Post 4 - Glaze testing

Conventional advice is to begin glazing with commercial mixes and that is the approach I took. By this time I’d also pretty much committed to one type of stoneware clay as the best for the next stage of exploration.

Now an electrical kiln operates in an atmosphere where the oxygen in the heated air is at normal levels (oxidising) and so various components of a glaze when in molten form tend to be relatively more stable than the alternative. In a fired kiln (timber or gas) the oxygen is consumed and the atmosphere in the kiln is a reducing one – which tends to extract the oxygen out of many glaze components which are basically metallic oxides. This can cause interesting colour responses. With modern analytical tools and a thorough understanding of the interaction between the key components it is possible to achieve just as interesting results in an oxidising atmosphere.

Glazes are amazing mixtures of clay, minerals, silica and metallic oxides nearly all of which have individual melting temperatures well beyond the maturity or vitrification temperature of clay. The really interesting thing is that when you mix these powders together, they then melt at a temperature below the vitrification temperature. These eutectic compositions have been the subject of plenty of scientific investigation and are well understood. So the ancients did pretty well to get started on that path in the 8^th or 9^th century BC.

That makes me a late starter and here is my first  effort at testing variously sourced glaze recipes. At this point I’m principally chasing glaze formulations which have a matte or preferably satin finish. I’m not keen on the shine of high gloss and the satin surface is just as easy to keep clean. That said there are some pots which look quite good in gloss. It’s an interesting call to make.

Just for fun I thought I’d post my winged griffin. Modelled on an old European stove tile - clear Italian Green gloss over terracotta.