Is Custer Feldspar discontinued?

Custer Feldspar is the newest ceramics component to face extinction… and you may find some of your favorite clays and glazes are getting a facelift because of it (who are we kidding, this is shaking up the ceramics world). 

First, what IS Custer Feldspar? Custer feldspar is/was a type of potassium feldspar, a group of rock-forming minerals composed of silicate minerals rich in potassium, sodium, and calcium. It was named after Custer County in South Dakota, where it was primarily mined.

It has been used as a staple material in ceramics for decades. It was primarily used in ceramic glazes and clay bodies to enhance properties like vitrification and workability. In glazes, it acts as a flux, promoting the melting of other ingredients at lower temperatures. It contributes to a glossy finish and good color development. 

In clay bodies, Custer Feldspar helps to increase plasticity, reduce shrinkage, and improve firing properties. It also helps ensure the proper fusion of particles during firing, resulting in stronger, less porous, ceramics.

Because of all these good things, Custer Feldspar was a key component in many of our glazes and clays—as it was in the majority of clays and glazes produced commercially and available in the U.S. It was also, importantly to us, an ingredient of our famous Sea mix 5 clay.

Unfortunately, the mine that produced Custer Feldspar stopped production last year because they had mined all the feldspar from the area. It is no longer available.

Our replacement for Custer Feldspar

The journey to a good replacement for Custer Feldspar—both for the clays we 

A bag of Feldspar G200, a good replacement for Custer Feldspar

manufacture and as a raw ingredient to recommend to our customers—has been a bit bumpy. We originally moved to G200 Feldspar, a similar feldspar mined from Georgia. We even used it in our new Sea Mix 6 clay to great fanfare—as it actually performed better than the old Custer did. But, unfortunately, G200 is also facing production challenges due to mine depletion. This new sourcing issue shortly after the failure of Custer Feldspar has had significant repercussions on the ceramics industry, affecting glaze and clay body formulations and production processes.

So, what does a pottery supply company do? 

Since we heard the news about G200 Feldspar, we’ve been experimenting with alternatives, and we have chosen Feldspar G200eu, a feldspar mined in Spain that is identical in chemical composition to G200. Our tests have shown that it performs better than Custer Feldspar and is extremely similar to G200. In our new, updated Sea mix 6, we’re seeing better whiteness at lower firing temperatures, greater vitrification at mid-range, and more firing flexibility. We’ve even taken it to cone 7 with no defects. And it has the same color and feel that folks have come to expect. 

The search for a good replacement for both Custer Feldspar and the old G200 instigated a deeper investigation into the properties of our clay. We performed many rounds of test tiles, measuring and examining them to the limits of the tools at our disposal. The result is clays and glazes that perform even better than ever before. And who can complain about that?