Making Your Own Raku Glaze
If you are making your own raku glaze, you may need to experiment with the firing temperature. There are many different raku glaze recipes floating around the internet and most of them do not have a recommended firing temperature.
If you consult a glaze book, such as ‘The Glaze Book’ by Stephen Murfitt, you can find numerous raku glaze recipes. And helpfully, he includes a recommended range of cooking temperatures for each recipe.
Different Raku Firing Techniques
There are numerous different raku techniques and potters tend to have different firing temperatures for each.
Firing Temperature for Horsehair Raku:
When making horsehair raku, potters usually have a lower raku firing temperature. There are several reasons for this.
First, with horsehair raku, you remove the pottery from the kiln and apply the horsehair. Unlike most normal rakus that enter a hot reduction chamber, horsehair raku cools in the open air.
This makes it more likely that the pottery will crack due to thermal shock. If you fire your pots to a temperature of about 1025F (550C) this is hot enough to burn the hair. But this temperature is just below the quartz inversion temperature, where pots are most vulnerable to cracking.
However, the higher incidence of cracking is not the only reason for making horsehair rakus at a lower temperature. If you put horsehair in a very hot pot, the hair will burn before it sticks to the pot.
Temperatures up to about 1400F (761C) should be low enough for horsehair to adhere. This is approximately cone 017.
Firing Temperature for Naked Raku:
Bare raku is a process in which a thick slip is painted over bisque pottery. The slip cracks when it dries in the kiln. Some areas of the pot are protected by the remaining slip. And some areas where the slip has cracked are exposed. When this is placed in a reduction chamber, the exposed parts of the pot will darken through carbonisation.
Normally a temperature around cone 015 is sufficient for the lining to shrink and crack. Thus, the typical raku firing temperature for naked raku is around 1450F (787C).
Firing Temperature for Copper Matte Raku:
When I learnt to make copper mat raku, I was told that the best target temperature was around 1922F (1050C). Like a diligent student, I stuck to this guideline. However, since then I have heard other potters suggest different target temperatures. Some were much lower than the temperature I fired at. Others argue that the maximum temperature for copper mat raku should be around 1750F (954C).
I mention this because raku firing is unpredictable, personal and open to experimentation. What works for me may not be so good for you. While it's good to have a raku cooking temperature in mind, it's also good to be open to experimentation.
How to Measure Raku Firing Temperature?
Many potters, myself included, use a pyrometer to monitor the temperature of my raku kiln. Some potters rely on the appearance of the glaze. They look up the chimney of the kiln and check what the glaze looks like. At some point the glaze will bubble and boil. Then when it starts to mature it will settle and take on a glossy wet appearance.
I must say that it is not possible for me to see the inside of my kiln so clearly while making raku. Even with oven goggles, the chimney gives off too much heat for me to look inside. So I rely on my pyrometer and it serves me well.