dhPotter Posted October 30, 2015 Report Share Posted October 30, 2015 In searching the forum for calcine I saw some threads about using half un-calcined EPK and the other half calcined EPK in a glaze recipe to help with hardpan problem. Say a glaze calls for 15% EPK. if you use 10% un-calcined EPK and 5% calcined EPK, does this actually equal the originally called for 15% EPK? In other words, after calcining EPK it looses its water content, which to me, translates to weight loss. Wouldn't this make the calcined EPK lighter then the un-calcined EPK? Would an adjustment need to be made to the recipe? Does it really matter because of the small amount? Link to comment Share on other sites More sharing options...
Chantay Posted October 30, 2015 Report Share Posted October 30, 2015 Good question. I have several receips that call for calcine material. But, the receipe states the amount, so I follow it, and it works. All of them call for 50/50. Not sure if this helps. Link to comment Share on other sites More sharing options...
oldlady Posted October 30, 2015 Report Share Posted October 30, 2015 the only recipe i have seen for calcined epk was for kiln wash. bentonite works to prevent hardpanning glaze. just make sure you mix it into the dry ingredients and stir it around so it does not gel when water is added. Link to comment Share on other sites More sharing options...
PeterH Posted October 30, 2015 Report Share Posted October 30, 2015 This gives EPK's Loss On Ignition as ~14% so 50:50 mix will be up to 7% out. http://edgarminerals.com/EPK-Clay.html Link to comment Share on other sites More sharing options...
Min Posted October 30, 2015 Report Share Posted October 30, 2015 Calcined kaolin in glazes reduces the shrinkage of the glaze therefore reduces cracking of the glaze while it’s drying and crawling of the glaze while it’s firing. Since it’s calcined it has lost it's chemically bound water content and sulphur impurities, loss on ignition, LOI for kaolin is approx 15%. So to do the math on subbing calcined for raw kaolin it would be1point 15 grams of raw kaolin for every gram of the calcined kaolin. Other way around would be 0 point 87 grams of calcined kaolin for each gram of uncalcined. Link to comment Share on other sites More sharing options...
rayaldridge Posted October 30, 2015 Report Share Posted October 30, 2015 I would think that calcining part of the EPK would make hardpan problems worse rather than better, since one of the reasons you would add plastic clay to a recipe is to make the glaze suspend better. The worst glazes in that respect have no plastic clay in them. I don't use them. Link to comment Share on other sites More sharing options...
dhPotter Posted October 30, 2015 Author Report Share Posted October 30, 2015 Thank You all for responding. Min, don't ya just love math? My first foray into calcining. Going to make some for kiln wash. Time to clean shelves and re-apply. When I put kiln wash on, I slopped it on thickly, thinking it would smooth out in the melt. It didn't. I was so ignorant of kiln wash I actually just dusted the shelves with alumina hydrate thinking this would do the job. Then testing glazes became a nightmare with the runs and sticking of test tiles. Now, I think the slopping on has made the surface uneven, making some mugs come out with a rocker bottom. I always concave the bottoms slightly. Link to comment Share on other sites More sharing options...
Chantay Posted October 31, 2015 Report Share Posted October 31, 2015 I have found that if I put the kiln wash on too thick it cracks n pops up. If your using a good mix it doesn't need to be to thick. Glaze drips will just pop off. Link to comment Share on other sites More sharing options...
TJR Posted October 31, 2015 Report Share Posted October 31, 2015 Lot of information here. I use 3% Bentonite in all my glazes to prevent settling or "hardpanning". You could also use one tablespoon of Epsom SSSalts instead. Ray, you are correct. Calcining the EPK will reduce it's plasticity. It will not help with hardpanning. TJR. Link to comment Share on other sites More sharing options...
Tim T Posted October 31, 2015 Report Share Posted October 31, 2015 As others have said, calcined EPK (or anything else) will be lighter, and you need to adjust for this. I calacine china clay to 1000, just putting it in with my biscuit firing. I have an unglazed, wide and shallow container that I put the material to be calcined into, so there is a large exposed area for giving off the water. If you weight the china clay before and after calcining, that'll give you a more exact correction than a theoretical LOI figure. E.g. if you start with 1000g of china clay, and after calcining it is 87, then multiply the uncalcined weight by 87/100. Tim Link to comment Share on other sites More sharing options...
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