" Flux, or worse yet rosin core solder is nasty stuff and should be
avoided." --
JK Sinrod
www.sinrodstudios.com
www.MyConeyIslandMemories

I clean and cool the tips with a dip of rosin and solder. Been doing
over 20 years, I've seen no problems. Rosin was a common stained glass
flux. A number of windows (100+years old) I've restored still had
rosin residue between the leads, with no bad effect. The problem with
rosin, as I see it, is that it is hard to clean and a bit messy to work
with.

Vic