heh
Your supposed to prevent the acid from getting warm - in fact, you start with COLD acids (plural - takes two kinds - and no, I won't give the formula - I have an OLD encyclopedia that has the instructions!!) - the reaction makes the Nitro unstable, particularly until it's washed, so you have to keep it cold (ICE) until the reactions are done, and it's washed - and yes I do mean washed - the 2 acids plus the glycerine end up making nitro, which is heaver than water - thing is, there is usually some excess acid floating around (of one type or the other), and it is LIGHTER than the nitro - and you want to wash it away, as it is what tends to make nitro "unstable"

Back when I was a kid (and I'm NOT old), I knew an elderly gent who had been a blaster, and he used to laugh at the movies where people had to ship nitro. Nitro was almost NEVER shipped according to him - easy enough to make if you had the raw materials, so you would ship THOSE, and any blaster knew how to make the nitro. My great-grandfather (who I never met) was in the business - my father had stories of skipping out of school - go to the dynamite shack, get some of the "sweat" (nitro ) and rub it on your forehead - get a fever - get kept home.....