Monthly Tech-Tip | No tracking! No ads! |
AMACO and Crysanthos. 1.26 (67.5% water) and 1.22 (68% water)! The former is well below their recommended specific gravity of 1.4 (it still paints well but needs more coats and more time to dry and apply them). The Crysanthos, although having a lower specific gravity is more viscous and goes on thicker (so it likely contains more gelling agent). When doing underglaze decorative brushwork it is important to get adequate thickness with eachbrush stroke, so a higher specific gravity is better. This may be reason enough to consider making your own (by adding stain powders to a base and using Veegum CER to gel the slurry, slow down its drying and harden it well at the dried state).
I am using this on Polar Ice porcelain. This adds 20% black stain, 1.5% CMC gum and 5% bentonite to a 90:10 MNP and Nepheline Syenite mix. We mix this to have as high a specific gravity as possible (the CMC is an enabler for that). The lower water content is a key to this working well, it enables putting it on much thick enough in a single brush stroke (the commercial one we have won't do that). The freedom to fiddle with the recipe enables really tuning the way it goes and and fires.
Amaco V-303 Terra Cotta underglaze was applied in one coat, at leather hard stage (on these Plainsman Snow mugs). It appeared to be thick enough on application and after bisque (left) but the clear overglaze on the right has difused into it to melt it enough to lose the opacity. A higher specific gravity (lower water content) would make it possible to apply it thickly enough in one coat, this is especially important because each layer rewets the piece and adds a lengthy drying period. An appropriate specific gravity is thus the difference beween practical and impractical.
URLs |
https://www.amaco.com/clay_how_tos/207
AMACO recommended specific gravities for Potters Choice glazes |
---|---|
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. |
Glossary |
Brushing Glaze
Hobbyists and increasing numbers of potters use commercial paint-on glazes. It's convenient, there are lots of visual effects. There are also issues compared to dipping glazes. You can also make your own. |
Glossary |
Specific gravity
In ceramics, the specific gravity of slurries tells us their water-to-solids ratio. That ratio is a key indicator of performance and enabler of consistency. |
Buy me a coffee and we can talk