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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 Fe2O3LOI
Cornwall Stone 1.810.144.303.300.1516.3373.760.20  
Custer Feldspar 0.30 10.282.91 17.3569.000.12 0.04
Nepheline Syenite (Canada) 0.350.034.8010.60 23.5060.200.08  


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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