This chart compares the decompositional off-gassing (% weight Loss on Ignition) behavior of six glaze materials (there are many more). Each was heated through the range 500-1700F at 350F/hr. It is amazing how much weight some can lose on firing. For example, 100 grams of calcium carbonate generates 44 grams of CO2! Magnesium carbonate even more. But the main message of this chart is that some late gassers overlap early melters. That can be a problem (causing glaze bubbles, blisters, pinholes). The talc in this test is an example: It did not finish gassing until 1650F. Yet many fritted and Gerstley Borate glazes have already begun melting by then.
There are two takeaways here. The biggest weight loser is not the biggest troublemaker. What matters is how gas release overlaps glaze sealing. Calcium carbonate illustrates this; its high carbonate content can still produce a clear glaze (when gases can clear ahead of any melting). And the talc, although having a small LOI, is a potential troublemaker when the glaze melt cannot effectively clear the bubbles.

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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).
| 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. It is glaze-like, needing only a kaolin addition to create a low-temperature glossy glaze. |
| 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 | Volatiles |
| Glossary |
LOI
Loss on Ignition is a number that appears on data sheets test results of ceramic materials and bodies. It refers to the amount of weight lost 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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