Substituting Cornwall Stone — Digitalfire

Calculating a recipe of materials having the same oxide chemistry as Cornwall Stone

Cornwall Stone is quite different from feldspars. Consider Custer Feldspar and Nepheline Syenite.

Here is a wide range of analyses from data sheets and textbooks.

Fluorine has not been included. To get an average I entered each of these as a material into desktop Insight and added equal parts of each to a recipe. A detail report (below) reveals an average formula and recipe.

Here is an example glaze recipe in which our L3617 Cornwall Stone substitute could be used..

WHITING............. 37.70
FERRO FRIT 3134..... 55.80
KAOLIN.............. 35.50
SILICA.............. 50.70
CORNWALL STONE...... 20.30
                     -----
                    200.00

There are two ways: Weigh up a batch of the Cornwall substitute, mix it thoroughly, then treat it as a raw material (if you are confident enough in being able to shake the mix enough in a plastic bag to believe it is mixed well). Otherwise, retotal the Cornwall substitute to 20.3 and combine it with the above recipe. Actually, both of these approaches have a small issue. The L3617 recipe contains materials have that an LOI, its total calculated LOI is 2.4%. That means the most correct amount to substitute for 20.3 is 20.3/(100-0.27/100) or 20.8.

L3617 Cornwall Stone substitute vs. real Cornwall Stone

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This melt fluidity comparison demonstrates how similar the substitute L3617 recipe (left) is to the real material (right). 20% Frit 3134 has been added to each to enable better melting at cone 5 (they do not flow even at cone 11 without the frit). This substitute is chemically equivalent to what we feel is the best average for the chemistry of Cornwall Stone.

Two Cornwall Stone shipments compared to two substitutes

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This is a cone 11 oxidation melt flow test. Shown (left to right):
-A new shipment of Cornwall Stone 2011
-L3617 Digitalfire calculated substitute recipe
-An older Cornwall shipment
-The H&G substitute 2011 (far right, mislabelled on the picture).
These do not flow well here, a small frit addition was later employed 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 Stone than the new Cornwall is.