Once a rip is started, there's a better than 
average chance it could get worse. I have a 
couple cels with rips and what I usually do is to 
put a small piece of clear tape right at the end 
of the rip, just as a little support to prevent 
it from ripping further. This isn't a "repair" 
per se, but only lessening the chance of the 
damage increasing. Of course, I take extra care 
when handling it, and (if possible) make sure to 
bag it in a way that is extra protective of the 
ripped area.

There is no way I would ever try to melt the cel 
back together with anything. All it will do is 
warp. o_0;; (I've seen the disastrous results of 
acetate starting to melt.)

Many Sharp Smiles,
--Drac