Is it actually TIN???? or do you just mean a very old fashioned photograph,,,,a daggerotype? (sp?)
I only ask because tintypes (true ones, printed on metal) were quite cheap, and tended to be used by beach photographers and itinerant type photographers,,,,,,and were usually low quality,,,,most true photography studios didn't use them much,,,,
If it's a real tintype, it needs to be stored in glass, but it should be fine to scan,,,
I *highly* recommend scanning all of your tintypes. Then, email a full copy of all of the tintypes to your family members and ask them to back up their copy. That way even if your entire house is destroyed in a hurricane or flood, you have copies of all of those images. As long as you have a digital version saved, you have something to pass down to future generations. If you don't have a scanner, there are many services that can do this for you for relatively little money.
Scanning a tintype once or twice won't hurt it. Scanning it hundreds of times will.
Just be very careful putting it on the scanner...you could scratch both the tintype AND the scanner glass. If you set it down gently and then don't move it while it's touching the glass you should be fine.
I have a "real" tintype stored under glass. I was going to scan it and am glad to see that if I do this only once or twice it will be okay. Honestly, I hadn't even thought of any damage the scan might do. Yikes! TFS.