Its an old topic but a good one!!
Problem with mounting the print touching the glass is it doesn't allow for air flow or allow the print to breathe. Moisture/condensation can (will!!) cause the print to stick to the glass ruining the print also possible causing mould etc.
The sticking issue is true for traditional photographs made with gelatin emulsions. Gelatin undergoes what material scientists refer to as the glass transition temperature (Tg) at about 24C/75F and reverts to a gel-like state when humidity goes above approximately 70%RH for extended periods of time. Gelatin's Tg value is highly moisture dependent (i.e., lower RH requires higher temperature, and vice versa). This 75F/70%RH temp/RH level is very commonly encountered in typical real world conditions, especially during summertime periods. All photographs that have ever stuck to the photo frame glazing have been subjected to such a real world temperature and humidity cycle event. The adhesion then takes place when the gelatin goes back to below Tg temp/RH conditions and the gelatin returns from the rubbery gel state back to a relatively brittle and dry polymer state. Gelatin is after all a very close chemical cousin to traditional hyde glues used in traditional wood working.
The room temperature/moderately high Relative humidity value that triggers the Tg transition for gelatin photographs is not necessarily true for other media, so not all works of art on paper will stick like traditional photos to glass. Many
but not all(e.g., the "swellable polymer media" which are pretty much no longer on the market) inkjet media are not going to stick to glass like traditional silver gelatin photographs. Adding a spray coat like Premier Print Shield which is an acrylic polymer will tip the odds as well much in favor of not sticking. However, there may be cockling and mould damage at prolonged high humidity levels with all works of art on paper, coated or not. That said, artwork can also be protected from high humidity cycles in most typical print viewing environments and thus kept safely away from humidity-induced issues by creating sealed microclimates such that seasonally induced high moisture spikes inside the picture frame are eliminated. Said microclimate methods take more discussion than can be reasonably described here, but I do it all the time, and with prints on display for over two decades, I have had no issues whatsoever with placing photographs in direct contact with glass or acrylic. This is my preferred method with inkjet media in these modern times because I like the borderless, no over mat look for most of my photographs. The visual effect can be very similar to "front facing" techniques like Diasec yet retain some of the surface texture of the fine art media which I often find preferable to the totally slick glossy appearance of Diasec. The prints stay flat, no cockling, no hinge mounting techniques necessary, and the picture framing costs are lower.
cheers,
Mark
http://www.aardenburg-imaging.com
