•The secret to cool bodies and glazes is a lot of testing.
•The secret to know what to test is material and chemistry knowledge.
•The secret to learning from testing is documentation.
•The place to test, do the chemistry and document is an account at https://insight-live.com
•The place to get the knowledge is https://digitalfire.com

Sign-up at https://insight-live.com today.

Carbon trap glazes

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. As the firing continues, 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 which will melt earlier than the rest of the glaze, thus facilitating the carbon trapping.

More carbon needs to burn out than you might think!

More carbon needs to burn out than you might think!

Hard to believe, but this carbon is on ten-gram balls of low fire glazes having 85% frit. Yes, this is an extreme test because glazes are applied in thin layers, but glazes sit atop bodies much higher in carbon bearing materials. And the carbon is sticking around at temperatures much higher than it is supposed to (not yet burned away at 1500F)! The lower row is G1916J, the upper is G1916Q. These balls were fired to determine the point at which the glazes densify enough that they will not pass gases being burned from the body below (around 1450F). Our firings of these glazes now soak at 1400F (on the way up). Not surpisingly, industrial manufacturers seek low carbon content materials.

Soda fired porcelain vessel by Heather Lepp

Soda fired porcelain vessel by Heather Lepp

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".

In Bound Links

  • (Glossary) Glaze Bubbles

    As glazes melt, gases from decomposition of organics, carbonates, sulphates and hydrates are generated (if the body was glazed green, or unbisqued, many more of these gases will be present). If glazes are already melting while the gases are being generated, bubbles form and suspend in the glass melt...

  • (Glossary) Shino

    Shino glazes were developed in Japan and their aesthetics are covered on many other web sites and books. But they are described by artists using the language of art. However this page is about understanding the chemistry and the mechanisms of the color development with a view to achieving it in func...

By Tony Hansen

Feedback, Suggestions

Your email address


Your Name


Copyright 2003, 2008, 2015 https://digitalfire.com, All Rights Reserved