Safely Displaying a Hand-tinted Photograph

I have a circa 1887 (5 x 7 inch) glass plate from the photography studio of an ancestor. It is positive (not negative) and has been hand-tinted (we think, with oil-based paint). It is currently wrapped in acid-free paper, placed inside an acid-free paper sleeve, kept inside an acid-free box on a shelf in a glass-front cabinet. Safe for sure, but I would like to have a display box built for it so I can safely remove it from the cabinet and show it off from time to time. I was thinking of a small wooden structure with frosted plexiglass on one side and clear on the other (and the removable glass plate in a safe slot in the middle), to allow one to hold it up to the light. Would a wooden box exude gasses that will damage the emulsion or paint? What if a "finish" is put on the wood? Would that make it worse or better? Do you have other advice for me? This is an extremely dear and precious artifact, but as is no one can ever see it because I'm a nervous wreck every time I unwrap it. (And, yes, I've already made some lovely prints from it as I am a digital restorer of works-on-paper but have never had anything else like this.) When complete, I would make a little black felt "cozy" to cover it up and make sure no UV rays get to it between "shows." Thanks, Rita

What a great idea to be able to enjoy your hand-tinted positive!

As you know, you want to be sure that anything you use for your viewing box to be chemically stable to not cause the image layer of your glass plate to deteriorate. Ideally, the frame would be metal rather than wood but I can understand that this can be expensive. If you are using wood, you will want to seal it to prevent the volatile organic compounds (VOCs) from the wood having an impact on the image. The best way to do this is to seal the wood with a product called Marvelseal. It is a true vapor barrier and will prevent the transmission of VOCs. You can seal only the inside if you want to leave the exterior wood for aesthetic reasons. If you use the Marvelseal you would also want to consider using Volara foam as a gasket to provide extra protection against airborne pollutants. 

Not as effective, but easier to use is water- (not oil-) based polyurethane. This will block a good percentage of the VOCs but not all of them. If you use poly, be sure to read the can and let the surface cure completely before putting the frame together and put the glass plate inside. 

Good luck with your project and if you have any further questions, please don't hesitate to ask.