Glass mounts are mainly a physical-protection choice, not a longevity upgrade. Based on Kodak guidance, glass does not significantly extend the useful life of slides beyond helping protect them from dirt and scratches.

The bigger factors for archival storage are light, heat, and humidity. If slides are stored properly—cool, dry, and away from light—glassless mounts in archival slide pages/binders are generally fine, especially if they’re only handled or viewed occasionally.

Glass mounts do have a potential downside during projection: heat from bright projector lamps can cause moisture to condense inside the mount. Keeping slides dry and using silica gel in storage can help reduce that risk. Also, the glass should be clean before mounting.

So, if your slides mostly live in Print File-style archival pages and aren’t projected often, glassless mounts are usually an acceptable long-term storage choice. Choose glass if you want extra protection against surface damage from handling, but don’t expect a major preservation advantage over well-stored glassless slides.