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We are trying to achieve a body sufficiently vitreous for functional ware but open enough to still have the characteristic variegated patching of light and dark brown (with reduction iron speck of course). Our base recipe is a two-part mix of red and buff fireclay. Mugs #1 and #2 have 12% Custer feldspar but #2 has more of the white clay (it’s more refractory and more plastic - so #2 has a porosity of 3.5% while #1 has 2.8%). #3 has the same clay mix as #2 but it uses 12% Nepheline Syenite - it’s greater fluxing power has turned the color a solid brown (and dropped porosity to 1.5%). The next step: We will target 3% porosity in #3 using a reduced percentage of Nepheline, likely about 9%. We have depended on 100% clay mixes in the past and the body has suffered variations in density (as measured by our SHAB test) and has seldom hit the 3% porosity target, having feldspar in the recipe will enable accurately controlling it in future (so we are more comfortable recommending it for light duty functional ware).
Glossary |
Maturity
A term used in the ceramics industry to signify the degree of vitrification in a fired clay. Mature clays are dense and strong, immature ones porous and weak. |
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Materials |
Feldspar
In ceramics, feldspars are used in glazes and clay bodies. They vitrify stonewares and porcelains. They supply KNaO flux to glazes to help them melt. |
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