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.
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.
The mug on the left is a brush-on version of a boron-based clear glaze for cone 05. Three coats were applied and the often-encountered clouding occurred. The one on the right is an 85:15 lead bisilicate:kaolin mix. Three coats were also applied. It is an absolutely "knock your socks off" crystal-clear hyper-glossy surface that transmits the terra cotta color beautifully. And my lead testing kit passes it with no detectable lead release. Underglaze brushwork here we come! I have sought this effect for decades, this is it! Recent realizations about the slipware tradition in the UK (and their standard use of this same glaze) motivated me to get some of the frit. All I could get was a sample of the frit shards, but these milled down to a powder easily in our ball mill.
Of course, to be safe, I would still glaze the insides of pieces with a boron clear, likely as thin a layer as possible of G1916Q (with 2% added iron as a fining agent for the micro-bubbles). And, I will obviously fire these lead glazed pieces with the kiln exhaust system turned on.
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.
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.
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.