|Monthly Tech-Tip |
Alternate Names: Cornish Stone, China Stone, Corn Stn, China-stone
Also known as: Pegmatite. Graven. Gowen. Manx stone. Meldon stone. Growan. Petuntse. Amakusa.
Cornwall stone is a low iron feldspar material used primarily as a flux in clay bodies and glazes. Is mined in the Cornwall area of the UK. It melts 1150-1300C. It has a more diversified selection of fluxes than other feldspars but also has one of the highest silica contents. By itself it does not melt as well as feldspars (melt flow begins around cone 11 oxidation). It is popular in engobes for its adhesive power during and after firing and in glazes for its low shrinkage and minimal contribution to defects.
Long time users often comment on the difference in color between different batches of materials they receive. The parent ore materials are much 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 where some of the alkalis and fluorine have been leached and washed away creating a softer material. Cornish stones tend to be classified into major types according to the amount of flux present. Although Cornish stone is quite variable in composition, its low iron makes it an attractive material. Understandably, documentation on this material has provided a wide range of chemistries over the years, so no one really knows what a truly representative analysis should look like (the one provided here is an average of many that we have collected).
Its diversity of oxides make it similar in composition to common stone, thus its name. It is common to see synthetic substitutes for this material since it is easy to blend other feldspars to approximate the analysis of Cornish stone. These substitutes have the advantage of having no fluorine (which is suspected in various glaze faults that do not appear when using the substitutes). 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. The logic for the chemistry we show can be found in an article on this site (link below).
These flow tests demonstrate how similar the substitute 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). Links below provide the recipe for the substitute and outline the method of how it was derived using Digitalfire Insight software. This substitute is chemically equivalent to what we feel is the best average for the chemistry of Cornwall Stone.
Cornwall Stone as it changes over time. 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 after water has been added, the tan one is the easiest.
This is a cone 11 oxidation melt flow test. Shown (left to right) are the new shipment of Cornwall Stone 2011, the L3617 calculated equivalent (a recipe, see link), the older Cornwall shipment we have been using and the H&G substitute 2011 (far right, mislabelled on the picture). These do not flow well here, a small frit addition is needed 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 than the new Cornwall is.
Although Cornwall Stone is used as a glaze flux, by itself it does not melt well at this temperature. However a blend of Cornwall:silica:clay does melt well. An old stock (two left balls) is being compared to a February 2014 and October 2014 shipment (right balls). The Oct 2014 shipment is slightly browner in color and not melting quite as much as the others. To better test this at cone 10, a little frit needs to be added, perhaps 5%.
Substituting Cornwall Stone
How to create a blend of materials to chemically substitute for another (Cornwall Stone is used as an example).
|Materials||BPS Cornish Stone|
|Materials||H&G Cornwall Stone|
|Materials||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.
L3617A - Synthetic Cornwall Subsitute
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|