|Monthly Tech-Tip |
A type of ceramc glaze.
Key phrases linking here: carbon trap glazes - Learn more
Glazes with variegated patterns of grey and black from carbon trapped below the surface.
The effect is created by fuel firing without adequate oxygen in early stages to build up soot (carbon) on the surface of ware. The firing must continue in reduction, to maintain the carbon. The carbon trap glaze begins to melt before the carbon sitting on the surface burns away. Carbon is a refractory material and will stay in a glaze as long as there is no oxygen to combine with it. Typically this type of glaze includes soda ash or other soluble alkaline fluxes which will migrate to the surface of the raw glaze as it dries, forming a crust of alkalis that will melt earlier than the rest of the glaze, thus facilitating the carbon trapping.
A danger with these is trapping carbon in the body, especially if it fires vitreous, resulting in brittle ware that can crack spontaneously on kiln cooling and opening.
This is a small cup-sized object made from Plainsman P600 (simply composed of Tile #6 kaolin, nepheline syenite and quartz). It is valued as a product-of-the-process piece, consigned to the "kiln God" as unglazed. It exhibits carbon-trap, soda glaze deposition and flashing. The soda-vapour atmosphere of the kiln glazed one side of the vessel early enough in the firing to trap carbon under a crystal-clear glass. Often such glazes are crazed, but this one likely is not because the body contains 25% quartz, giving it a high thermal expansion. The other side of the piece exhibits tones of red, brown and yellow on the bare, vitreous porcelain surface - this is characteristic of "flashing".
More carbon needs to burn out than you might think!
Suspended micro-bubbles in ceramic glazes affect their transparency and depth. Sometimes they add to to aesthetics. Often not. What causes them and what to do to remove them.
Traditional Japanese high feldspar glazes having cream to orange color flashing or blushing. Potters today seek to emulate the Shino appearance using a wide range of recipes.
Ceramic glazes are glasses that have been adjusted to work on and with the clay body they are applied to.
Carbon trapping explained at CeramicArtsNetwork
|By Tony Hansen|
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