Cellulose nitrate (nitrocellulose) was the dominant base material for 35mm motion picture film manufactured prior to 1951. Inherently unstable and highly flammable, nitrate film deteriorates chemically over time, producing corrosive nitrogen oxide gases that accelerate decomposition and drastically lower ignition temperatures.
Five Recognized Stages of Nitrate Decay
Curators must continuously monitor physical film collections for signs of chemical breakdown, categorized standardly into five progressive phases:
- Stage 1: Amber discoloration, faint odor of nitric acid, and minor image fading.
- Stage 2: Emulsion becomes sticky; film reels adhere together in container cans.
- Stage 3: Softening of base, appearance of gas bubbles, and pungent acrid odor.
- Stage 4: Film turns into a viscous, dark brown mass and structural integrity collapses.
- Stage 5: Shock-sensitive brown powder residue remain.
"Nitrate film contains its own oxygen supply when burning, making standard carbon dioxide and water fire suppression systems ineffective once ignition occurs under ambient pressure."
Recommended Vault Environmental Parameters
To arrest chemical decomposition, national archives maintain specialized cold storage units with redundant climate control hardware:
Storage Temperature: Maintained strictly between -2°C to 2°C (28°F to 35°F).
Relative Humidity (RH): Maintained between 20% and 30%. Excessive dryness leads to extreme emulsion brittleness, while high humidity accelerates acid production.
Acidity Gas Extraction & Air Scrubbing
Storage vaults require positive-pressure HVAC units equipped with activated carbon scrubbers. Air must be replaced at a rate of 10 to 12 total volume exchanges per hour to continuously extract off-gassed nitrogen dioxide before it causes catalytic degradation on adjacent canisters.