Not trying to be a pain, but....
Galvanized metal and flame are not good for ones health. The fumes given off create a whole new set of rules concerning ventilation, so at least don't do this in the house.
The galvanized sheet metal commonly used for duct work is fairly thin, 26-30 gauge. Considerably thinner than the 16 ga, black iron or stainless that make up my boiling tanks. My tanks discolor and even warp slightly at the spots where the flame is hottest. I wonder how long the thinner stuff will last before fatigue causes cracks, etc.
Yes, you may boil water in a paper cup, but how many times? Ditto, on the aluminum.
In my opinion, if you are going to do this more than once or twice, buy or have built a decent boiling tank. The cost per use on mine is down to almost nill. In fact, in a couple of cases, it is nill, because the tanks were made by the guys in our shop out of scrap, and as a favor.
JMO,
Jim
