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Why Calcine China Clay?

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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!



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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.



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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.

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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.

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