broken tap in alum. for CNC conversion..HELP

I have a small HSS tap broken off in a piece of 7000 series aluminum
that I want to dissolve out using aluminum sulphate(aka ALUM), by
boiling the piece in a solution of the alum at around 120 degrees F
until the tap dissolves ,turning into a rusty looking color, at which
point it can be flushed out with water without affecting the aluminum
part surrounding it.. but I dont know what it is or where it can be
found easily.

I was hoping that someone could tell me what it is and maybe give me
an idea of where it can be found.
Id rather not have to make the part all over agin, because I have
quite a few hours in it.
And being that I live in the "sticks", a tap extractor can not be
found anywhere closer than 100 miles or so from my location.If I was
looking to buy a pig or some chickens..I could buy them all day long!
But I dont think they serve my purpose too well.

Ive googled and it seems that aluminum sulphate is used in a few
different things......
1) as an astringent for swollen mucus membranes.(local pharmacist
looked at me like I was crazy when asked about it)
2) used in certain types of fertilizers(what kind??where to look??)
3)found in most baking powders (although ive yet to see a baking
powder label that indicates this)
But no information as to where it can be found or what to look for.

Any help in this matter will be greatly appreciated.

I sure would like to avoid machining another part again.......

Thanks
Jim