A five-material cone 6 clear glaze that does not craze on stoneware

A clear glaze sounds like the easy one. It is not: clears that are perfectly transparent also tend to craze, because the alkaline fluxes that make them melt early also give them a high expansion coefficient. This recipe sits at roughly 7.2 x 10 to the minus 6 per degree Celsius, which is inside the fit range of most cone 6 stoneware, and it is built from five materials you can buy anywhere.

Base

A five-material cone 6 clear built around Custer feldspar and Ferro frit 3134. The frit supplies the boron that makes cone 6 maturity possible in an electric kiln without waiting for the feldspar to dissolve.

Fired surface

Glossy, colourless, slightly blue where it pools. Fires perfectly transparent over a white body and picks up a faint green cast over iron-bearing stoneware.

Why frit and not more feldspar

Custer feldspar is the backbone of most cone 6 glazes, but it is a slow melter: on its own it needs cone 9 or 10 to dissolve completely. Frit 3134 is a pre-melted borosilicate that flows from about 1050 degrees Celsius, and at 20 percent in this recipe it does the job of bringing the whole glaze into solution at cone 6 without making it so fluid that it runs off the pot.

Cutting the frit to 12 percent gives an under-fired, slightly rough surface at cone 6 — which is fine if you want a semi-matte, and wrong if you want transparency. Raising it to 30 percent makes the glaze run at the foot and craze more.

Both directions have been tested at cone 6 on the same clay body. The 20 percent version is the only one that came out of the kiln glossy and crack-free on a 6 mm wall.

Mixing, sieving and specific gravity

Mix the dry materials with water to 1.44 specific gravity — that is about 600 ml of water per kilogram of dry powder. Sieve through 80 mesh twice. The kaolin in the recipe is the material that lumps; two passes through 80 mesh removes the lumps that would otherwise fire out as pinholes and light patches.

Add 0.2 percent Epsom salts if the glaze settles into a hard cake overnight. This clear settles slowly anyway because of the 16 percent kaolin, but the Epsom salts make a one-second dip deposit 0.4 mm rather than 0.3 mm, which matters when the glaze has to cover a dark body in a single coat.

Application and what it does over decoration

Dip for one second, moving the pot slowly in and out. The glaze is viscous enough that a single dip covers a white body fully; on a dark stoneware, dip twice with 20 minutes of drying between, which takes the layer to about 0.7 mm and reads as a milky white rather than a clear.

Over underglaze decoration, this clear is neutral: it does not shift cobalt, iron or copper colours, and it does not bleed the decoration at the edges. That makes it a useful topcoat when you want the drawing to stay exactly where you put it. Over black iron oxide it goes slightly brown; use a dedicated black clear if that matters.

Testing fit before you commit

Throw four tiles in the body you actually use, bisque them, and glaze each differently: one at 0.3 mm, one at 0.4, one at 0.6 and one double-dipped. Fire them in the same load. Then soak each tile alternately in boiling water and in iced water five times, and look for crazing under a loupe.

A glaze that crazes after five thermal cycles will craze on a mug within a few months of use. If the 0.4 mm tile survives and the 0.6 mm one does not, the recipe is fine and the application was the problem — which is the outcome in most cases where a clear glaze is blamed.

Steps

1. Weigh the five dry materials and combine them thoroughly.
2. Add roughly 600 ml of water per kilogram of dry powder while stirring, aiming for 1.44 specific gravity.
3. Sieve through 80 mesh twice, discarding the residue on the second pass.
4. Add 0.2 percent Epsom salts dissolved in warm water if the bucket settles hard overnight.
5. Dip a test tile for one second and weigh it dry to confirm a 0.4 mm deposit.
6. Dip the pot for one second, moving it slowly, and dry for 20 minutes before handling.
7. Glaze four fit-test tiles at 0.3, 0.4, 0.6 and 0.8 mm and fire them in one load.
8. Thermal-shock the tiles five times between boiling and iced water and check under a loupe for crazing.

Recipe and schedule numbers here were recorded on the workshop’s own kiln. A different kiln, a different thermocouple position or a different clay batch will move the result, so treat every firing schedule as a starting point and keep your own log beside it.

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