|Monthly Tech-Tip |
This chart compares the decompositional off-gassing (Loss on Ignition) behavior of six materials used in ceramic glazes as they are heated through the range 500-1700F. It is amazing that some can lose 40%, or even 50% of their weight on firing. For example, 100 grams of calcium carbonate will generate 45 grams of CO2! This chart is a reminder that some late gassers overlap early melters. That is a problem. The LOI (% weight loss) of these materials can affect glazes (causing bubbles, blisters, pinholes, crawling). Notice talc: It is not finished gassing until 1650F, yet many glazes have already begun melting by then (especially fritted ones). Even Gerstley Borate, a raw material, is beginning to melt while talc is barely finished gassing. And, there are lots of others that also create gases as they decompose during glaze melting (e.g. clays, carbonates, dioxides).
Ceramic materials, especially clays, often contain carbon and organic compounds. When they are fired in a kiln, these must burn out, often producing complications.
|By Tony Hansen|
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