I recall using that last year as a deodorant and found it resembled a block of alum but never thought to use it on the face....
It was quite useless as the bottom broke off so it no longer twisted. 
That's it. Dump ~200g in 250mL cold water, give it a good stir and leave it for a while. Most of it will go into solution quite readily, and chuck out any solids left behind.
The MSDS says potassium aluminium sulphate. It's a mineral. They probably dig it out of exactly the same hole in the ground as the stuff you pay $30 for 100g, which is no more likely to be 100% pure. It also comes naturally as other forms of double sulphate of that cation, typically Na or NH4 instead of Ka, but the astringent property is the same.
Sort of, yes. But the feel is a bit different. I think alum is a bit more effective in dealing with razor burn or bumps and definitely with weepers/nicks, but you have to rinse it off after say 5-10 minutes, otherwise the residue will drive you mad.
Unfortunately skin type makes that almost impossible for some.
I love how the Chemist Warehouse add states "no aluminium". Pretty sure that alum, by definition is a salt of aluminium.
I leave the alum on only while I rinse and dry my brush and razor and then rinse off my face with cold water. As you say if you leave it on to dry it drives you nuts with irritation.
