|Monthly Tech-Tip |
Even after two weeks it is still sticky. This was purchased at an import store. What could this black goo be? It is likely a sealer that they use to make the porous clay water tight, perhaps an organic sugar. The clay is porous (and thus also weak) because they want to save energy by firing their kilns as low as possible. A water soluble sealer can be OK if the vessel is not used for storage. But it is not OK because there is another problem: The glaze is crazed. That is what is permitting the water to be absorbed into the body. That water is dissolving the sealer and bringing it out. There is yet another issue: The glaze could very well contain lead. Lead makes glazes melt low, so it is a great for saving energy. But not so great for producing safe ware.
Crazed ceramic glazes have a network of cracks. Understanding the causes is the most practical way to solve it. 95% of the time the solution is to adjust the thermal expansion of the glaze.
Clay Body Porosity
In ceramics, porosity is considered an indication of density, and therefore strength and durability. Porosity is measured by the weight increase when boiled in water.
A term used in the ceramics industry to signify the degree of vitrification in a fired clay. Mature clays are dense and strong, immature ones porous and weak.