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I've got a bunch of Kodak slides that I'm scanning, I've noticed that for ones from the late 1950s only, there is a white film on the slide on the side with the image (not the glossy side). If I wipe the slide with a microfiber cloth that is used to clean my glasses, it wipes away easily and is gone.

Before wiping:

enter image description here

After wiping:

enter image description here

What is this, is there any harm in removing it?

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    \$\begingroup\$ Just for the record, the image side is the emulsion side and the glossy side is the film base. \$\endgroup\$
    – Stan
    Commented Jul 1, 2019 at 0:10

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The emulsion side of Kodachrome was coated with a clear protective lacquer. Perhaps it has oxidized. Additionally, this coat may attract dust and such as it can gain an electrostatic charge. Lacquer is used to protect the Kodachrome emulsion. Lacquer is made from “guncotton”. Ordinary cotton is treated with nitric acid and solvents to make lacquer. As time goes by, its composition changes; it outgasses solvents and plasticizers. In other words it disintegrates. Lacquer is a cousin of the cellulose nitrate film base that was replaced by “safety film” in the late 1950’s. Cellulose nitrate film burns with explosive vigor. Cellulose acetate (safety film) will burn if lit by a flame, but will self-extinguish.

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    \$\begingroup\$ Kind of looks like water marks from condensation/evaporation, possibly moving dust and other stuff around. \$\endgroup\$
    – xiota
    Commented Jul 1, 2019 at 1:36

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