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The G1916Q recipe uses common Ferro frits and can be made to fit any low fire clay body (the thermal expansion is adjustable). And it melts well down to cone 05. And we have a strategy to reduce clouding and micro-bubbling. These five test tiles were fired using the 04DSDH schedule (drop-and-hold) firing schedule. All exited the kiln without crazing. The L215, L213, L210 and L212 samples subsequently survived a 300F/Icewater test without crazing, but the Buffstone did not - it needs a lower thermal expansion (by increasing Frit 3249 and reducing Frit 3110, e.g. 15:5 instead of 10:10). The L213 would not likely survive a cold-to-hot test without shivering - it needs a higher thermal expansion (by increasing Frit 3110 at the expense of Frit 3249).
Tin oxide is a powerful opacifier, but the 5% in this Frit 3124 based Arbuckle Majolica cone 04 recipe on the left is not sufficient. Adding 10% zircon (e.g. Zircopax, Superpax) produces the one on the right. Would just using 15% zircon be better? No. More zircon means a stiffer melt and more issues with crawling (notice the small bare spots on the right edge). This could be managed by the addition of CMC gum or a higher clay content to get better adhesion and dry hardness). But another improvement would be worth trying first: Use 4% tin and 8% zircon. That should opacify enough and present fewer problems with crawling.
These terra cotta clays were bisque fired at cone 04 and glaze fired to 04 using the 04DSDH schedule. The glaze is G1916Q, an expansion-adjustable cone 04 clear. That schedule alone is often enough to get transparent, defect free glazes in many situations. But not in this case. The solution was to add a fining agent. In this case we added 2% red iron oxide (to the top glaze). The particles of iron floating in the melt acted as a congregating points for bubbles, helping them to escape. And we got a bonus: a more interesting aesthetic. A 1% addition also worked, but not as well (we have settled on 2% iron and screening the glazes to 100 mesh). Screening out the larger particles slightly degraded the fining performance (so we have to accept the tiny specks). Iron does not always workin other situations. Other fining agents we have used at cone 6 do not work in this situation (e.g. 2% Zircopax, Alumina). Of course, this glaze will fire amber on a white body.
Recipes |
G1916Q - Low Fire Highly-Expansion-Adjustable Transparent
An expansion-adjustable cone 04 transparent glaze made using three common Ferro frits (low and high expansion), it produces an easy-to-use slurry. |
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