|Monthly Tech-Tip |
The soluble salts dissolved in the water of plasticity of this red body have migrated through the white engobe during drying of these earthenware cups. The cups were upside down so all the solubles have been left on the outside surface. The red body is made using a high percentage of Redart clay (a widely available commercial low-fire low-plastic clay in North America). It is plasticized using added ball clay. The brownish material is organic, because after bisque firing it has disappeared.
The most common commercially 200 mesh available raw terra cotta clay in North America. It fires red, has low plasticity and matures a low kiln temperatures.
A common problem with dry and fired ceramic. It is evident by the presence of a light or dark colored scum on the dry or fired surface.
In ceramics, certain compounds in clays and glazes can dissolve into the water, then on drying these are left on the surface.