For a pottery studio, this is one of the harder glazes to produce and use with control. Many potters develop a love/hate relationship with this one. Judy and I have spent over twenty years working with various copper red glazes, application techniques and firing schedules and our customers have liked some of the results.
Reduced Copper Red – Technical
Here is how we do it…..
The way we got ours was by mixing up 9 or 10 found recipes and firing them in several firing. We tested them on the vertical as well as on the horizontal all over the kiln before committing to the top performing recipe.
Here’s one to try, we used it for years.
Nepheline Syenite 42
Pot Feldspar 9
Tin Oxide 1
Copper Carb 0.3
This is the easiest to achieve. We have found most fine white stoneware clays work pretty well. Porcelain is better and the better the porcelain the better the red.
A good application
This you’ll gain from experience. Copper red must be thick and most copper reds run due to the low alumina content. If you don’t damage some kiln shelves during this phase then you are running the glaze too thin. Now-a-days we treat heavily glazed copper red pots in the similar way as a crystalline pot, they are lifted off the kiln shelves with blocks and placed on saucers made from the same clay. If the glaze is too thin it will be white or show white blemishes. We have poured, dipped, sprayed, brushed and trailed our red glazes.
You can reglaze and refire copper red pots. Refiring a copper red pot won’t make a green glaze go red. Refiring a red pot in an oxidising atmosphere won’t remove the red.
The first thing to notice with copper red is the way it highlights edges by thinning and turning white. Copper red will happily highlight unintentional imperfections too. The pots have got to be confident. You can’t have any blemishes. If you want to highlight edges then make them neat. Some times I might rub the unfired glaze thin on some edges to ensure highlighting.
Copper red needs an early and nice reduction. We start ours at 850 centigrade. We measure the reduction by ensuring a consistent flame out of the kiln bung holes but not up the flue (as this is too much and certainly no smoke). This is best done in the evening since the bung hole flame can be almost invisible in daylight. We don’t use an oxyprobe, although we own one, because its measurements don’t relate well to the conditions required by copper red. We keep this reduction up to 1200 centigrade after which it doesn’t seem to matter what you do. Just watch the reduction as it naturally increases as the temperature increases slowing the kiln unnecessarily. We gradually open the damper as the temperature rises while maintaining that bung hole flame. Fire to cone 10 but the top temperature is not very important (from cone 9 to cone 11 depending on the clay and the effects you’re searching for). The speed of the firing doesn’t seem to have a great impact on the results.
If the glaze is white then it was too thin or the reduction was started too late, the copper has escaped before being locked into the glass matrix. Watch the bag walls since any hot spots in the kiln can easily be too hot when the pyrometer is reading 860 giving white blemishes on the hot sides of the pots. If the result is green then the reduction was too light and if liver brown then reduction was too strong. Because our kilns are imperfect pieces of equipment, it is possible to get any combination of these problems in the one firing or even on one pot. I’ve spent a lot of time fiddling with the primary air controls on the gas burners, bag walling and the way the the kiln is set to even out the results