Cold starts are becoming a fairly popular trend. As people begin to familiarize themselves with cold starts we have noticed one trend that we would like to put an end to: COLD STARTING WITH THE CAP ON.
Unless your cap is made out of quartz or another material that won't break from thermal shock, do not put the cap on first and torch second. This is a recipe for disaster. Borosilicate glass caps cannot withstand the extreme temperature changes quartz nails can endure. The sudden change from cold to hot can cause regular glass caps to break from thermal shock.
When doing any cold start technique involving a borosilicate cap, always place the cap on after the torch has been removed from the nail.
What about on reheats?
It is still wise to remove the cap in the instance of a reheat. Leaving a borosilicate cap on while any flame is being applied is a gamble being taken at the risk of the user.