Quite simply, the weight of a formula. Typically, in glaze chemistry, when we refer to formula weight it is assumed we are talking about the weight of the fired formula of a glaze (without LOI and volatiles). However is is possible to also talk about the formula weight of a material (although materials are normally evaluated as analyses). In this case, the weight specified includes the volatiles (e.g. CO2, carbon, CO, H2O, etc) that burn away during firing.
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.
In ceramics, the chemistry of a glazes are expressed as formulas of oxides. There are direct links between the oxide chemistry and the fired physical properties.
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