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This is G2926B clear cone 6 glaze deflocculated with Darvan. Because the Darvan is thinning it, 2.5kg of glaze powder is suspended in only 1100g (1100ml) of water (half the normal amount). While the slurry in the bucket flows well and appears like it should work, a one-second dip produces twice the desired thickness. It dries slowly and it is very difficult to prevent runs. The lesson: Make sure the specific gravity (SG) of your glazes is right. What should the SG be? Measure it when your glaze is working well. Or take note of it in instructions that come with the recipes you use. For bisque ware: 1.43-1.45 with a flocculant (like Vinegar or powdered Epsom Salts) added to gel the slurry slightly.
Why? Glaze fit. Do these yourself and they might end up being glaze compression demonstration pieces. These are available on Aliexpress (as Drip Pottery, Drippy Pottery or Goopy Glazes) and they are made by a manufacturer that has close control of body maturity (and thus strength) and the capability to tune the thermal expansion fit of glaze-on-body. Glaze fit has to be better than normal because of the absence of an outside glaze. Too low an expansion and the compression (outward pressure) will fracture body (especially for thin-walled pieces). Too high and it will craze. And the glaze is thick, it will shiver or craze with far less forgiveness than a thin layer. And how did they get the glaze on this thick? They likely deflocculated it, up to 1.7 or more, glazed the inside, let it dry, then glazed the outside. And applied the glaze to preheated ware. If done right these pieces are a visual and technical achievement. However hobbyists, for example, often just brush multiple layers of commercial glaze that only by accident fits the body they are using. No wonder their pieces often end up as time bombs or crazed bacteria farms.
Glossary |
Once fire glazing
Refers to the practice of firing ceramics in one firing (rather than two) to produce a fully glazed product. This practice requires more technical expertise. |
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Glossary |
Specific gravity
In ceramics, the specific gravity of slurries tells us their water-to-solids ratio. That ratio is a key indicator of performance and enabler of consistency. |
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