Calculating a Synthetic Minspar

Project Name

Calculating a Substitute for Minspar 200

Project Codenumber

UnAssigned

Notes

What happens when you cannot get a specific feldspar for use in a glaze recipe? In some cases, there is a similar feldspar that can simply be substituted, this is the case when the chemistries of the two materials are very similar. When a recipe only calls for 5 or 10%, then substitution is likely not a problem, even when the new feldspar is a little different that the old. However, when the recipe calls for a significant percentage (e.g. 30%+), which is normal since feldspars are the backbone of many glazes, then the situation becomes much trickier. That is the case with the recipe we studied, "Perfect Clear", it calls for 40%.

That being said, feldspars are a little like Mother Nature's Frits, each one is different. Feldspars are almost a glaze in themselves, just needing silica and alumina to shift their chemistry toward that of a typical glaze. But that is where the biggest differences are: The percentage of silica and alumina.

Here is the comparison of Minspar, Custer Feldspar and Nepheline Syenite, the three we have in stock:

Custer has a lot less Na2O and Nepheline has a lot more. Nepheline has significantly more Al2O3 less SiO2 than the other two. So, it seems that ny employing a blend of Custer and Nepheline diluted by the right blend of other materials it would be possible to produce a mix having the same chemistry as Minspar.

So that's what I did. As you can see from all the tests, the substitute I calculated in my account at insight-live.com is "bang on" the very first try! Who gets the credit? The magic of glaze chemistry once again!

Minspar

Total:100.00

Auto Unity Formula

Notes

*I targeted this Minspar chemistry in calculating the substitute.

Pictures

Minspar vs. L4441B synthetic substitute

Orton Autofire test kiln. Ball wts. 9.24 grams.
PLC6DS firing schedule.
Top temp. of 2205F. with 10 minute hold.
S.S. cone 6 @ 82.5 degrees Orton Cone Template.

As can be seen, the flow properties of the calculated substitute are almost identical.

Minspar Calculated Substitute

Total:100.00

Auto Unity Formula + Analysis

Notes

*These materials were used to supply the oxides needed to produce the same chemistry as Minspar 200. The trickiest part of the calculation was realizing that I needed a highly concentrated source of Na2O that was also low in Al2O3 - that is the reason for the small amount of frit.

An interesting aspect of this recipe is that it produces a feldspar that has some plasticity.

This recipe is also a testament to how different Nepheline Syenite and Minspar actually are. If we stocked another soda feldspar the recipe would have been much simpler than this.

Pictures

Minspar vs. L4441B synthetic substitute

Orton Autofire test kiln. Ball wts. 9.24 grams.
PLC6DS firing schedule.
Top temp. of 2205F. with 10 minute hold.
S.S. cone 6 @ 82.5 degrees Orton Cone Template.

As can be seen, the flow properties of the calculated substitute are almost identical.

Perfect Clear

A well known cone 6 transparent

Total:100.00

Auto Unity Formula

Notes

*This is a popular transparent cone 6 glaze. We use it for comparing feldspars in-glaze.

While this 10:20:30:40 recipe is convenient, it is much less than "Perfect", we use this as a demo of how such recipes can often be improved using some glaze chemistry.

40% feldspar in a cone 6 glaze is highly unusual. And high feldspar is normally a red-flag for crazing. But this glaze is "rescued" by it 30% Gerstley Borate (GB). GB has a high B2O3 content and that has a low thermal expansion (countering the high expansion of all the sodium/potassium (KNaO) in the feldspar). But 30% GB content means that the slurry will gel badly, only about a 1.35 specific gravity can be achieved (without having a bucket of jelly).

This glaze is guaranteed to have issues with drying cracks, and therefore crawling. It is not just the high water content needed to keep it from gelling, but the added ball clay. GB is plastic, like a clay, 30% is plenty to suspend a glaze, no added clay is needed. But this has 20% highly plastic ball clay also. You could likely dewater the slurry and throw this glaze, like a clay, on a potters wheel!

The low Si:Al ratio produces a less glossy surface than it would if that ratio was higher. To counter that the very high feldspar content is pushing the KNaO levels much higher than typical cone 6 glazes. That is the reason why this is as glossy as it is (while having such a low Al:Si ratio). But again, the high KNaO means issues with crazing on many clay bodies.

The high LOI, 11% which is mainly from from the GB, means gas bubbles will be generated, if they come when the glaze is melting then bubble clouds will appear in the glass.

Pictures

G3925 Perfect Clear on M390, M340

Left: Uses L4441B Minspar subsitute
Right: Uses Minspar (at 40%)
Results are the same. The glaze was a little thicker on the left-top, so was clouding a little.

G3925, G3925A glaze comparison

G3925, G3925A glaze flow comparison

Fired to cone 6. The calculated substitute for Minspar is used in G3925A, 40% of the recipe. The flows are identical.

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