LOI (Loss on Ignition)

Notes

-The LOI summarizes the oxides within a raw material that burn away as "products of decomposition" during firing. Some material producers separate the different components of weight lost during firing as H₂O, CO2, SO₃, even pure carbon, etc. Since products of decomposition escape as gases during firing, fired glazes do not have an LOI, it is thus not expressed as part of the oxide chemistry.

-The formula weight of a material having an LOI is derived as the sum of the products of oxide weights and their amounts divided by (100-LOI)/100. For example, if the weight of the ceramic oxides (those that survive firing) is 90 and the LOI is 5%, then the total formula weight is 90/((100-5)/100) = 94.74.0

LOI horse race with a surprising winner and loser

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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.

LOI test of common materials flags copper carbonate

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These are pure samples (with 2% binder added) of (top left to bottom right) strontium carbonate, nepheline syenite, cobalt carbonate, manganese dioxide, bentonite (in bowl), 6 Tile kaolin, New Zealand kaolin and copper carbonate. I am firing them at 50F increments from 1500F and weighing to calculate loss on ignition for each. I want to find out at what temperature they are gassing (and potentially bubble-disrupting the glaze they are in or under). Notice how the copper is fuming and spitting black specks on the shelf, this happens right around 1500F (these stains on the shelf darkened considerably when the kiln was fired higher). Yet copper carbonate does not melt and fully decompose until much higher. No wonder it is implicated in cases of glaze blistering and bubbling.

Desktop INSIGHT MDT dialog showing kaolin LOI

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The LOI appears below the material name and alternative names (beside the weight). The formula that goes with that LOI is the bold numbers in the blanks beside the oxide names on the right.

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