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These porcelain mugs were decorated with the same Amaco Velvet underglazes (applied at leather hard), then bisque fired, dipped in clear glaze and fired to cone 6 (using a drop-and-hold schedule). While the G2926B clear glaze (left) is normally a great glossy transparent for general use, its melt fluidity is not enough to clear the micro-bubble clouds that dull the colors below (while these bubbles are a product of the LOI of body and glaze, underglaze colors can really generate them too). However, the G3806C recipe (right) has a more fluid melt that better enables bubble escape. But such glazes have downsides. The melt fluidity requires care not to get it too thick (or it will run). Its high flux content means it is not as durable. And, its high KNaO content raises the thermal expansion (COE) considerably (and thus the danger of crazing). This porcelain has a high enough COE to fit this, but this glaze crazes on others (that's why I always use the G2926B on the insides of ware). To get even better transparency a thinner layer could be applied (by mixing it as a brushing glaze).
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The mug on the right is terra cotta slipware fired at cone 04 using underglazes and a leaded transparent over-glaze (lead glazes are still commonly used in many parts of the world and considered safe there). Mug on the left: This potter wants to use the same technique on cone 6 stoneware. This is a typical transparent glaze (fluxed using a frit or Gerstley Borate). The result is micro-bubble clouding, boron blue, washed-out colors and surface defects. Because it is a dipping glaze it went on too thick and didn't cover well over the colored brushstrokes. However, achieving better warm browns is possible. A more refractory underglaze (made with stains, not iron oxide) that does not bleed. A more fluid melt transparent glaze that is better able to shed bubbles. A drop-and-hold firing would reduce surface defects. Finally, careful control of the glaze thickness and quality of laydown. To achieve the latter, it might be worth preparing the transparent as a brushing glaze, at least for application on the outsides (enabling a dense and even laydown over the whole surface).
Recipes |
G3806C - Cone 6 Clear Fluid-Melt transparent glaze
A base fluid-melt glaze recipe developed by Tony Hansen. With colorant additions it forms reactive melts that variegate and run. It is more resistant to crazing than others. |
Glossary |
Transparent Glazes
Every glossy ceramic glaze is actually a base transparent with added opacifiers and colorants. So understand how to make a good transparent, then build other glazes on it. |
Glossary |
Underglaze
An intensely pigmented highly opaque non-melting ceramic material mix meant to adhere best to leather hard pottery and fire-fit the body. Often transparently overglazed. Starter recipes. |
Glossary |
Glaze Bubbles
Suspended micro-bubbles in ceramic glazes affect their transparency and depth. Sometimes they add to to aesthetics. Often not. What causes them and what to do to remove them. |
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