Lustres
There are several kinds of lustres, but the true lustres possess a pearly iridescence in addition to their colour. The copper and silver lustres of the eighteenth-century Staffordshire potters were thin metallic films over the whole surface of the glaze. Gold or silver solutions were used. Only where the gold was fired on a white clay is there any iridescence, and then hardly so pronounced as to deserve the name of lustre. The bismuth and other lustres made by the modern potter are combinations of metallic oxides and resinates dissolved in ethereal oils. These are painted on the glaze, transparent or opaque, but having almost invariably a lead base, and then fired at a dull-red heat. The medium disappears, and the metal in a finely divided state is deposited on the surface of the glaze. This, however, gives a painted look very different from the lovely irradiance of the Persian Hispano-Mooresque or Italian work.
With these the lustres were fired in a reducing atmosphere, one supercharged with combustible gases, the metals decomposed and fixed to the semifluxed glaze. The manipulation was not infallible and was attended with much risk; but the successful pieces are unrivalled, and according to Piccolpasso “paid in gold.” De Morgan, who more nearly approached the works of the Italian masters than any other
modern potter, used a very similar medium and method with his fine lustres. In the last few years lustres have been brought to such perfection, in preparation, application, and firing, that accident is practically eliminated. Glorious colours and gorgeous effects are obtained. Yet it may be said without senseless adulation of the merely mediæval that nothing has been produced superior, or even akin in spirit, to the work sent out from Persia, Spain, or the botega of the inimitable Maestro Georgio of Gubbio.
Lustres may be bought ready to apply. They are then painted on the glazed pot, which is re-fired at a dull-red heat in the ordinary oxidizing atmosphere. Once the painted look has been contrasted with the lustrous appearance of the reduced effects there is only one kind of lustre for the artist. Fortunately, perhaps, their preparation is not easy, and the correct method of reducing is a difficulty to be overcome by practical experiment alone. Hainbach gives many practical receipts for lustres that are not beyond the scope of a craftsman. The reducing atmosphere can be obtained in an open kiln by the introduction at the right moment of any combustible giving smoke free from all traces of sulphur.
In firing with a muffle kiln the introduction of coal-gas free from sulphur is a matter attended with some risk at the necessarily low temperatures
required by the lustres. It should be approached with caution, and each mixture will have a varied firing point, the correctness of which is established only by trials.
Lustres applied with skill and restraint enhance the most beautiful glaze, but in unskilled hands they inevitably vulgarize and cheapen. The fresh, unsophisticated renderings of the Persians or the Moors and the virility of the Italians should be studied, but not merely imitated before working in this medium.