Substituting Cornwall Stone

Description

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

Article

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

	CaO	MgO	K₂O	Na₂O	TiO2	Al₂O₃	SiO₂	Fe₂O₃	LOI
Cornwall Stone	1.81	0.14	4.30	3.30	0.15	16.33	73.76	0.20
Custer Feldspar	0.30		10.28	2.91		17.35	69.00	0.12	0.04
Nepheline Syenite (Canada)	0.35	0.03	4.80	10.60		23.50	60.20	0.08

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

# MATERIAL	WEIGHT	CaO	MgO	K₂O	Na₂O	Fe₂O₃	TiO2	Al₂O₃	SiO₂
100 NUMBER 1	640.93	0.21	0.06	0.29	0.43	0.01	0.01	0.97	7.72
101 NUMBER 2	682.90	0.23	0.03	0.34	0.37	0.01	0.02	1.02	8.27
102 NUMBER 3	680.57	0.29	0.02	0.27	0.40	0.00	0.02	1.06	8.19
103 NUMBER 4	705.21	0.27	0.02	0.32	0.37	0.01	0.02	1.10	8.47
104 NUMBER 5	717.95	0.22	0.04	0.32	0.40	0.01	0.02	1.09	8.73
105 NUMBER 6	696.23	0.22	0.02	0.16	0.59	0.00	0.01	1.11	8.45
106 NUMBER 7	718.73	0.27	0.04	0.31	0.36	0.01	0.01	1.13	8.66
107 NUMBER 8	736.85	0.28	0.04	0.31	0.35	0.01	0.02	1.12	8.97
108 NUMBER 9	1118.35	0.18	-	0.56	0.22	0.01	0.03	1.85	13.68
109 NUMBER 10	963.42	0.15	-	0.39	0.42	0.01	0.02	1.61	11.70
110 NUMBER 11	685.63	0.26	0.01	0.48	0.25	0.01	-	1.13	8.10
111 NUMBER 12	738.13	0.18	0.02	0.35	0.43	0.01	0.01	1.15	8.82

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.

DETAIL PRINT - CORNWALL STONE AVERAGE
MATERIAL	PARTS	WEIGHT	*CaO	*MgO	*K₂O	*Na₂O	*Fe₂O₃	*TiO2	Al₂O₃	SiO₂
WEIGHT OF EACH OXIDE			56.10	40.30	94.20	62.00	160.00	79.70	102.00	60.00
NUMBER 1	500.00	640.93	.16	.05	.22	.33	.01	.01	.76	6.02
NUMBER 2	500.00	682.90	.17	.03	.25	.27	.01	.01	.74	6.06
NUMBER 3	500.00	680.57	.22	.01	.20	.29	.00	.01	.78	6.02
NUMBER 4	500.00	705.21	.19	.01	.23	.26	.01	.01	.78	6.00
NUMBER 5	500.00	717.95	.15	.03	.22	.28	.01	.01	.76	6.08
NUMBER 6	500.00	696.23	.16	.01	.11	.42	.00	.01	.79	6.07
NUMBER 7	500.00	718.73	.19	.02	.22	.25	.01	.01	.79	6.02
NUMBER 8	500.00	736.85	.19	.03	.21	.24	.01	.01	.76	6.09
NUMBER 9	500.00	1118.35	.08	-	.25	.10	.01	.01	.83	6.12
NUMBER 10	500.00	963.42	.08	-	.20	.22	.01	.01	.83	6.07
NUMBER 11	500.00	685.63	.19	.01	.35	.18	.00	-	.82	5.90
NUMBER 12	500.00	738.13	.12	.01	.24	.29	.01	.00	.78	5.97
TOTAL	6000.00		1.90	.21	2.70	3.13	.07	.11	9.43	72.42
UNITY FORMULA			.23	.03	.33	.39	.01	.01	1.16	8.92
PER CENT BY WEIGHT			1.81	.14	4.31	3.30	.20	.15	16.33	73.76

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.

Related Information

L3617 Cornwall Stone substitute vs. real Cornwall Stone

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

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.

Links

Materials	Cornwall Stone

By Tony Hansen
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