Lowering the Thermal Expansion of G2926B

M370 Transparent Liner Base

Perkins Fritted Clear Plus Silica

Total:110.10

Auto Unity Formula

Notes

This is the standard cone 6 whiteware clear base glaze made by Plainsman Clays. Mix it at the proper specific gravity and gel it correctly to get a very workable slurry. Search "G2926B" at https://plainsmanclays.com for mixing-with-water info. The page for this glaze at the Digitalfire Reference Database also has lots of pictures and notes (see link below).

This recipe is a descendent of Perkins Studio Clear. It sources boron from a frit instead of Gerstley Borate and adds 10% silica. The recipe totals 110 for this reason (this is not an issue, just recalculate to the total you want).

This recipe has very high Al2O3 and SiO2 content yet still fires smooth and glassy. Notwithstanding this, it begins to melt below cone 02.

On porcelains the cone 6 surface is brilliantly glossy and fires without defects. This glaze may not have enough melt fluidity to pass bubbles on stonewares that produce excessive gases of decomposition. This responds to moderate additions of most stains to produce homogeneous colors that bleed very little. But again, because this glaze does not have high melt fluidity certain colors, or high percentages of colors, can produce surface defects and matting.

URLs

G2926B at PlainsmanClays.com

Pictures

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="M370 Transparent Liner Base" keywords="Perkins Fritted Clear Plus Silica" id="56342" key="rZEyYQSd" date="2018-11-01" codenum="G2926B"> <recipelines> <recipeline material="Nepheline Syenite" amount="18.300"/> <recipeline material="Ferro Frit 3134" amount="25.400"/> <recipeline material="EPK" amount="19.600"/> <recipeline material="Wollastonite" amount="6.900"/> <recipeline material="Silica" amount="37.600"/> <recipeline material="Silverline Talc" lookup="Talc" amount="2.300"/> </recipelines> </recipe> </recipes>

G2926B Reduced COE (Li2O)

Total:100.00

Auto Unity Formula

Notes

This is the best of a series of tests to lower the thermal expansion of G2926B. Separate trials of substituting ZnO, SrO, Li2O for KNaO were tried (in each the brilliant surface was being marred by an orange peel like surface). This version is a hybrid, diversifying fluxes by adding Li2O and SrO. It sources SrO from Frit 524 and Li2O from Spodumene.

The calculated expansion is dramatically lower, 5.6 vs. 6.4. This should thus be much more resistant to crazing. Of course, there is an issue: Spodumene introduces bubbles into the slurry in the bucket (washing it before use could solve this). Color response might also be different.

Pictures

G2926B vs J shock test

There was no crazing at 320F, but at 340F 2926B has five cracks and this has one at the edge.

It is interesting that 2926B has performed differently in different firings in this test.

2926J on P380

The surface is brilliant and clear.

G2926B vs J Cone 6 Fluidity

G2926J is flowing a little more. Fired character is very similar to B.

G2926B Reduced COE (MgO)

Total:100.00

Auto Unity Formula

Notes

This is an adjustment to G2926B to reduce its thermal expansion. It increases MgO at the expense of KNaO. Even though MgO levels are quite high, no loss of gloss has happened! The effect on lowering the thermal expansion is dramatic.

This looks very promising. With 10% added zircopax it produces a white of even lower thermal expansion. We have subjected various Plainsman P300 pieces (a difficult-to-fit-glazes-to body) to 300F:IceWater thermal shocking and there is no crazing.

Ferro Frit 3249 can be substituted for Fusion F-69 (although F-69 is more reliable for ceramics). Likely any kaolin can be used.

Although there is only 12.5% kaolin this will suspend perfectly if you follow the same procedure as explained for G2926B (mix to 1.43-1.44 specific gravity) and gel with epsom salts.

Pictures

G2926S after 300F:IceWater test on P300

No crazing after the test.
The transparency and clarity of the glass is great, this looks very promising.

P300 after 300F:IceWater test

S is not crazing after two days. B crazing in the cold water.