In a glaze melt, oxides do not act alone, they interact. For example, while one material might not melt well by itself, when combined with others the mix as a whole can melt at a significantly lower temperature than any of the ingredients in the mix. Cornwall stone is an example, by itself it does not melt enough to even be a glaze at cone 10, but a glaze can be made using this material in combination with kaolins, silica, etc. Oxides that are even refractory by themselves can be powerful fluxes in combination with silica and alumina, a good example is the material calcium carbonate. By itself it is completely refractory and yet at cone 8-10 it is the principal flux in stoneware glazes. Thus, due to interaction, the function of the oxide is changed completely.
Cornwall Stone off different dates compared to substitute L3617
This is a cone 11 oxidation melt flow test. Shown (left to right) are the new shipment of Cornwall Stone 2011, the L3617 calculated equivalent (a recipe, see link), the older Cornwall shipment we have been using and the H&G substitute 2011 (far right, mislabelled on the picture). These do not flow well here, a small frit addition is needed to better compare them. However they have melted enough to see some differences in whiteness and degree of melt. Notice the L3617 is more like the old Cornwall than the new Cornwall is.
Calcium carbonate and dolomite are refractory when used pure
Examples of calcium carbonate (top) and dolomite (both mixed with 25% bentonite to make them plastic enough to make a test bars). They are fired to cone 9. Both bars are porous and refractory, even powdery. However, put either of these in a mix with other ceramic minerals and they interact strongly to become fluxes.
Frits melt so much better than raw materials
Feldspar and talc are both flux sources (glaze melters). But the fluxes (Na2O and MgO) within these materials need the right mix of other oxides with which to interact to vitrify or melt a mix. The feldspar does source other oxides for the Na2O to interact with, but lacks other fluxes and the proportions are not right, it is only beginning to soften at cone 6. The soda frit is already very active at cone 06! As high as cone 6, talc (the best source of MgO) shows no signs of melting activity at all. But a high MgO frit is melting beautifully at cone 06. While the frits are melting primarily because of the boron content, the Na2O and MgO have become active participants in the melting of a low temperature glass. In addition, the oxides exist in a glass matrix that is much easier to melt than the crystal matrix of the raw materials.
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