|Monthly Tech-Tip |
These work well from cone 04 up, they are better than any commercial transparent brushing glaze we have used. And far better than glazes made using raw sources of boron (like ulexite, colemanite, Gerstley Borate). These glazes have lower thermal expansion and do not craze on any body we have tried (yet are ultra gloss and ultra clear). They are G1916QL1 and G3879C. We developed them for use on the dolomite-based (rather than talc-based) L4410L low temperature art clay body. These are a good demonstration of the technical and economic sense it makes to use highly fritted glazes at low temperatures. Having a good base glaze is the key to adopting low temperatures for your production. You would likely agree that no stoneware glaze has melting patterns like these shown in this melt fluidity test! These recipes and all details about their development and adjustment are openly available.
Potters are used to making dipping glazes that they weigh out and mix from recipes. Hobbyists commonly use bottled commercial brushing glazes. Did you know that a dipping glaze can be turned into a brushing glaze by the addition of Veegum (or Veegum CER) and water? Do you know what a base-coat dipping glaze is? Here is a quick overview: Dipping glazes need to go on to bisque ware evenly, be thixotropic enough to hold on at thickness and drain and dry quickly. But they don't need to dry hard. Brushing glazes need a cohesive slurry that dries slowly and hardens well on drying. They also must adhere to the body really well so that multiple layers can be applied (individual layers go on thin). Base-coat dipping glazes are in between, they need to dry fast enough and gel well enough to make application by dipping possible (although less practical) and they need to adhere well enough to tolerate another layer without letter go of the body.
Both of these mugs are fired at cone 04 using the C04PLTP schedule. The one on the left is made from L4410P (Plainsman Snow) with Amaco V-303 Terra Cotta underglaze brushed on the outside base. It is cover-glazed with Spectrum 700 brush-on clear. The mug on the right is made using a red terra cotta body (L4170B with 10% 65 mesh silica sand and 3% bentonite added to convert it from a casting to throwing body). The white engobe brushed on the upper outer surface is L3685Z2 (it is blender mixed for brushing with a 1.5% Veegum and 1.5% CMC gum additions). It paints on to the leather hard clay in one coat and almost no brush strokes. The clear cover glaze is G1916Q dipping (a glaze you can make yourself). The terra cotta piece on the right has two obvious advantages: It fires much stronger and the engobe fires whiter. This method of "white engobe on terra cotta" produces a surface of porcelain-whiteness on a body that fires at cone 04. And, it has much higher strength than white burning bodies. Plainsman L215 and L210 terra cottas could also be used with this engobe.
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.
A type of red firing pottery. Terra cotta clay is available almost everywhere, it is fired at low temperatures. But quality is deceptively difficult to achieve.