As you probably know, many of yesteryear's photographs were not taken care of as they should have been. They became dirty, torn, or worse. You might be able to correct a minor problem, but if the damage is too severe, professional help is required.
A dirty photo can be cleaned, if the proper materials are used, and if proper care is taken to protect the surface. To clean a photo, begin by gently brushing off any loose dirt with a camel hair brush. Use a circular motion, beginning in the middle of the photo and working outward. Clean the brush frequently by flicking the bristles against your hand or the edge of the table. For any remaining dirt, use a cotton swab dipped in a non-lubricant film cleaner, available from a photo dealer. Wear cotton gloves, and work in a location with good ventilation. Take extra precaution if you are pregnant or have breathing problems.
Old and torn photos can be mended on the back with Filmoplast P tape, or an acid-free linen tape. These tapes are of archival quality and will hold the picture together for years to come. Do all repair work on the back of a photograph. The emulsion surface should never be mended with tape, or anything else for that matter.
If you have old photos that have been mounted and framed and you want to remount them or remove them from their mounts, proceed with extreme caution. Early photographs were mounted for permanence. If you attempt to separate the print from its mount, you'll probably shred both.
The best way of handling this situation is to gently strip away any overlay material, and leave the print attached to its mounting board. If you wish to trim the board for any reason, make sure that the work is done on a firm, flat surface. In order to precisely cut a straight line, use a metal ruler or other absolutely solid straightedge. It should have a cork or rubberized backing in order to protect the picture and prevent slippage. Place the straightedge on the print so that if the cutting blade slips, it will cut to the outside, not toward the photo itself.
For cutting, use an X-acto knife, or a very stiff single-edge razor blade with a very firm spine. Whatever type of blade you use, make sure that it has never been used for cutting anything else because the blade must be, well, razor-sharp. In fact, before you have finished cutting all four edges of the mounting board, you will probably have changed blades at least once. If the blade shows the least sign of dullness, change it. A dull blade will tear the material rather than cut it. Chances are, you will have to score the material several times before you get through the material.
These are the problems that can be easily solved. You won't be able to repair prints that have been damaged by moisture, heat, chemicals in the air, or have faded from sunlight. These problems can be solved, but only by a professional. There are ways, though, of protecting photographs, old and new, from potential hazards.
Avoid exposing valuable photos to sunlight and/or artificial light. Ultraviolet and other rays will damage photographic emulsions. If you wish to display such a photo, have a duplicate made for display and store the original for archival permanence. Duplicates should not be made with a copy machine because the bright light can damage the print. A digital camera would work nicely for this purpose if the prints wouldn't be exposed to bright light. Using a copy stand for stability, use very subdued window light, and very gently press the shutter release.
Don't store unprotected photos in wooden boxes, wooden drawers or on wooden closet shelves. Even 100-year-old wood continues to emit fumes which are detrimental to photographic emulsions.
Never store photographs in an attic or basement, or any place where they could possibly be exposed to heat, moisture, chemical fumes or dust.
Old photographs are mementos of the past, just as the pictures you take today will become. Begin now to preserve your photos for future generations. They'll thank you for it.