Is it impossible to get a good clear over-glaze for cone 6 brush decorated stoneware?

The mug on the right is terra cotta slipware firing 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). Mug on the left: This potter wants to use the same technique on cone 6 stoneware. Like pretty well all cone 6 recipes, this transparent depends on a boron frit to melt it. The result is micro-bubble clouding and boron blue when only slightly too thick. Also surface defects, washing out of colors and bleeding of brushwork. Actually, these commercial underglazes are an issue, by cone six they melt and diffuse into the clear overglaze. So is it possible to glaze-fire cone 6 stoneware and achieve results like the one on the right? At this time I do not know how! The path could be stained non-melting underglazes, separate body and glaze firings (the latter being much lower) and a transparent brushing glaze whose thickness and laydown can be carefully controlled.

Related Pictures

An incredibly white engobe on terra cotta that produces durable and decoratable low fire ware

Two terra cotta slip-cast vessels with white inner surface

I found seven secrets with recipe, process, glaze and firing to make durable terra cotta.
1. A lot of Zircopax, in this case 20% (for whiteness, opacity).
2. The whitest burning materials: New Zealand Halloysite as the kaolin and nepheline syenite as the feldspar.
3. 3% Veegum to gel the slurry (enabling low specific gravity for thin and even coating).
4. The recipe, L3685Z2, has 55% kaolin, that will certainly produce drying cracks. But 1% CMC gum stops that and makes it brushable. It even works on on bisque, I pour-applied it to the insides of these two slip-cast pieces, it drained to perfectly even coverage (in a very thin layer).
5. A terra cotta casting/throwing body to fit the engobe to (has the same fired shrinkage at a target temperature, e.g. cone 04): Initially I am using L4170B.
6. A clear glaze that fits and is transparent: Notice how much whiter the left one is, G3879. At the same thickness as the G1916Q on the right, it is more transparent, better transmitting the white of the engobe.
7. The right firing curve: The 04DSDH drop-and-hold schedule for defect free surfaces.

Lead bisilicate with his ugly borosilicate cousins at a cone 05 party

Boron and lead transparent glazes on terra cotta

The middle front mug is glazed with an 85:15 lead bisilicate:kaolin mix, the G3971 recipe. It is an absolutely "knock your socks off" crystal-clear hyper-glossy surface that transmits the terra cotta color beautifully regardless of whether the clay is smooth or coarse or the glaze thick or thin (this one was applied as a brushing glaze in three coats on L215). My lead testing kit passes it with no detectable lead release. The other pieces are done using brush-on versions of boron-based clear glazes (commercial and made from a recipe). At almost any thickness and whether on L215 or L4170B clouding occurs. The worst one is a commercial three-coater on the right, the best is G1916W (it has 2% added iron as a fining agent for the micro-bubbles). My terra cotta plan: Glaze the inside functional surfaces with that and the outsides with the leaded one (and using a kiln exhaust system).

Fired slipware, a UK tradition of terra cotta and lead glaze

The traditional UK slipware is possible because of the brilliant gloss and hyper transparency of glaze made using lead bisilicate frits. The lead glaze interacts with the colors in underlying slips, dissolving and feathering them (as enabled by the time and temperature of the kiln). Interactions with iron produce warm colors. Ware is bisque fired after the decorating and drying (lower left), then dipped in the leaded glaze. Photo courtesy of Russell Kingston, Lynmouth Slipware Pottery.

Links

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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