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Photography Question 
Dave Hartman

Cleaning slides

I have some old slides I'm going to get prints made from. I noticed there's something on the emulsion side that looks kind of like crystalized something-or-other. Anyone have any ideas of what it might be? These slides are more than 50 years old.

What should/can I use to clean them? I have some old Kodak film cleaner. Would that be a good thing? I'm a little nervous to clean then but may have to try it on a less important slide to see what happens.

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5/21/2008 4:42:24 AM

Alan N. Marcus   Hi Dave,
The color images that make up the side are tiny oily dye droplets dispersed in gelatin. Under the microscope, gelatin resembles a pile of transparent spaghetti. The dye droplets are entwined in the spaghetti noodles All films, color negative, and black & white, utilize gelatin as the binder.

Film makers have tried but failed to avoid gelatin. Gelatin is flexible, transparent, and permeable (water & chemicals can percolate in and through the gelatin structure) thus allowing chemical contact with the light sensitive goodies that comprise the film. Add gelatin’s low solubility, this substance becomes the ideal binder (glue) to hold the light sensitive silver crystals and other compounds to the film base. Additionally, gelatin contains needed impurities that enter into the light sensitive reaction greatly increasing ISO. Silver salts without these impurities are unresponsive to light.

Gelatin is a byproduct of the food industry. Cattle skin, bones, and connective tissue is rendered to a colorless jelly. This is the same gelatin you eat as a dessert. Because it is a protein it is a favored food of microbes and algae. During the slide developing process, the final chemical bath contains a weak solution of formaldehyde, an embalming fluid. The formaldehyde serves two functions..

1. The formaldehyde forms a peptide bond that tacks together the gelatin strands at every points of contact. This attachment locks the dye in place preventing its movement otherwise it will pool forming oversized and commingled dye puddles.
2. The formaldehyde serves to make the gelatin distasteful as a food for microbes and the like.

As time goes by, the formaldehyde which is a gaseous compound, outgases thus the ability to inhibit growth of microorganism is lost. Likely the damage is done and irreversible. You can buy film cleaners at a camera store and try to clean off some of the stuff.

Kodachrome slides were additionally coated with clear lacquer that can craze over time You can remove with ordinary lacquer thinner from the hardware store. Coat a well washed “T” shirt with thinner and wipe from center to the edges. In a pinch try using denatured alcohol. Do not use water.

Try to clean one or two with least valuable images as a test. Consider making or having a digital scan made. Consult a well established local photofinisher.

Best of luck,
Alan Marcus (marginal technical gobbledygook)

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5/21/2008 7:51:45 AM

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