Transparent over-glaze for cone 6 stoneware

Is a good one even possible on brush decorated ware?

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).

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Underglazes require a fluid melt transparent

But extra melt fluidity comes at a cost

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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).

The right transparent glaze makes this underglaze design shine!

A cone 6 plate with underglaze fish design

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The underglaze decoration on this cone 6 porcelain plate, by Dana Powell @danaspottery, is undeniably impressive. But the transparent overglaze is equally amazing. Why? It is not easy to get adequate transparency, crisp brushstroke edges and a defect-free surface at cone 6. While a number of factors are at play, perhaps the most important is a controlled thickness and proper adherence and coverage over the underglaze brush strokes (true for any recipe). Dipping glazes tend to go on too thick on the porcelain and don't stick well to underglazes (underglazes are gummed and, when dried, have almost no porosity to adhere an overglaze). A commercial low-specific-gravity gummed brushing glaze is an option; layers go on thinly, enabling careful control of thickness (of course, it can be time-consuming to paint all the layers needed). However, the brushing action can smear the underglaze. Mixing the transparent as a base-coat dipping glaze could be the best answer. With the right amount of gum and water (by experimentation) it will drain fast enough to dry in a minute or so and spread out evenly into a thin, dense and even laydown of the right thickness, one that adheres equally to body and underglaze (of course, a method is needed to hang the draining plate on edge).

Why dipping transparent glazes may not cover underglazes well

Underglaze fails converage by transparent

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Underglaze brushstrokes were applied to this test cup at the leather hard stage (lower left) and then bisque fired on. On the lower right a ball of the pure underglaze emerged from the same bisque firing, notice that it is certainly fusing enough to seal the surface of the bisque. Notice what happened on the upper left: Although the bisque piece was immersed in a dipping glaze the underglaze is not covering. Yet on the upper right a just-applied transparent brushing glaze has covered well (although three coats were needed with plenty of drying time between). Why is this? Brushing underglazes contain a lot of gum, it makes them spread well like paint - and dry hard, impervious and non-absorbent. And the frit they contain makes them fire even denser. Manufacturers of brushing underglazes assume that gummed brushing glazes will also be used over them and thus no coverage issues will occur. This being said, a thixotropic dipping glaze may cover. Or a first coat dipping glaze.

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Glossary	Engobe Engobes are high-clay slurries that are applied to leather hard or dry ceramics. They fire opaque and are used for functional or decorative purposes. They are formulated to match the firing shrinkage and thermal expansion of the body.
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.

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