Copper Red Glazes
Copper Red Glazes are one of my favourite glazes. The earliest known copper-red glaze occurs on wares made in Shanxi during the Tang dynasty (AD 618 to AD 907). The secret of to how to make these lovely reds was then lost for several hundred years and there is a story about a royal potter taking the secret with him to the grave leaving a disgruntled Emperor.
Copper is one of the oxides that provide colour to glazes. Copper provides a green to turquoise colour to glazes in an electric kiln but can turn a wonderful red in a reducing atmosphere found in a gas or a woodfired kiln. This wonderful red is not guaranteed as copper is quite volatile and the red colour develops as it is cooling, after the pot has reached Cone 10 (approx. 1300 C). If the timing of creating the reduction atmosphere in the kiln is too late then you can end up with cream or pale grey coloured pot with perhaps a smudge of red.
The other complication is that the atmosphere in the kiln is uneven and some areas will have better reduction than others and some areas will be hotter because the air flows around and over the shelves. The best temperature to start reductions seems to be around 800 C. Copper red glazes nearly always “break” on a rim or texture and become cream or pale grey. There are several types of Copper Red Glazes, Ox-Blood or “Sang de Boef”, Flambe and Peach Bloom. Even the names are evocative.
Below are two tiles with identical glazes, the left one fired in a reduction atmosphere in a gas kiln and the right in an electric kiln.
Electric kilns usually have a programme that controls the firing (the rate of the rise of temperature in the firing eg rise from 800 to 1200 degrees Celcius at a rate of 100 degrees an hour) so they are switched on and then you can walk away and come back to find the firing cycle completed. A gas kilns like I use, needs constant monitoring of the temperature over the 10 – 12 hour firing cycle and the gas is increased or decreased manually as required along with the opening or closing of the damper and adjusting air vents to control the atmosphere in the kiln.
So what is reduction firing? In an oxidising atmosphere (ie. electric kiln) there is plenty of oxygen across the firing cycle but in a reduction firing the unburnt gas is starved of oxygen ( by closing down the damper and the air intake valves) so it seeks to take oxygen from materials in the glaze thus altering their composition. It is quite fun to open the spy hole in the kiln door and seem the characteristic flame associated with reduction, shooting out the spy hole. There is also a particular smell and flame noise to alert you that the kiln is in reduction.