LOI is not important? Think again!

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).

Related Pictures

Talc was making this glaze "puffy" - here is how I fixed it.

The recipe on the right employs talc to source the MgO needed to create a silky matte texture. However, talc is a late gasser, potentially able to produce micro-bubbles in the glaze after it has begun melting. The reduction in the definition of contour edges on the tile in front and the reduction in melt fluidity tipped me off. Using my account at insight-live.com I did the calculations needed to source the MgO using dolomite instead, producing recipe G3933E. While dolomite has a far higher LOI than talc it starts releasing the gasses of its decomposition much earlier and finishes well before talc. The mug in the back confirmed my suspicions, firing with a much smoother surface (and with far better definition of the incising).

Links

Materials	Light Magnesium Carbonate A refractory feather-light white powder used as a source of MgO and matting agent in ceramic glazes
Materials	Ferro Frit 3124 A commonly available calcium borosilicate frit.
Materials	Calcium Carbonate In ceramics, calcium carbonate is primarily a source of CaO in raw stoneware and porcelain glazes.
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	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?
Temperatures	Gerstley Borate stops gassing (760-)
Temperatures	Calcium carbonate, talc finished gassing (815-)
Temperatures	Talc has finished gassing (900-)
Temperatures	Decarbonation (200-1000)
Temperatures	Comparison of frit melts at 1650F (900C) (1650-)
Temperatures	Common frits begin melting (760-)
Articles	Firing: What Happens to Ceramic Ware in a Firing Kiln Understanding more about changes taking place in the ware at each stage of a firing helps tune the curve and atmosphere to produce better ware
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.
Oxides	LOI - Loss on Ignition
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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