I tend to get a bit geeky on art preservation, so I don’t really know how deep you want me to go with this @rooeytoo. The main criteria is to follow archival procedures from the very beginning of the artwork creation and follow them through to final display method.
Was the painting an oil or dye? Acid free masonite or ph neutral cotton? There are many variables, and the substrate is just as important as the colorant. Oil must be allowed to dry before framing behind glass. This could take months depending upon the humidity conditions. But one rarely sees oil paintings framed behind glass because it is already a very stable colorant, hundreds of years. Even longer if a UV varnish is applied after drying. I’m assuming your painting was dye based. The problem with dye is that it out-gases for a few days. The surface layer literally becomes gaseous and will adhere to the glass in front of it, creating a latent mirror image transparency right on the glass. It doesn’t matter if the glass is touching the painting or not. I’ve seen it happen with glass that was an inch away. Is this what happened to you?
Though dye based artwork is typically more colorful and vibrant than oil based (pigment), it requires a much greater degree of care when preparing for archival longevity.
Use acid-free substrate.
Get some cheap ink-jet paper and lay it over the artwork, allowing the outgas to be pulled away rapidly. You’ll see the image actually transfer to the inkjet paper after a few hours, and this step should be repeated with new inkjet paper as many times as it takes until the out gassing stops.
Coat the artwork with a UV spray or varnish. I prefer Timeless Veneer for quick jobs, but use Glamour II for heavy production because I can mix my own ratios, and also mix the gloss with matte to produce my own signature luster surface. These are water based UV coatings, so there is little smell and dry times are very fast. Apply with roller or HVLP spray gun. Premier Art also makes a great product available in spray cans. Call them to ensure best product line for your intended purpose, as all of their veneers are not water based.
@BarnacleBill makes a great point with taping the glass sides together sealing the interior from moisture. The glue turning brown could have been from a number of reasons. It may not have been ph neutral acid free. Neither may the artwork have been. In this case, it was the sealed glass acting as the only preservant, and after 35 years, I’d say it did a fabulous job. Could have been longer if acid free materials were used.
All glass is UV protective. But art glass has extra up to 99%. Plexiglass must be ordered special for UV protection.
BTW @YARNLADY, glue is not necessary to mount the artwork directly to the glass for smaller pieces. Your handkerchief may offer enough thickness to provide a compression fit between the two sheets, especially if sealed as @BarnacleBill suggests. But if you do need to glue, either to glass, or to another acid free substrate, the Timeless and Glamour II make excellent archival glues, as well as UV protective veneers.
I’ve done a considerable amount of art handling for the Pulitzer Foundation and have made many archival canvases for the St. Louis Contemporary Art Museum. Check out my cover shot on September 2010 issue of American Art Collector Magazine for artist Jeff Aeling.