LOI is not important? Think again!

Thursday 31st July 2014

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 insight-live.com/material/173">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).

Pages that reference this post in the Digitalfire Reference Library:

Light Magnesium Carbonate, Frit, Talc, Gerstley Borate, Calcium Carbonate, Dolomite, Ferro Frit 3124, LOI, Firing: What Happens to Ceramic Ware in a Firing Kiln, Talc was making this glaze "puffy" - here is how I fixed it., Pinholing, Glaze Bubbles, LOI, 760-Gerstley Borate stops gassing, 1650-Comparison of frit melts at 1650F (900C), 760-Common frits begin melting, 200-Decarbonation, 900-Talc has finished gassing, 815-Calcium carbonate, talc finished gassing, Glaze Blisters

This post is one of thousands found in the Digitalfire Reference Database. Most are part of a timeline maintained by Tony Hansen. You can search that timeline on the home page of digitalfire.com.