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Substituting Custer Feldspar in a Cone 10R glaze recipe

A locally available feldspar contains much less KNaO and Al2O3 and much more SiO2 than Custer Feldspar. Here is a step-by-step method, using my account at insight-live.com, to alter the recipe so that the overall chemistry of the glaze is maintained.

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Transcript/Notes

In this step-by-step with Lily:

I want to use a new feldspar that has a significantly different chemistry.
Notice I already have my G1947U recipe open and I've already added the new feldspar into my materials.
So let's compare it with Custer feldspar.

more..

As you can see, the latter has much lower SiO₂ and much higher Al₂O₃.
And the combination of Na₂O and K₂O is much lower in the new feldspar.
Let's duplicate the original recipe and click the "Edit button".
I'll change the title of the recipe first. I will enter its code number later when I print out this recipe for testing.
I will remove the notes at the bottom.
I'll change the name of the feldspar to the new one, being careful to enter the name as I defined it earlier.
I'll click the "done editing" button and turn on the calculation of the unity formula.
In comparing that with the original, notice the SiO₂ is higher and the Al₂O₃ is lower.
And, the sodium/potassium is quite a bit lower.
Let's turn on "calculation mode" - and then I'll increase the amount of the new feldspar until the K-N-a-O closely matches the original.
That happens with an addition of 10.
We don't need to worry about the Al₂O₃ - it is about the same.
To reduce the SiO₂ I'm reducing the silica in the recipe, by two-at-a-time, until we get down to near 4 point zero SiO₂.
There we have it!
Using your account at insight-live.com, this is generally going to be the method that you'll use when replacing a feldspar with another having a significantly different chemical analysis.

Links

Materials

Custer Feldspar (pre-2000) The most common potash feldspar used in ceramics in North America, a standard for many decades, but no longer available. Its chemistry shifted around 2000.

Replace Custer Feldspar in a cone 10R glaze:

Two minutes of glaze chemistry

This picture has its own page with more detail, click here to see it.

My G2571A and G1947U cone 10R glaze recipes are both in use in Ecuador. One user has been importing Custer feldspar, but now needs to substitute a local one. That material is much higher in SiO₂ and much lower in Al₂O₃ and KNaO (so a pound-for-pound substitution is likely not going to work). Here is how I did the chemistry. It turned out to be a matter of increasing the new feldspar to match the KNaO (which also matched the Al₂O₃) and then reducing the silica to match the SiO₂. The amount by which I had to change the feldspar is a testament to how different the chemistry of these materials can be from country to country.

If you do this calculation, please note the following: In August 2026 the name for Custer Feldspar was changed to "Custer Feldspar (post-2000)" (both here and in the Insight-live reference materials). There is also one for "Custer Feldspar (pre-2000)".

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