Posted 09 June 2014 - 08:27 AM
2 products that do similar things to clay/slip/glaze, but are chemically completely different. Sodium silicate is just that, Darvan 7 is a long chain polyacrylic acid. Whereas Darvan 811 is a short chain polymer, with a low molecular weight, less appropriate for glaze, more so for clay and slip. A polymer that you would probably not be able to recreate without an advanced chemistry degree and a lab.
My experience with them is that Darvan created a more stable slurry and is much less caustic to both you and mold hydrocal/plaster. However, as John says above, if you are applying it to stretch clay and get a cracked effect, Sodium Silicate is the one to use. Both are inexpensive and easy to purchase from any well stocked pottery supplier. If you are in Philadelphia, where Standard is, you can get Darvan at the Ceramic Shop which I believe is on on Amber Street. Or order online from the Ceramic Shop, Axner, Clay King, Bailey, etc.