On the left is a 2013 shipment of PV Clay. These fired test bars are from cone 10R and 5 oxidation down to 1. This is PV theoretically "as it should be", a plastic feldspar. It is melting at cone 10R and reaches almost zero porosity at cone 4 (by cone 3 it has 4.5% and by cone 2 it has 11.5%). Fired shrinkage is 10% by cone 4, dropping to 6% by cone 2. Dry shrinkage is about 5%. The substitute blend on the right, L3894, is 38% silica, 44% Custer feldspar, 12% Pioneer kaolin, 3.5% bentonite and 2.5% dolomite (it calculates to a very similar chemistry). And these fired bars exhibit shrinkage and densities very similar to the PV Clay at every temperature. However the drying shrinkage is about 1% lower and the fired color is darker. So an improvement could be made by using a cleaner feldspar (e.g. Minspar) and a whiter burning, more plastic kaolin. By using #6 Tile kaolin, for example, the plasticity would be improved so much that the bentonite could be reduced, further whitening the color. Of course, if the less vitreous version of PV was better for you, then your substitute should have less feldspar and more kaolin.