I'm confused by what I'm reading in various places.  I have a glaze recipe that calls for Kaolin.  Are there any acceptable substitute?  The recipe I'm working on is Cream breaking Rust.

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I think you will find the Digitalfire Materials Database very useful.

http://digitalfire.com/4sight/material/index.html

Kaolin is a hydrated alumina-silicate composed of 40.21% alumina and 47.29% silica, and 12.5% water.  In theory you could use alumina hydrate and silica in place of kaolin, but kaolin or other clay is usually the primary ingredient which suspends the glaze. 

More to the point, 50 pounds of kaolin costs only $16, comparable to the price of silica.  Alumina hydrate is five times more costly.

http://digitalfire.com/4sight/material/kaolin_925.html

Each molecule of Kaolin (Al2O3 · 2SiO2 · 2H2O)  loses the two water molecules, 12.5% of its weight, between the temperature of 986 F and 1,166 F,  forming metakaolin.  Most kiln firing programs slow during part of this temperature range to allow time for this super-heated steam to escape.

Kaolin, originally sourced and used in Kao-Ling China, has roughly 60% more alumina than other clays.  Not surprisingly it's also called China Clay.  The particle size of kaolin is much larger than clays so kaolin is not at all plastic.  In America the most commonly used kaolin has the brand name of EPK (Edgar Plastic Kaolin) which is mined in Edgar, Florida.  http://digitalfire.com/4sight/material/ep_kaolin_291.html

http://digitalfire.com/4sight/material/ball_clay_80.html

Looking at the two reference pages above for EPK and Ball Clay, you can see you could substitute any white clay for kaolin, but the resulting glaze will be somewhat different.

English Kaolin has more flux and titanium dioxide so fires whiter.  http://digitalfire.com/4sight/material/english_kaolin_289.html

Chinese porcelain was originally made from a mixture of Kaolin and a feldspar called Petunse.  China Stone is a feldspar similar to Petunse which is found in many other locations around the world.  http://digitalfire.com/4sight/material/petunse_1116.html

Wow Norm, thanks for the indepth response.  I did find my kaolin.  But your response left me thinking about a couple of ways to alter and test a few glazes. 

On another topic.  Do you know where I can find info explaining how the slow cooling in electric cone 6 changes glazes.  I have never read any info explaining why this works, only that it does.

Thanks again, chantay

If you cool glass quickly, it freezes into a random structure, analogous to gassy lava spewed from a volcano.

If you cool glass slowly, crystals of various types have time to form. The formation of an orderly structure takes time.  Crystals on the surface of the glass refract the light making it look more matte and less shiny.  Depending on the chemistry of the glaze you grow different crystals of of various types of minerals or gems.

We are limited with our kilns by the fact that they work at atmospheric pressure.  Some crystalline formations, like diamonds, require extreme pressure in addition to slow-cooling.

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