Copper, silver and gold have been used to create lustres - a metallic sheen - on or within ceramic glazes from the earliest beginnings of the technique.
Lustred ceramic making began in the Middle East over a thousand years ago. It would appear from the earliest known pieces that the initial intention was to give ceramic objects the appearance of solid gold or silver. It would seem likely that finely powdered metals would be incorporated into the unfired glaze mixture to be fused into the molten glaze during firing. This could work if precisely the right conditions were met during the firing. However, these conditions are pretty exacting. If a glaze containing the metals were fired to maturing point and merely allowed to cool naturally then a disappointing whitish or grey stain would result, or green in the case of copper. The metals would have been oxidized in the fire and to be converted back to the desired shiny metallic state `reduction `would be necessary – ie the atmosphere within the kiln chamber would need to be held oxygen-free for a specific length of time. This is usually achieved by loading the kiln with fuel and sealing all entrances and exits to the kiln to prevent oxygen from penetrating, thus for combustion to take place oxygen is taken from the metals thereby reducing them back to their metallic form. This has to be done at precisely the right point during the cooling – too much reduction causes the glaze to load with carbon and blacken. This is rather difficult to get right and the mountains of rejected shards at ancient kiln sites evidence endless trial and error.
From this experimentation, it would easily become apparent that as well as creating bright metallic surfaces of copper, silver or gold other rather pleasing effects were happening. Where the metals overlapped or were applied thinly, shimmering surfaces and rainbow iridescence may have been produced and thought desirable and it is a small step to purposely use these decoratively. From `happy accidents` new techniques were discovered. Early kilns, being largely wood-fired, and hard to control rather favoured random happenings. However judging reduction temperatures must have been rather tricky having only the colour of the kilns interior to go by. Even with modern temperature controls, it remains problematic and alarmingly easy to get wrong – humidity, wind speed, type of fuel, the pressure within the kiln, and speed of firing have to be taken into account and success depends on intuition, instinct and considerable experience.
The knowledge was hard-won and not surprisingly closely guarded and regarded as mysterious and connected with alchemy. Then as now whenever glaze recipes are passed on some crucial element is omitted. Each generation seems to have to invent its own glazes and methods and guard its own secrets, and although modern chemical analysis can reveal the exact chemical composition of a glaze, it cannot inform us how the glaze was fired and therein lies the secret. I have now made lustres in four different kilns and each one has needed substantially different firing regimes and each one favoured different colours. The first kiln favoured reds whilst the current one makes better blues than any of the others.
This technique, known as in-glaze reduction lustre, where metals are suspended within the glaze, is not the only method of lustre making. There are several others. Each producing different effects. However, for me, it is the most direct and creates the widest range of colours and surfaces. It is the one used to make all the pieces in this exhibition. Nor are copper, silver, and gold the only metals capable of forming a lustre, but they are the most reliable and to my eye the most pleasing. I find silver the most versatile and as well as making solid silver surfaces it can make a range of delightful blues, turquoise, greens, yellows and orange and very rarely an amazing jewel-like emerald. Copper is the most reliable and in addition to the familiar bright copper a range of reds and purples. Gold used on its own is the most problematic. It volatizes rather easily and disappears into the ether. Its greatest value is to brighten and extend the range of the other two metals. However, the heavenly purple it very occasionally produces makes it irresistible to attempt. Thus with copper, silver and gold, the spectrum can be covered.
Sadly it is not possible to produce precise lustre colours at will. A kiln is a very blunt instrument to perform a very delicate operation. All who attempt lustre despair at the seemingly capricious nature of the process, and must, as I do, take comfort from the much-quoted statement by the sixteenth-century chronicler of ceramic techniques when on writing of lustreware he states ‘—the art is so uncertain that out of a hundred pieces hardly six are good … but when these wares are good they must be paid for in gold’.
Sutton Taylor