel has cooled, it is rubbed with damp earth, or clay, or wool, to remove the earthy medium in the pigment and bring out the lustrous sheen. Silver gives a yellowish tone to the lustre, and copper a red tone. The colour of the lustre naturally depends on the relative quantities of copper and silver, but is also affected by the firing temperature and the kiln atmosphere: there is a very small margin of error in both, to within 20°C.
It is very easy with lustre wares to underfire, resulting in a non-reflecting stain on the glaze, or to overfire, leaving a dull thin lustre where the metallic oxides volatilised and were carried away with the other gases. Uneven firing is also common. Test pieces were probably removed from the kiln during firing to see when the pots were ready. Nevertheless there was probably a high rate of failure, judging from contemporary experiments by the lustre-potter Alan Caiger-Smith.
This technique of pottery decoration originated on glass, possibly in Egypt and spread to Iraq. From the above description, it is clear that it is a complicated and delicate technique and the formula seems to have been a secret held only by a select few: there was no clue in the finished product as to how it was produced, so the transmission of the technique must have been through the migration of the potters themselves. The high cost investment in the raw materials, fuel and labour also suggest that there were significant financial rewards for those who knew the lustre formula, and this encouraged them to keep that formula a family monopoly.
"Minai" is a translation of the Persian word meaning "enamelled". This was a technique that seems to have become obsolete by the time Abul-Qasim was writing, as he says "they have passed into oblivion at this time" (which was 1301), but seems also to have been practised at Kashan in the C12th and C13th. Some of the most magnificent Persian vessels have been produced in this technique, for example the Shah Nameh beaker, and the dish with an elaborate battle scene, both in the Freer Gallery in Washington DC.
Parts of the design were painted in pale blues, purples or greens on the raw glaze before the first firing, then black outlines and other supplementary colours (such as black, red, white) were mixed with a vitreous flux and painted on to complete the design. Gold-leaf was also applied to the decoration, but this was attached using gum arabic, or some other adhesive, and was not fired. The vessel was then fired again in a reducing atmosphere, so that the glaze softens just enough to hold the colour of the applied pigments. There are some rare experimental examples of minai combined with lustre, but it is a highly risky process because of the cost involved and the frequency with which it seems to have been unsuccessful.