I’ve been working on some new glazes and these two have worked really nicely on this little dish (7cm diameter). Both the grey and the green use a titanium mineral called Rutile. Green glazes often use copper or chromium which I wanted to avoid. Here the green is created by the interaction of the rutile with a little cobalt (which usually comes out blue). I love the brilliance of the lighter green against the deeper tones.
For the technically minded, the green is sensitive to the underlying base glaze chemistry. The grey here is my standard magnesium silky matt base glaze with added rutile. Using cobalt with this would produce blue instead of green. To get the green I’ve had to use a different base glaze.
I still have more testing to do on this glaze before I’m happy with it – although I love the colours it’s very powdery and doesn’t apply well to unfired pots so I have to do more work on the physical chemistry to make it a better fit for my raw glazing processes. Oh well, back to the test tiles…
An interesting article in the Northern Potters Association bulletin this month about using less energy in firings. It suggests, amongst a range of other measures, considering dispensing with biscuit firings by raw glazing. This is exactly what I do.
Along with some other potters I do just one firing, applying glaze to the “raw” unfired clay (hence the term “raw glazing”). This is an uncommon technique as the majority nowadays do two firings. The first firing, called the biscuit or bisque firing” is to harden the clay to make a solid and consistent surface to work on when applying glaze and decoration. This is then fired a second time to melt the glaze.
Single firing is actually a much older technique going back to antiquity and probably how most pottery was fired through human history. However single-firing is not without it’s challenges – which is why the second firing was introduced – and so it does require a carefully selected approach to the way glazes are formulated and applied. This for me is where the fun and interest lie!
How much energy does it save? The short answer is, as with all things in ceramics, it depends… Single-firing omits the bisque firing, but as this would typically be a relatively low temperature firing, and often the single firing may need to be slower to compensate, in the end the savings are not quite as clear cut – perhaps nearer to 35% saving than 50%. There may also be other savings that can be made by changing clays, firing temperatures and timings etc. However single-firing is still a winner for the environment and your budget too.