C. 1920 Lithograph/Guide on Preserving Antique Prints.

Another Reinthal & Newman lithograph c. 1920 that I picked up with the other one. This one shows the facsimile signature of the original work, albeit dark from air-brushing their production design before reproduction.

The key to preserving antique art like this lies not only in preserving it from outside factors (mainly, bright light and humidity), but also by preserving it from within.

Some pieces just cannot be saved, whether by poor storage, handling, or that the materials used were of low quality (paper/pigments). This one is fortunate that everything, from the pulp backing to the paper it was printed on, was of decent quality and shows no worrisome acid-damage. It did not experience any moisture (worst case is mold in which it needs to be dried immediately) damage either and has not been long in a sunny room.

When I get framed antique prints, often I'll put a piece of white acid-free paper between it and the cardboard or wood backing behind it. The backings often transfer acid to the printed paper, which not only makes it fall apart with time (and vibrations travelling through the walls), but also causes darkening of the images. The white paper absorbs best as it's not full of dyes and will diffuse it throughout itself.

Rice paper is not recommended, as it is often slightly acidic. Sometimes I remove that from old books, as it begins yellowing the paper it touches (which means it's destroying it). Rice paper was originally added only to prevent ink transfer from engraved plates into the neighbouring page.

I tend to rip backing paper off the cardboard backs, as it risks staining the wall as it deteriorates. It usually is the lowest quality of paper meant to hide the nails.

As for the cardboard backings (and, if wood, paper is usually recommended between it and the image), if it's dirty brown or deep tan, (not the grey-tan of normal, safe cardboard), or shows variegated lines of colour, or is expanding/warped, it's time to replace it.

Remove nails with pliers. A strong hand can push them back into the same holes. Cut new cardboard using old as a template. If you clean the glass before putting it back on, avoid cleaners designed to remove stains and make sure you wipe the chemical off with 100% clean, dry cloth or paper towel and let sit a few minutes anyway (use sparingly the cleaner). Grab with cloth or paper towel so as not to add smudges. Latex is to be avoided as it powders and adds its latex dust.
Do not breath on the art as saliva contains acids that stain paper. If it's pastel, don't touch it at all. You'll permanently smudge it.

As for plaster frames, I may write on that at a later time.