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I wish I could simplify this but I don't think it is a really simple issue.

CaSO4?+2H2O heated to near 300 F becomes CaSO4? +.5 H2O? It looses the water in several stages starting at about 127 degrees ( I am told and I am not sure of this number. This does not jibe with my old Handbook of chemistry and physics) Heated about 326 or so it becomes dead burned plaster or the anyhydrate CaSO4?. This compound if pure is quite stable even into stoneware temperatures (mellts at 1460) Unless ground fine dead burned plaster is VERY slow to rehydrate. [ Flooring plasters used to be made with dead burned plaster which were soaked and then spread on a floor. As the mixture dried to a paste it was pounded with flat mallets compacting it and also breaking up the big chunks exposing more surface to become plaster of paris. I have never seen this, only read about it.]

What happens above that temperature (326 F) is complicated. I believe it can be reduced to CaSO3? making the sulphur easier to volatilize giving you CaO?. Calcium Sulfite melts at a little about quartz inversion. 1120 or so. The edges of either dissolve into solution. As they do so the suplhur also can go into solution and be liberated as gas bubbles apparently a source of pinholes. Judgeing from plaster popouts I have seen chunks of plaster are not much easier to dissolve into glaze than chunks of limestone. Plaster expands as it sets, so if a chuck of plaster rehydrates inside your fired clay it creates pressure. If your clay cannot restrain that pressure or release the pressure somehow through small cracks into pores the piece will break. In stone ware usually plaster or limestone smaller than 30 mesh will not create problems although it can in soft porcelain bisque. It depends. Josh Deweese ( I think its him) talks about a village in Italy that wets their bisque and their lime does not blow. I think this is because the slaked lime gets squeezed into pores as it slakes, but really I am stumped on this.

So, plaster in your clay may or may not be a problem depending on size of chunks, firing schedule and atmosphere, porosity of bisque and glaze ware and the strength of your clay. I could stand some better information on the conversion of calcium sulfite into quicklime. I know it happens, but I don't know at what temperature.

Common scum on clay is calcium sulphate. I believe that clays that scum are more prone to pinholing. I don't think barium carbonate helps this. After it reacts with Calcium Sulphate the barium sulphate is stable like calcium sulphate but can be reduced.

I like the way scum looks in very light soda and salt firing and wood. I often soak bisque in calcium chloride or epsom salt solutions to get nice scum. (Don't mix the two).

I someone has more pieces to this puzzle I would love to know them. Louis

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Page last modified on September 23, 2011, at 11:22 PM