|Monthly Tech-Tip |
-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.
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).
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.
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.
|By Tony Hansen|
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