Cornwall Stone

Alternate Names: Cornish Stone, China Stone, Corn Stn, China-stone

Oxide	Analysis	Formula
CaO	1.81%	0.24
K2O	4.31%	0.34
Na2O	3.30%	0.40
TiO2	0.15%	0.01
Al2O3	16.33%	1.19
SiO2	73.76%	9.11
Fe2O3	0.20%	0.01
MgO	0.14%	0.03
Oxide Weight		742.30
Formula Weight		742.30

Notes

Was also known as: Pegmatite. Graven. Gowen. Manx stone. Meldon stone. Growan. Petuntse. Amakusa.

Cornwall stone was a low iron feldspar material used primarily as a flux in clay bodies and glazes. Is was mined in the Cornwall area of the UK. It melted 1150-1300C. It had a more diversified selection of fluxes than other feldspars but also had one of the highest silica contents. By itself it did not melt as well as feldspars (melt flow began around cone 11 oxidation). It was popular in engobes for its adhesive power during and after firing and in glazes for its low shrinkage and purity.

Long-time users often commented on the difference in color between different batches. The parent ores are more complex than other feldspars, and tend to be a mix of varying types of igneous rock in different stages of decomposition. Earlier stages of the ore materials are bluer (from fluorine) and contain more fluxes than newer rocks (some of the alkalis and fluorine have been leached and washed away creating a softer material). Cornish stones tended to be classified into major types according to the amount of flux present. Although Cornish stone was quite variable in composition, its low iron made it an attractive material. Documentation on this material provided a wide range of chemistries over the years, so no one really knew what a truly representative analysis should have looked like (the one provided here is an average of many that we collected).

Its diversity of oxides made it similar in composition to common stone, thus its name. Now it is common to see synthetic substitutes for this material (since it is easy to blend other feldspars to approximate the analysis). These substitutes have the advantage of having no fluorine (which is suspected in various glaze faults). Hammill & Gillespie makes one of these, H&G Cornwall Stone. It is described in an article in Ceramics Technical Nov 2011, the chemistry given there differs from the chemistry we show here.

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 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 we 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.

CORNWALL STONE Average Calculation
MATERIAL	PARTS	WEIGHT	*CaO	*MgO	*K₂O	*Na₂O	*Fe₂O₃	*TiO2	Al₂O₃	SiO₂
Oxide Weights			56.1	40.3	94.2	62.0	160.0	79.7	102.0	60.0
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
Precentage Analysis			1.81	.14	4.31	3.30	.20	.15	16.33	73.76

This became the target for our L3617 Cornwall Stone substitute recipe. Here is an example glaze recipe in which it 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 to use it the L3617 Cornwall substitute: Weigh up a batch, 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.

Cornwall stone powder variation

Left: Traditional blue material, could be 20 years old.
Center: A shipment we got in Feb 2014.
Right: A shipment in Oct 2014.
Front: 10 gram balls prepared for a melt-flow GBFL test.
The blue powder is the most difficult to form (therefore the least plastic) after water has been added, the tan one is the easiest.

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.

Which is the better flux? Cornwall stone or nepheline syenite?

Left: Cornwall plus 10% Ferro Frit 3134. Right: Nepheline Syenite plus 10% of the same frit. These are fired at cone 6.

Melt fluidity: Cornwall Stone vs. Nepheline Syenite

Three Cornwall Stone shipments (from 2011 and 2014) fired at cone 8 in melt flow GLFL testers and compared to Nepheline Syenite. Each contains 10% Ferro Frit 3134.

An original container of MFW30/G Spray Dried Flux

A Cornwall substitute from the UK. LagunaClay distributed this in North America until increases in price motivated them to produce their own at a much lower cost.

Links

Articles	Substituting Cornwall Stone Calculating a recipe of materials having the same oxide chemistry as Cornwall Stone
Materials	Potash Feldspar
Materials	Soda Feldspar
Materials	DF Cornwall Stone
Materials	BPS Cornish Stone
Materials	H&G Cornwall Stone
Materials	Petunse
Materials	SG919 Laguna Cornwall Substitute
Hazards	Fluorine Gas Can you be exposed to fluorine gas in ceramics. Yes, if you are using materials that contain fluorine. These are not many, but worth knowing about.
Typecodes	Generic Material Generic materials are those with no brand name. Normally they are theoretical, the chemistry portrays what a specimen would be if it had no contamination. Generic materials are helpful in educational situations where students need to study material theory (later they graduate to dealing with real world materials). They are also helpful where the chemistry of an actual material is not known. Often the accuracy of calculations is sufficient using generic materials.
Typecodes	Feldspar The most common source of fluxes for high and medium temperature glazes and bodies.
Oxides	K2O - Potassium Oxide
Glossary	Oxide Interaction In ceramic glazes each oxide generally makes a specific contributions to the melting and freezing behavior of the glaze. However interactions are also important.

By Tony Hansen
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