|Monthly Tech-Tip |
These brush-strokes of gummed glaze are painted onto an already-fired glaze (the gum enables adherence and drying without cracking). Notice the brush strokes hold their character. The brown glaze on the left has 1.6 specific gravity (SG) and about 1.5% CMC gum. The white one has the same gum content but an SG of 1.5. Its brush stroke has flattened and it is running downward. Is this because of the lower SG? No - commercial glazes with an SG down to 1.3 still hold in place well. How? Because they also have Veegum to thicken and gel them. That reveals a secret: Gum needs particle surface area to work its magic. A bentonite addition supplies that. The dried strokes on the right demonstrate that - 2% bentonite has been added to that one.
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.
A clay of incredibly small particle size. It has the highest plasticity of any known clay and acts as a suspending and gelling in slurries.
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.
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.