Formula Weight
The total weight of oxides in an oxide formula. If the formula is re-totalled, the formula weight changes accordingly.
Key phrases linking here: formula weight, molar mass - Learn more
Details
In ceramics, this is the total weight of oxides in an oxide molecular formula (a formula normally expresses the chemistry of a fired glaze, without the LOI present in the unfired raw materials). Since an oxide formula compares moles, the formula weight (or, more correctly, molar mass) can be derived by simply summing the products of the oxide amounts and oxide weights.
When an oxide molecular formula is retotaled (e.g. to unify RO or R2O3) it becomes a unity molecular formula (UMF), the formula weight changes accordingly. Thus, the formula weight is not like an atomic weight; it is simply the weight of the formula as presented.
It is also possible, although much less practical, to express the chemistry of a material as a UMF. In this case, the formula weight includes the volatiles (e.g. non-ceramic oxides like CO2, CO, H2O, even pure carbon) that burn away during firing. Thus, a material with 10% weight loss on firing will have a formula weight that is 10% higher than that for the ceramic oxides alone. The MDT (materials database) in the original Digitalfire desktop Insight used this technique. However. materials are much better expressed as a percentage chemical analysis.
The formula weight of an oxide molecular formula is not really something one needs to know. It never comes up in glaze chemistry except when one encounters a material whose makeup is expressed as an oxide molecular formula. Knowing the weight is what enables deriving the LOI in the percentage chemical analysis.
Related Information
Links
| 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 |
Oxide Molecular Formula
In ceramics, the chemistry of fired glazes is expressed as an oxide formula. It expresses the relative molar proportions of the oxides contributed by a glaze recipe. There are direct links between the oxide chemistry and the fired physical properties. |
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