The Zero3Engobe finds its inspiration in the ceramic tile industry. Thousands of tons of white engobe are applied to tiles made from red or brown burning bodies, after pressing, to prepare them for glazing. The white surface enables the use of glazes of any color.
This engobe is not required in the Zero3 system, but like tile, it is employed for practical reasons. A typical use case for functional ware is application to inside food surfaces at leather hard state (and covering with the clear transparent Zero3 glaze after bisque firing). Or, as in tile, application to the red-burning body, by dipping or painting, to create a white base onto which coloured underglaze decoration is done. If you are using this engobe on another clay body, especially if not firing to cone 03, do an EBCT test to be sure they are compatible (have the same firing shrinkage), adjust the frit percentage as needed.
This is an engobe, it must be applied at stiff leather-hard stage (if you apply it to dry or bisque it will flake off). It will rewet pieces and they will require significant time to re-stiffen to the same state. Even coverage can normally only be achieved by preparing a slurry of the correct rheological properties and dipping or pouring. It is vital that you have a good mixer and can effectively measure specific gravity. To mix this use the process described on the “Engobe” glossary page at the Digitalfire Reference Library. A paint-on version of this can be made by adding 500g powder engobe to a mix of 400g water and 75g of Laguna Gum Solution (makes 1 pint). This produces brush-stroke coverage, which can be very attractive.
A drop or two of Dettol might be needed to store this for long periods (to prevent bacteria or mold).
Recipe info: The silica helps match body thermal expansion and glaze fit. The frit makes the fired hardness of the engobe more closely match the body. Nepheline Syenite is included as a filler. The bentonite is needed for good adhesion to the leather hard body and for dry hardness.
Engobe applied to leather-hard mug
This was done by simply filling and rotate-pouring the engobe, then pressing the lip into it. Since the engobe gels the mug could be turned over immediately without creating any drips.
Notice that the outer surface of the handle has been waxed to slow down its drying. This is because the engobe rewets the mug body, thereby setting it behind the handle. This mug has thin wall so this this rewet adds a day to the production process. This next day the brushwork is done on the outside.
Zero3 body with G2931F Zero3 glaze
The F version of the glaze employs Ulexite to source the boron (instead of frits). These Zero3 stoneware mugs were fired to cone 03 with underglazes. The right mug has the Zero3 engobe inside (under the glaze).
Pulling badly on L212 on EBCT at cone 04
6100 - Red
1400 powder, 1067 water, 5.6 Darvan produced 1.52 SG.
Too runny, took alot of epsom salts. Perhaps water could be reduced.
Pulling badly at L212 on EBCT test.
This slip is part of a project to find a white slip to fit Plainsman terra cotta bodies (non vitreous).
The development project for this engobe has been difficult. We have been testing to make the drying, fired shrinkage, thermal expansion and quartz inversion behavior compatible with the body. We wanted this slip to be as vitreous as possible and that turned out to be the problem. A vitreous slip has a high fired shrinkage (early versions were 8% or more). The problem is that the terra cotta body is only around 3% at cone 03 (the target temperature). We did a long series of tests reducing the frit at each stage and retesting. But the flaking off at the rims of ware persisted, always at the slip/body interface (rather than the glaze/slip interface). In the end (two trials after this one, and a dozen before), we ended up reducing the frit to only 5% (it was 30%!). The lesson is that this entry level terra cotta body is not vitreous so neither should the slip be. But remember the advantages: Low fired shrinkage, adequate strength, bright colors, red body, crystal clear glaze and fantastic thermal shock (approaching flameware).
It is very important to mix this properly. It must be gelled so that it hangs on to leather hard water that has been dipped, does not drip or curtain. In the first test we mixed it to 1.43 specific gravity then added Epsom Salts until it gelled enough to stop immediately after stirring was stopped. Thus it has more water and is then flocculated (you may not be accustomed to mixing a glaze or slip as thin as this might be, but trust us, if there is not enough water it cannot be gelled and will not go on properly). The proof of its correctness is how it behaves when dipped, it should go on a smooth as silk and stay put. It will dry slowly however. Ware needs to be fairly firm or it could soften excessively after slip is applied. For painting, you will need to tune each color so that it paints well. If it does not paint well the problem is with how you gelled it. Again, 1.43 specific gravity for dipping, adjust for painting if needed (but it still needs to gel or brush strokes will run).
This is flocculating at 1.35 SG so it only needs a tiny bit of vinegar or Epsom Salts. Application is very good. For stain additions be ready to add a little vinegar to gel sufficiently for painting.