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G2931H - Zero3 H High Expansion Variant

Modified: 2018-12-06 09:20:32

A cone 04-02 clear glaze developed from Worthington Clear. This employs Ulexite instead of Gerstley Borate and has a lower thermal expansion.

Material Amount Units
Ferro Frit 319530.00KG
Ferro Frit 313433.00KG
Ferro Frit 311017.00KG


This is a high-expansion version of the Zero3 Clear glaze (code G2931K), originally developed from the Worthington Clear Gerstley Borate based recipe. While G2931K fits the vitreous Zero3 porcelain and stoneware and also non-vitreous low-talc bodies (like Plainsman L212, L215), it shivers on high-talc bodies (e.g. Plainsman L213). That means that this G2931K variant is the most similar to commercial low fire glazes (which are designed to fit high-talc bodies).

This version increases the thermal expansion of Zero3 K glaze from 7.4 to 8.0 (by adding increasing KNaO at the expense of MgO, ZnO).

Do not try to use this with too little water. Mix it to 1.43 specific gravity and add vinegar or Epsom salts to gel it so that it stops moving (after vigorous stirring) in 2 or 3 seconds. When mixed right it applies very well to low or high porosity bisque (in a very thin layer if needed), it does not drip or move.

The soak:soak:slowdrop firing schedule is critical to a defect-free surface.

Related Information

Three low fire bodies need three different clear glazes. Why?

Glaze fit. The left-most clay mug contains no talc (Plainsman Buffstone), the centre one about 25% talc (L212) and the right one is about 45% talc (L213). Talc raises thermal expansion. The centre glaze is G2931K, it is middle-of-the-road thermal expansion (Insight-live reports it as 7.4) and fits the low-talc bodies (and Zero3 porcelain and stoneware). But it crazes on Buffstone and shivers on L213. The lesson is: Forget about expecting one clear or base glaze to fit all low fire bodies. But there is a solution. I adjusted it to reduce its expansion to work on zero-talc porous bodies and raise it to work on high talc bodies. How? By decreasing and increasing the KNaO (in relation to other fluxes). The three fire crystal clear and work the best in a drop-and-hold firing.


Plainsman Zero3 L,K,H and Firing Schedule
Plainsman Zero3 red body, engobe, glaze and firing schedule
Ceramic Arts Daily 15 Low Fire Glazes
Recipes G2931L - Low Expansion Low-Fire Clear
A cone 04-02 clear glaze developed from Worthington Clear. The latter was based on Gerstley Borate, this one employs Ulexite as the melter an is easier to use.
Recipes G2931K - Low Fire Fritted Zero3 Transparent Glaze
A cone 04-02 clear glaze developed from Zero3 which in turn was developed from Worthington Clear. This employs frit instead of Ulexite.
Recipes L3924C - Zero3 Porcelain Experimental
Glossary Zero3
The designation for a group recipes for body, glaze and engobe (by Tony Hansen), that potters can use to make low fire stoneware and porcelain

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By Tony Hansen

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