The right transparent glaze makes this underglaze design shine!

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).

Related Pictures

Transparent over-glaze for cone 6 stoneware

Is a good one even possible on brush decorated ware?

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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).

These Stoneware Potters Do Something Unusual:

They make certain pieces using the low fire process

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Come 6 stoneware is super for most pieces. But there is some things you just cannot do. For example, you can decorate the underside! The one on the right is the back side of the plate. This is Plainsman Snow clay, it has 25% absorption at cone 06-04, this level of porosity is so high that the body can actually absorb the molten glaze if it is melt-fluid enough. However, when fired at cone 06 the body does not absorb any of the glaze. And the plates stay flat when fired on stilts. These are done by the team of Micah & Jeremiah Wassink of Creston, BC (at Pridham Studio). They make matching mugs, but fire those at cone 6 using underglaze decoration with a clear overglaze. But these plates are decorated using a combination of heavily pigmented viscous-melt low-fire glazes and a black underglaze and then finished with a thin layer of transparent glaze. Since the glaze is not crazed, water is not able to penetrate into the body. And the layer of clear glaze isolates the heavily pigmented colors from food. Even though they are low fire, these pieces are functional and food safe.

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Links

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	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.

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