Monthly Tech-Tip | No tracking! No ads! |
On the left, the brush-strokes of gummed glaze, which I batched myself, have been freshly painted onto an already-fired glaze. Notice the brown brush stroke holds its character. It has a high specific gravity (SG): 1.6. And contains 1.5% CMC gum. The white one to its left, whose brush stroke has flattened and it is running downward, has the same gum content but an SG of 1.5. Is it running because of its lower SG? No. Commercial glazes with an SG below 1.3 still hold in place well. How? Because they also have a gelling agent (e.g. Veegum - it has an unfortunate name, it is not a gum). That reveals a secret: Gums and gelling agents need each other. CMC Gum needs particle surface area to work its magic and bentonite, the gelling agent here, supplies that. And, the gelling agent needs the gum to slow down drying and enable a hard and crack-free dried surface. The dried strokes on the right demonstrate that - 2% bentonite has been added to the drippy one on the far left. They held in place because of the bentonite and hardened without cracking because of the CMC gum.
Materials |
Bentonite
Bentonite can make a clay body instantly plastic, only 2-3% can have a big effect. It also suspends slurries so they don't settle out and slows down drying. |
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Materials |
Veegum
A clay of incredibly small particle size. It has the highest plasticity of any known clay and acts as a suspending and gelling agent in slurries. |
Materials |
CMC Gum
CMC gum is indispensable for many types of ceramic glazes. It is a glue and is mainly used to slow drying and improve adhesion and dry hardness. |
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. |
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