So I dropped my last intact alum block and it shattered into a million pieces. This was a sign that it was time to play around with melting and re-forming the shards. Over the years I'd saved broken pieces and leftover ends from a number of blocks, and together they weighed about 5 oz.

Alum melts around 200F, so I figured a double boiler would be the perfect way to do this. My original plan was to melt the alum in the mold I intended to use (a stainless measuring cup) but the method I tried used was poorly conceived and dumped the alum into the simmering water. On to plan b...

I shook the excess water off the alum, put it back in the measuring cup, and put it on a rack above a little water in a pressure cooker. 10 minutes at 15psi and the alum was completely liquid. The only problem was that when it cooled it was firmly adhered to the measuring cup. Tapping didn't work, prying didn't work, heating the outside of the cup - didn't work. Back into the pressure cooker.

The next time I lined another measuring cup with plastic wrap, then poured the melted alum in. Once it solidified, the plastic wrap helped it to pop right out. I now have a 5-ounce, ~2.5" diameter alum block. The size makes for an easy grip and quick application. The part that formed against the measuring cup is very smooth.

The top side, on the other hand, looks like a white version of the Malpais of New Mexico. Rough as hell, with jagged crystals forming in every which direction. And there's even a "lava cave" in the middle that I am guessing comes from the water that got mixed in with the alum. It doesn't bother me enough to re-melt the stuff, though.

TL;DR: If you drop your alum block it's easy to melt and re-form it. But it's probably best to wait until you have quite a bit to work with.

edit: words, added link to image