Bob Rosinsky from Top Dog Imaging walks through the extraordinary process of restoring a badly damaged 1870s tintype (such as the Billy the Kid image that was the subject of a recent record-setting auction) using modern, high-tech techniques, such as a polarized strobe and ultra-high-rez camera. He hints at even more advanced techniques employing X-rays, UV and infra-red light.
My standard operating procedure is to use an ultra-high resolution camera combined with a top-of-the-line macro lens to photograph tintypes. I use strobe lights to illuminate the artwork. Strobes produce "hard" light, much like the sun on a clear day. In addition to the strobes, I place a polarizer over the camera lens and polarizer gels over the strobe lights. This eliminates all reflections and enables the camera to pick up a greater tonal range along with more detail...Restoring a Photograph from the 1870s (via Kottke)
One advantage of using a scanner to digitize a tintype is that it will smooth out surface imperfections and micro details thus reducing the amount of time it takes the retouch artist to produce a clean, albeit low fidelity, image -- somewhat analogous to hearing a Beethoven symphony on AM radio.