|Monthly Tech-Tip |
The freshly opened transparent low fire glaze on the left has a specific gravity of only 1.34 (thus has a high water content). Yet it is viscous and holds in place because they add alot of gum. It needs three coats to go on thick enough and takes quite a bit of time to dry each one. For low SG glazes like this dipping is pretty well impossible. That being said, highly fritted transparent glazes do need to be applied thinner and they actually do not work well at higher specific gravities, AMACO is right to produce it at this low specific gravity. The center Potter's Choice glaze, made by the same company, is 1.52. And thus goes on nice and thick with each coat (and is thus a better deal). That glaze likely contains lots of clay so little or no gelling agent (e.g. Veegum) is needed. The Celadon glaze on the right is in between, 1.46. Glaze manufacturers can produce at a broad range of specific gravities, they adapt the percentage of gum (e.g. Veegum and CMC gum) to the nature of the powdered materials in the recipe to impart the needed rheology and brushing characteristics.
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.
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.