Jorge has a handle on this problem. The heat that causes the negative to "pop" is almost all infra rays. Conductive heat from the lamp should not be a factor to an great degree (lol). The heat adsorbing glass being discussed absorbs IR, not heat, and they work. Like Jorge says, negative pop may be traced to an overly powerful lamp.

Someone mentioned the heat produced by slide projectors. This heat is conductive and radiated heat, and it is a lot! Kodak used a system of mirrors and heat absorbing glass in the 35mm Carousel projectors and slides still pop. Things are a little less intense in an enlarger. I use a transformer (Variac) on mine to control the luminance and this reduces heat as well. It really extends bulb life too.