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So, the feldspar industry has been in a bit of a flux (ha!) over the past few years. Feldspar is an important melting agent, kinda like a natural frit, which promotes fusion and vitrification within a glaze and clay body.
I've sung the praises of feldspar before (Feldspar Rocks!), so I won't go into all the technical aspects here. This is more of a mineral update. Some time ago we all learned that G200 was no longer being produced, although G200HP, which made up about 70% of G200, would still be available in its place. Now production of G200HP has ceased and the mine is closed. That was at the end of last year. At that time Custer Feldspar was the only commercially available potash feldspar. In anticipation, we squirreled away a lot of G200HP and tried to let folks know that our stocks would soon be depleted. Well that time has come. By the time you read this, porcelain production will have used up every last full bag of G200HP.
Custer has been mined in South Dakota for about 80 years and long-term supply looks good. It's pretty similar in composition to G200 though it does have a higher iron content. The differences in potassium and sodium levels are within a percent or two, according to the chemical analyses provided by Pacer.
New on the North American market is a potash feldspar from Spain, G200EU. It is very similar to the original G200 feldspar. Of course, due to shipping costs it is more expensive than any other feldspar we use. We will stock this feldspar for retail sale and it's also available for custom recipes on request. However, we are hesitant to depend on it for clay production due to location and cost. G200EU comes in 44# bags and smaller weighed out quantities.
In order to understand the difference in the feldspars beyond their chemical analysis sheet we ran melt tests at cones 6 and 10 oxidation with G200, G200HP, G200EU, and Custer feldspars.
Testing at cone 6 oxidation
Testing at cone 10 oxidation
Custer and original G200 appeared most similar in terms of color, while G200EU and original G200 are the most similar in terms of melt, and should substitute out for one another without problems. However, anytime raw material changes occur it becomes extremely important to test. Sometimes potters are working right on the edge without knowing it, and these slight differences in oxide levels can cause unpredictable results or failure. Please contact us if you have any questions about feldspar futures!