Negative 'pop'

I discovered that 35mm negs pop, 6x4.5 negs pop, but LF negs do not! I use glass carrier in my MF enlarger, glassless in the LF. When I put a 6x6 neg in the LF enlarger, it popped... Yet another reason to shoot LF ;)

With my enlarger I bet 4x5 is the most likely to pop. When in 4x5 the negative is closest to the bulb. When in 35mm it's pretty far away.

4x5 film usually has quite a bit thicker base than the medium format stuff. No pop there. Then theres the carrier with the tensioning rig made on it.

I have LPL 4550 XLG witch has a fan. I print 6x7. The megatives still pop. Wih color I pre-heat them until they pop. Also, doing that gives more consistent color balance since lamp color changes depending on whether enlarger's lamp was pre-heated or not. With B&W this does not always work. Even if I pre-heat, the film may still pop after that or pre-heat time becomes very long. Glass negative holder fixed it all.

How long are yor exposures and how open are your lenses for normal enlargement?

It just maybe you have too powerful a lamp.

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.