|Monthly Tech-Tip |
Alternate Names: Cornish Stone, China Stone, Corn Stn, China-stone
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.
|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.
|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|
This became the target for our L3617 Cornwall Stone substitute recipe. Here is an example glaze recipe in which it could be used.
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.
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.
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.
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.
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.
Substituting Cornwall Stone
Calculating a recipe of materials having the same oxide chemistry as Cornwall Stone
BPS Cornish Stone
H&G Cornwall Stone
SG919 Laguna Cornwall Substitute
DF Cornwall Stone
In ceramic glazes each oxide generally makes a specific contributions to the melting and freezing behavior of the glaze. However interactions are also important.
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.
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.
The most common source of fluxes for high and medium temperature glazes and bodies.
|Oxides||K2O - Potassium Oxide|
|By Tony Hansen|
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