Hey Jay,
It never occurred to me to use a cutting torch to cast with!
I’ve been posting that tip every time the subject has come up for
years! Whaddaya been ignoring me because I use a Little Torch? 
Kidding aside, cutting torches make great casting torches, with a
couple of modifications. There’s no need to wire the jet handle up,
just remove the fulcrum bolt and take it off altogether and open the
cutting jet valve all the way (if so equipped). An acetylene cutting
torch tip works fine with propane for casting everything jewelry
related including platinum, and as Brian says, it’s almost impossible
to overheat it. You can find cutting torches really cheap on EBay, so
you don’t have to pay retail if the deal with your student’s extra
torch doesn’t fly. I like the old discontinued version of the Meco
Weldmaster #63103. I have three, so if you want one, I’ll make you a
good deal (maybe towards tuition for me or one of my guys?). The main
problem with using most cutting torches for casting is that the head
is at a right angle or close to a right angle to the tube which can
make it really awkward to get it aimed right, so try to find one
that has an angled head.
Better yet, toss the entire upper half in the box that you keep your
old parts in, take the handle to a good welder’s supply place and
have them fab up a single tube that has a better angle to the head,
and have them modify a rosebud cutting tip for propane. They can make
the tube whatever combination of length and angle you want. I’m not
sure exactly what the mods to the tip need to be, but they do, and
the whole job’s not that expensive to have done. I (my brother
actually, Christmas 2003 - thanks, Bro!) have less than $150 in my
torch and it works better than any commercial casting torch I’ve ever
tried, especially for platinum, except for a high-end hydrogen setup.
If you (or anyone) would like photos and/or measurements to work
from, I’d be happy to send them to you.
Dave Phelps
precisionplatinumjewelry.com