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%.