|Monthly Tech-Tip |
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).
|Materials||Light Magnesium Carbonate|
|Materials||Ferro Frit 3124|
Pinholing is a common surface defect that occurs with ceramic glazes. The problem emerges from the kiln and can occur erratically in production.
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.
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.
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||Talc has finished gassing (1650F-)|
|Temperatures||Calcium carbonate, talc finished gassing (1500F-)|
|Temperatures||Gerstley Borate stops gassing (1400F-)|
|Temperatures||Common frits begin melting (1400F-)|
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|