|Monthly Tech-Tip |
Top are V-326 and V-388 underglazes, painted on and bisque fired at cone 04. Although the layer is thin the coverage is very good and the brightness is stunning. How can these colors be so bright? Using very high, and expensive, percentages of stain. That explains why these commercial underglazes are double or triple the cost of a typical commercial glaze. The bottom mugs are clear-glazed and fired at cone 05, the one on the left with Amaco LG-10, The one on the right is Spectrum 700. The latter produces better results over the underglaze and is more transparent and less yellowish on the body.
Can you make bright-colored engobes and underglazes like this? Yes. Start with 50% stain and 50% stain medium (the percentage needed varies by color and type of stain).
Amaco Velvet underglazes fire to a pleasant velvet surface at cone 6. The high saturation of color enables painting them on in one coat yet achieving good coverage. However, at low temperatures the surface is dry. If the velvet appearance could be achieved at cone 04-06 then aesthetics and bond with the body would be greatly improved. It can! These cone 05 fired tiles have a single coat of V-308 Radiant Red. The clay is Plainsman Buffstone. Each tile also has a single layer of G1916QL1 clear brushing glaze on the upper section. Spectrum 700 transparent low fire glaze has been blended at a 9:1, 8:1 and 7:1 (underlgaze:glaze ratio). Notice that as more glaze is added the color actually darkens! By 7:1 the surface resembles cone 6. 8:1 is likely optimal, adding more glaze starts to make V-308 translucent. Higher glaze content would also be an issue if the underglaze was applied too thickly, this is because it will try to impose its firing higher shrinkage on the body, setting up tension seeking release in fired pieces. A side-effect is that adding the glaze makes the underglaze more watery, thus it would be best to evaporate some water from the 700 glaze first.
It is a mistake to use pure stains for decorating ware. Stains need to be mixed with a ceramic carrier and a working medium to work and fire well.
A intensely pigmented and highly opaque brushing compound meant to be applied to leather hard pottery and covered with a transparent overglaze.