My sister sent me some old negatives and slides to scan. These slides and negatives date back as far as the 1940's, and some of them are in pretty bad shape, contaminated with dust and mold.
The obvious question was whether or not I should wash the more valuable negatives and slides in order to improve the picture, or simply rely on the scanner software to do what it can to reduce the effects of age.
I selected one slide that probably dated back to the early 1950's, but with a quite common image less likely to be missed if I destroyed it. I removed it from its cardboard frame and washed it with warm water and some dilute dish soap, rubbing it gently on both sides with my fingers for a few seconds. Then I rinsed the slide thoroughly in warm, running water, and followed that up with a drenching spray of distilled water on both sides, then hung it up to dry.
I did not destroy the image, but I did have to use tape to fasten the slide to the remnants of the old cardboard frame in order to get the PC's scanner driver to recognize the image for some reason. (The scanner's driver worked fine, and is the better choice anyway, I've learned).
Here's an animated GIF of before and after the wash (click on it to see its full width, and sorry for the jumpiness between images). I think it was worth washing the slide, although I don't know how well the scanner's driver would have done cleaning up the unwashed slide in software. I'll have to make that comparison with another "less-precious" slide.
It seems there are a few spots that appeared after the wash, but all in all I think it worked.
I've since read that one should not use the method I used because, apparently, water is bad for some film emulsions (no harm done here, apparently). The recommended methods include a light swipe with a soft cloth and alcohol, or special cleaning fluid one can buy. I picked up a bottle of 91% isopropyl alcohol at Walgreen's and will try that next. (The liquor section had a stock of 190 proof Everclear, which is 95% alcohol, but it cost much, much more than the non-drinking stuff.)
I already mentioned that the two scans in the animated GIF above were made using the PC's scanner driver (because I had not realized that you have to fill a checkbox in the software for it to use the scanner's drivers). I'll try to make the next test, wash-don't-wash-with-alcohol, a little better and more complete.
My conclusion, so far, is that washing a precious slide that's been contaminated with dust and damaged by mildew can be worthwhile, but it's too much trouble to do routinely. I should probably mention that the scanner I'm using is a Canon CanoScan 8800F. Here's a review if you're interested. I'm not competent to recommend once scanner over another, but I like this Canon. It's done a fine job for me.
I should also mention that the image above was taken at my Grandparents farm, Finca San Rafael, on the outskirts of Guatemala City, where I did some of my growing up. It was a beautiful place, and I was extremely fortunate to live there for some of my formative years, but now it's all bulldozed and subdivided.