wenders Posted May 6, 2015 Report Share Posted May 6, 2015 Hello, all. As an advanced beginner, I know a few things about clay and glazes but have not really studied the chemistry. I did do some searching before asking this question but cannot seem to find an answer so I would appreciate replies! I have a straight-forward semi-transparent glaze recipe: - china stone 20 - carb whiting 23 - kaolin 30 - quartz 27 This is a wonderful glaze that I used at La Meridiana School (in Italy), and it fired beautifully on porcelain in both reduction (1280C) and oxidation (1260C). In my studio here, I have used EPK (for raku slip resist), but we also have a bag of calcined kaolin - which I am sure that I was the one to buy it but cannot remember why! My question, then, is what exactly is calcined kaolin and what is its use, i.e. can it be used where something calls for just "kaolin"? Please forgive my ignorance , and thanks! wendy h. Link to comment Share on other sites More sharing options...
Guest JBaymore Posted May 6, 2015 Report Share Posted May 6, 2015 Calcining a clay is accomplished by bringing it up to a temperature at which the chemically combined water in the clay crystal is driven off. (around at least red heat.... approx. 1000 F.) Thus the theoretical formula for kaolin .... Al2O3 . 2 SiO2 . 2 H2O ....... becomes ...... Al2O3 . 2 SiO2. The reason for calcining clay is mainly to reduce wet to fired shrinkage. So this allows a high percentage of clay in a glaze recipe that is intended to use on a bisque form. Subbing some calcined kaolin for all or part of a problematic glaze recipe is the common use. If you use the same weight of calcined kaolin as is specified for the non-calcined kaolin in the recipe... you will be supplying MORE alumina and silica in the FIRED glaze melt. This is because you will no longer be weighing out the water weight that is present in the non-calcined kaolin when mixing the batch. To adjust to keep it the same alumina and silica in the fired glaze... you have to subtract the weight that is coming from water in the clay crystals when using calcined kaolin instead. Easy with glaze calc software. Hope that helps. best, ...............john Link to comment Share on other sites More sharing options...
Cavy Fire Studios Posted May 6, 2015 Report Share Posted May 6, 2015 So, since John is the chemistry guru here, do you think my ^04 shiny clear might benefit from replacing some of its kaolin content with the calcined variety? It crawls sometimes and that's kind of annoying. 15 silica 55 gerstley borate 30 EPK Link to comment Share on other sites More sharing options...
Recommended Posts
Archived
This topic is now archived and is closed to further replies.