Cornish stone is a partly decomposed granite from the south west of England, and it was for a long time the standard feldspathic flux in British glaze recipes. It brings feldspar, some clay and a little fluorine to a glaze in one material, which is why old recipes lean on it so heavily.
What it contributes
Feldspathic flux, some alumina from the clay content, and silica, all in one addition. That combination made it convenient in an era when recipes were built empirically rather than calculated, and it explains why so many classic British glazes are essentially Cornish stone plus whiting plus a colourant.
Why composition has changed
The material as supplied today is not identical to what older recipes assumed, because deposits vary and processing has changed, notably in the treatment of fluorine. A historic recipe reproduced exactly may therefore not give the historic result, which is a common source of confusion when working from old books.
Substituting it
Where Cornish stone is unavailable or a recipe misbehaves, the usual approach is to compare published analyses and build an equivalent from a potash feldspar plus a little china clay and silica. That is calculation rather than swapping, and it is the same discipline that applies to frits and to the lithium feldspars.
Buying it
Still stocked by UK suppliers as a fine powder, and inexpensive relative to the specialist materials on the same list. Standard powder handling applies: ventilated space, fitted respirator, wet cleaning. It keeps indefinitely if stored dry.
Questions people ask about cornish stone
What is Cornish stone?
A partly decomposed granite used as a feldspathic flux, supplying feldspar, some clay and silica in one material.
Why does my old recipe not work with it?
The material as supplied today differs from what older recipes assumed, so historic recipes often need adjusting rather than reproducing exactly.
What can I substitute?
A potash feldspar with small additions of china clay and silica, worked out from published analyses rather than swapped by weight.