In fact, silver sulfate is a white powder that you can buy from chemical supply companies. I have several bottles of it. In the meantime, I found a Youtube that demonstrates a good way to refine it. It turns out that silver sulfate is much more soluble in boiling water (1.33g per hundred mL) than water at lower temperatures. When it's in solution, you can add HCl or table salt in solution to convert it to silver chloride, which is much easier to refine. My preferred method is to put it in a 1 to 20 solution of dilute sulfuric acid, add a bunch of zinc plated roofing nails, and mix until all the silver cements out (usually about half an hour.) Then you just remove the nails with a magnet. I sold some of this to Manhattan gold and silver, and they gave me nearly the same price for it as they were paying for silver rounds and bars, so it must have been quite pure.