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LOI is not important? Think again!

This chart compares the decompositional gassing behavior of six materials as they are heated through the range 500-1700F. These materials are common in ceramic glazes, 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 your 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).

A chart showing weight-loss vs firing temperature for some common ceramic materials

Links

Materials Light Magnesium Carbonate
Materials Frit
Materials Talc
Materials Gerstley Borate
Materials Calcium Carbonate
Materials Dolomite
Materials Ferro Frit 3124
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 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.
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.
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 (200C-1000C)
Temperatures Talc has finished gassing (1650F-)
Temperatures Calcium carbonate, talc finished gassing (1500F-)
Temperatures Gerstley Borate stops gassing (1400F-)
Temperatures Common frits begin melting (1400F-)
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
Oxides LOI - Loss on Ignition


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