Calcined Kaolin

[CactusPots]

I started using a glaze that calls for 19.5% calcined kaolin and 4.5% EPK.  I just went with 24% EPK and the glaze seems fine. At some point, I'll run some EPK through a bisque load.  What would the purpose of the calcining of the kaolin be to this glaze?  Would it be more mat with the calcined EPK?  Would you calcine some other kaolin for this recipe?

[Bill Kielb]

Calcined Kaolin is in effect about 10% more concentrated than non calcined. So on that basis if the recipe calls for 100 grams of calcined that would equate to approximately 110% of non calcined. Calcining removes the chemically combined water but also remove some trace amounts of some minerals usually found in EPK. So the calcined version  of the recipe might be cleaner in some respect or fire with less imperfections. 

As far as gloss, by chemistry something fired to its appropriate melting temperature will generally follow the silica to alumina gloss ratio. Therefore 5:1 (or less) silica to alumina in UMF should be matte if fired to its proper melting temperature. At approximately 7:1 the glaze will generally be glossy, again fired to its proper melting temperature.

The catch, under fire something and it likely will appear matte, just not fully melted. Over fire it and it will run and potentially appear glossy and of course drippy. Slow cool something and you can force crystal growth in some recipes and they appear more matte.

so what does this all have to do with your recipe? Likely using EPK  for calcined did not affect the gloss ratio much as probably not a bunch of change in silica to alumina ratio. Just approximately 10% less clay than the originator of the recipe  had designed it with.

Why did they design it that way? They could have a great reason or maybe none. Many recipes were designed by adding and firing and looking and adding some more and firing. Not a bad thing just sometimes recipes contain all sorts of unnecessary components. Glaze calculators can be helpful in this regard.

here is a quick video that may explain the gloss thing a bit easier

 

[glazenerd]

Kaolin can have up to 15% natural shrinkage! most of which comes from combined molecular moisture. Specifications for natural clay are not expressed by "shrinkage" as is the case with blended clay bodies.LOI (loss on ignition) is used to express the loss of weight in comparison to the total weight. EX: LOI is 13%  so for ever 100 grams of material, 13 grams will be lost when heating up to bisq temperatures. Organic materials, lignite coal, and molecular moisture are examples of materials lost on ignition.

The primary reason kaolin is calcined (EPK) is control excessive shrinking of material during firing; up to bisq temps. Materials can flake off or crack when there is excessive LOI. Calcining does not effect material characteristics, nor does it lower melting temps.

T

[liambesaw]

So it doesn't crack when it dries. 

[Mark C.]

Yes less shrinkage so it stays on the pots and does not fall off.You can buy calcined EPK as well it goes by the name Glowmax from Laguna Clay.