The best way to opacify a low-temperature terra cotta glaze

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.

Related Pictures

Arbuckle Majolica glaze using Fusion Frit F-19

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Fusion Frit F-19 is said to be a substitute for Ferro Frit 3124, for this particular glaze that appears to be true. The Arbuckle Majolica glaze was applied on Plainsman L210 and fired at cone 04 (but not as thickly applied as would be customary on Majolica ware). On the left is the one with Ferro Frit 3124, on the right is the one with the F-19. The recipe is based on Ferro Frit 3124 and adds 20% of a feldspar neheline mix (likely to increase thermal expansion because it shivered on the original clay body). It is suspended by 10% kaolin. This may craze on your clay body, the G1916Q thermal expansion adjustable transparent base might be a better solution (it responds similarly to a zircon/tin addition to opacify).

G1916Q on L215, L212, L210, L213, Buffstone at cone 03

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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).

G1916Q transparent on terra cotta body at cone 06, 05, 03

Three clear-glaze terra cotta mugs with rich red color

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The body is Plainsman L215. We used the 04DSDH firing schedule. The G1916Q glaze is inexpensive to make so we have a 2-gallon bucket. It has dipping properties much like a stoneware glaze (because of the 15% ball clay it is thixotropic) so it is easy to apply quickly and evenly. For most terra cottas, body strength increases dramatically by cone 03. However, the most transparent and glassy glaze surface happens at cone 06. Terra cotta bodies need to be bisque-fired fairly low (e.g. cone 06) to have enough porosity to work well with dipping glazes. After cone 04 they generate increasing amounts of gases (as various particle species decompose within), for this reason the glazes can have more micro-bubble clouding or tiny dimples in the surface. This glaze has 2% iron oxide added as a fining agent to remove the bubbles. That iron also reddens the color and variegates the surface somewhat. Even though the surface character at cone 03 is not as smooth, it has a natural charm, and the color is very rich. And that piece has stoneware durability and strength.

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Glossary	Opacifier Glaze opacity refers to the degree to which it is opaque. Opacifiers are powders added to transparent ceramic glazes to make them opaque.
Glossary	Majolica Majolica is white opaque glazed red earthenware clay having colored overglaze decoration. But if you know more about what it is technically you will have more control of your product.
Glossary	Terra Cotta A type of red firing pottery. Terra cotta clay is available almost everywhere, it is fired at low temperatures. But quality is deceptively difficult to achieve.

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