Last week I mixed together a few variations on the Nepheline Syenite matt white which was itself a variation on a white base glaze in Jeremy Jernegan’s Glaze Handbook . This glaze has been working really nicely in reduction so far with a smooth, soft satin finish, blushes of pink and lilac and minimal running. I wanted to see if variations in the colour were possible with the hope of pale mint greens and turquoises reminiscent of the hazy, sunny colours in a Wes Anderson film. The additions of colourant to the base glaze were:
- 1 Cobalt carbonate, 2 Chrome Oxide = Turquoise
- 1 Cobalt carbonate, 2 Nickel Oxide = Grey-blue
- 2 Ilmenite, 2 Rutile = Brown
- 1 Cobalt Carbonate = Blue
The colours on the right indicate what colour the metals add however the glazes are all a little more brown and dark than I expected. The iron rich clay body I use is probably a factor in this, as is the fact that the glaze includes Nepheline Syenite which I already know adds a pinkish hue to this recipe. Swapping the feldspar back to the original potash or soda and using a porcelain slip underneath might remove the red tint from the colour and lighten the glaze. I’m hesitant to change the clay body itself. I’ve had problems over the past two years with jumping from one clay to another which results in lots of reclaim and the problem of keeping the different clays apart or the unpredictability if you mix them. I’ve decided this year to have fewer variables and hopefully learn something by working to get this clay to suit my artistic expressions.
Chrome oxide gives glazes a green colour (in percentages up to 2%) and mixed with the cobalt carbonate, a common blue colourant, gives turquoise. Cobalt carbonate is different to cobalt oxide in that it is a slightly less powerful colourant and since it’s a lighter powder, it can spread more evenly through the glaze.
Nickel oxide on its own produces green/brownish grey and in combination with chromium oxide creates more attractive shades of those colours, however here it serves to dilute the brightness of the cobalt.
Ilmenite is the name for the combination of iron and titanium oxide and as well as giving brown speckles in glazes is used in many crystalline glazes. Rutile is again titanium oxide but this time with up to 25% iron oxide.
Source: The Potter’s Dictionary of Materials and Techniques