this thread made my day
How didnt i notice it before?
It might be too late, but i indeed have a bit (really, only a bit, not much at all) of sympathy for the OP.
the use of the word chalk in climbing is totally misleading!
Geologically speaking chalk is a soft, sedimentary rock, made out of little "pellets" of Calcium Carbonate ( CaCO_3). So it is "simply" a limestone.
For example the cliffs of dover are made of this stuff. It is very soft (which makes for very "interesting" climbing i have heard) so it got used as writing chalk, since when dragged across a hard surface (such as a blackboard) it abraded and left a white line.
If you heat that stuff (or any other, harder limestone) it releases CO_2 and becomes CaO. CaO, or lime, has many uses, however putting it on your hands and getting it wet is none of them! (hint: OMG it burns
Then we have street chalk. Maybe sometimes it is chalk (as mentioned, historically it has been), but mostly it is not. what gets used here is calcium sulfate (Gypsum, CaSO_4*2H_2O) another sedimentary rock. Its similar in softness and color to chalk, but it is not chalk.
If you heat gypsum, it will loose H_2O, and thus either become Anhidryte (CaSO_4) or (dont know the english term) "dry Gypsum" CaSO_4*0.5H_2O. this is the stuff you can buy as plaster. If you ad water it first becomes a soft kind of slime (depending on the amount of water) of gypsum (the dry gypsum takes the water and becomes gypsum again) and when excess water is evaporated you have gypsum again. I guess it is cheaper to produce "chalk" sticks that way than out of real chalk.
Also: Gypsum + Water = Slimy -> not you desired chalk
And then there is climbing, or gymnastics chalk, which can be a variety of stuff (apart from real chalk, but it seems like that has also been used). . Mostly it is MgCO_3, magnesium carbonate, but there also all kinds of various Mg and Ca oxides, carbonates and hydrates flying around, as well as drying agents. However none contain sulfur as far as i can see.
i have one container that says "4MgCO_3 . ?Mg(OH)_2 . ??". the label has abrade, but i would guess the first ? is a number and the last is H_2O.
Metolius speaks about a safe drying agent added to highes quality magnesium carbonete, wheras Balck diamond says only "pure chalk" (which is bullshit, mineralogically speaking) but as a MgCO_3 atom (which is not possible) printed on the bag.
So to sum it up:
Buy climbing chalk!