Photographic slides contain three layers of organic dye — cyan, magenta, and yellow — suspended in a gelatin emulsion on a transparent film base. These dyes were never permanent. Over decades, they fade at different rates, causing colour shifts that worsen with every passing year.
Kodachrome is the most stable slide film ever produced, but even Kodachrome fades. The cyan dye deteriorates first, causing a gradual shift toward warm magenta tones. Ektachrome and Agfachrome fade faster, with cyan and yellow dyes both vulnerable, often producing a distinctive blue-purple cast. Fuji slide films tend to shift toward green as the magenta layer fades first.
Beyond colour fading, slides face a biological threat: fungal colonisation. In humid storage conditions, microscopic fungi feed on the gelatin emulsion, producing branching filaments that etch permanently into the image surface. Once established, fungal damage cannot be fully reversed — but it can be arrested, and the slide can be scanned before further damage occurs.
Mounted slides in carousels or magazines are additionally vulnerable to "Newton's rings" — rainbow-coloured interference patterns caused by the slide pressing against its mount. Glass-mounted slides can develop condensation between the glass and film, accelerating fungal growth in a sealed, humid microenvironment.