| Ratio and Expansion Calculations |
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This singles out the silica and alumina oxide molecules and gives their relationship. For example, if there is 5.0 SiO2 and 0.5 Al2O3, then the ratio is 5.0:0.5 or 10:1, or just 10.
This ratio is significant in stoneware glazes, for example, because high silica tends to produce glossy glazes when alumina is low and high alumina creates matte glazes when silica is low. It thus follows that the higher the Si:Al ratio the glossier a glaze will be.
However we must recognize that this ratio is not a general or fool-proof measure of gloss. Validity of the Si:Al ratio assumes a transparent glaze that is melting well and relatively free of other mechanisms that create matteness (e.g. high magnesia or calcia in low fire). Also remember that very low alumina glazes are fluid and encourage crystal formation during cooling; if these crystals are small they can completely cover the surface turning it matte.
This refers to the Silica:Boron combination campared to the amount of alumina. Since boron is also a glass former it needs to be considered in low and medium fire glazes. This ratio is not as clear an indicator of fired gloss as Si:Al because low fire glazes generally contain significant boron and thus operate by a 'different set of rules' than stoneware ones. For example, boron gloss glazes can take a lot more alumina into solution inthe melt than stoneware glazes even though alumina is very refractory. It is not typical to make a high alumina matte at low fire, generally mattes are made using high calcia or magnesia.
Thermal expansion refers to the amount by which a solid expands and contracts as it is heated and cooled. Glaze and body are attached to each other and must expand and contract together or else the glaze or body will crack to relieve the stresses..
The thermal expansion of a glaze can be predicted by feeding the oxide propertions and their expansions into a simple additive equation. INSIGHT maintains the expansion for each oxide in the oxides dialog.
Real-world expansion numbers are extremely small and refer to the amount by which an item expands per degree rise in temperature. INSIGHT removes the decimal to produce a simple number that generally falls between 5 and 8. This number is relative only. Thus if a glaze is crazing you need to adjust the formula to bring the expansion number down. If it is shivering you do the opposite. The amount by which you change it comes with experience.
INSIGHT has a place to enter the cost of materials in the Materials dialog. It assumes that you use the same unit of measure for each. The cost value that INSIGHT calculates is only as good as the costs in the materials database.
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