Pugaboo Posted June 27, 2017 Report Share Posted June 27, 2017 Okay another possibly stupid question.... Why would you Calcine China Clay? Background: I found a plastic bin marked, China Clay (calcined), in the storage room this past weekend at the art center and am wondering what it is used for and why it was calcined, etc. the little research I did seemed to suggest it helps prevent crawling of certain glazes. Quote: " Raw kaolin also supplies suspension to the glaze slurry and it hardens the dry glaze layer. However once raw kaolin percentages pass 20% in a recipe shrinkages can be to high (causing crawling). In these cases substituting part of the raw kaolin for calcined material solves the problem, maintaining the chemistry of the glaze but reducing the shrinkage and cracking. In other words, by substituting some of the raw kaolin for calcined the physical properties of the glaze slurry can be controlled without impacting the chemistry of the fired melt. Of course, mixing the raw and calcined materials must take into account the LOI of the raw material (12% less calcined is needed)." So far none of the recipes that I have found call for calcined China Clay... so if you need this is it normally recorded in the recipe or are you just kind of supposed to know? Thanks again! T Link to comment Share on other sites More sharing options...
Mark C. Posted June 27, 2017 Report Share Posted June 27, 2017 I use it in my kiln wash recipe I like the super duper recipe-used it for decades 1/2 alumina hydrate1/4 calcined epk 1/4 epk Link to comment Share on other sites More sharing options...
glazenerd Posted June 27, 2017 Report Share Posted June 27, 2017 What is being left out of the description is: what is being driven off is molecular water. A certain amount of carbons go with it, but kaolins have very low carbon ( sulfates) to begin with. New Zealand kaolin is the only kaolin that holds molecular water, all other kaolins do not have secondary layers: but then again NZ is a mineral clay. The reason NZ kaolin seems to clump when stored. For the most part, kaolins will be fine if you run them up to 400F for a short period to stave off moisture. Ball clays are an entirely different matter. While on the topic: the pottery industry needs to get use to a new term coming shortly: "mineral clay." For two decades the soil scientist rejected the idea that minerals like bentonite be classified as clay. For the same two decades, geologists insisted that minerals like bentonite be called clay; because they had clay like properties. ( primary and secondary platelets (2:1) The final compromise was the new term: mineral clay. So products like bentonite, NZ kaolin, and some smectites will start carrying this new label. So when you start seeing it in upcoming descriptions: a " mineral clay" is actually a mineral that has clay like properties. Nerd Link to comment Share on other sites More sharing options...
Mark C. Posted June 27, 2017 Report Share Posted June 27, 2017 My supplier does not carry New Zealand Kaolin only one kind of bentonite as well Link to comment Share on other sites More sharing options...
Tamas Posted June 27, 2017 Report Share Posted June 27, 2017 There are ~90 recipes using calcined kaolin over on Glazy, glazes and slips alike: https://glazy.org/materials/94 Link to comment Share on other sites More sharing options...
bciskepottery Posted June 27, 2017 Report Share Posted June 27, 2017 Agree with Mark, probably used for kiln wash. Using calcined clay reduces cracking when the wash dries. Link to comment Share on other sites More sharing options...
dhPotter Posted June 27, 2017 Report Share Posted June 27, 2017 This Cone 6 Dolomite Matte Base Glaze uses calcined EPK. https://digitalfire.com/4sight/recipes/cone_6_dolomite_matte_base_glaze_121.html Link to comment Share on other sites More sharing options...
Marcia Selsor Posted June 27, 2017 Report Share Posted June 27, 2017 I use calcines kaolin when I am spraying slips onto dry porcelain. I believe it helps keep the slip flat and not crack due to shrinkage. Marcia Link to comment Share on other sites More sharing options...
dangerouspottery Posted October 11, 2017 Report Share Posted October 11, 2017 Is Calcined Kaolin the same as Calcined China Clay? Link to comment Share on other sites More sharing options...
neilestrick Posted October 12, 2017 Report Share Posted October 12, 2017 23 hours ago, dangerouspottery said: Is Calcined Kaolin the same as Calcined China Clay? Basically. China clay is a type of kaolin, also referred to as grolleg and it come from the UK. It's unnecessarily expensive for use in glazes, though. Your typical domestic kaolins are fine for glazes. Link to comment Share on other sites More sharing options...
dangerouspottery Posted October 12, 2017 Report Share Posted October 12, 2017 i ask because a recipe i'm looking at calls for Calcined Kaolin. Instead of calcining it myself, i went to Ceramic Supply to ask for Calcined Kaolin, and was handed a bag of Calcined China Clay. Just want to make sure it's okay to sub in my glaze recipe. thanks Link to comment Share on other sites More sharing options...
Dick White Posted October 12, 2017 Report Share Posted October 12, 2017 China Clay and Kaolin are generic names that are often used interchangably. There are some specific technical/chemical differences between brands of kaolin, e.g., EPK, Tile6, Grollegg, but unless you are working with a highly nuanced recipe or situation, it probably won't matter which brand of kaolin/china clay was calcined and put in that bag. Link to comment Share on other sites More sharing options...
dangerouspottery Posted October 13, 2017 Report Share Posted October 13, 2017 THANK YOU! Link to comment Share on other sites More sharing options...
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