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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).
Materials |
Light Magnesium Carbonate
A refractory feather-light white powder used as a source of MgO and matting agent in ceramic glazes |
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Materials |
Ferro Frit 3124
A commonly available borosilicate frit is high in calcium. |
Materials |
Talc
A source of MgO for ceramic glazes, a flux or thermal expansion additive in clay bodies, also used in the manufacture of cordierite. |
Materials |
Calcium Carbonate
In ceramics, calcium carbonate is primarily a source of CaO in raw stoneware and porcelain glazes. |
Materials |
Dolomite
An inexpensive source of MgO and CaO for ceramic glazes, also a highly refractory material when fired in the absence of reactant fluxes. |
Materials |
Frit
Frits are made by melting mixes of raw materials, quenching the melt in water, grinding the pebbles into a powder. Frits have chemistries raw materials cannot. |
Materials |
Gerstley Borate
Gerstley Borate was a natural source of boron for ceramic glazes. It was plastic and melted clear at 1750F. Now we need to replace it. How? |
Oxides | LOI - Loss on Ignition |
Glossary |
Pinholing
Pinholing is a common surface defect that occurs with ceramic glazes. The problem emerges from the kiln and can occur erratically in production. |
Glossary |
LOI
Loss on Ignition is a number that appears on the data sheets of ceramic materials. It refers to the amount of weight the material loses as it decomposes to release water vapor and various gases during firing. |
Glossary |
Glaze Bubbles
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. |
Articles |
Firing: What Happens to Ceramic Ware in a Firing Kiln
Understanding more about changes are taking place in the ware at each stage of a firing and you can tune the curve and atmosphere to produce better ware |
Temperatures | Decarbonation (200-1000) |
Temperatures | Talc has finished gassing (900-) |
Temperatures | Calcium carbonate, talc finished gassing (815-) |
Temperatures | Gerstley Borate stops gassing (760-) |
Temperatures | Comparison of frit melts at 1650F (900C) (1650-) |
Temperatures | Common frits begin melting (760-) |
Troubles |
Glaze Blisters
Questions and suggestions to help you reason out the real cause of ceramic glaze blistering and bubbling problems and work out a solution |
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