Why cold
Some materials cannot be saved at room temperature. Color photographic prints fade through chemical processes whose rates roughly halve for every 5 °C drop in temperature; cellulose acetate film base hydrolyzes (the “vinegar syndrome”) on a similar Arrhenius curve; cellulose nitrate decomposes faster still. The math is unforgiving and the math has been settled since the 1990s; the question is not whether a collection of color slides should be stored cold, but how cold, and how soon.
For most institutional clients, the math arrives as a budget question. We are typically called in not to argue for cold storage but to design the cold storage that the conservator already won the argument for. Petra is the principal who runs these projects; she trained as an IPI eClimateNotebook trainer and has built and run the calculations on more cold-storage projects than the rest of us combined.
Setpoints we use
| Collection | T setpoint | RH setpoint | Notes |
|---|---|---|---|
| B&W silver-gelatin photographs | 13 °C | 35% | Cool, not cold |
| Color chromogenic prints | 5 °C | 30% | Standard cold |
| Color slides & transparencies | 5 °C | 30% | Same envelope |
| Motion-picture film, color | −4 °C | 30% | Cold, sub-zero |
| Cellulose nitrate film | 2 °C | 30% | Plus dedicated vault, fire-rated |
| Glass-plate negatives | 5 °C | 30% | Vibration sensitivity drives ramp rate |
| Magnetic media (open reel, cassette) | 13 °C | 30% | RH stability matters more than T |
| Wet specimens (alcohol) | 13 °C | 50% | Vapor-tight envelope; alarmed |
The setpoints in the table are starting points, not final answers. Petra runs every collection through the IPI Dew Point Calculator against the institution’s realistic equilibrium and against the equipment’s realistic capacity, and selects a final setpoint that is the most preservation-favorable point inside the envelope of achievable. We do not design a 5 °C/30% vault and then deliver a 9 °C/40% room because the chiller did not pencil. We tell you, at Gate M2, what we can deliver.
The dew-point envelope
The single most important constraint on cold storage design is the dew point of the room outside the vault. The vault wall must never reach the dew point of the surrounding air on either face, or the wall sweats; the vault door must never open into a high-dew-point ambient without a vestibule, or the warm humid air that follows the staff in fogs the boxes. Designing a 5 °C vault inside a Pittsburgh basement (which sits at 18 °C / 65% RH for half the year, dew point ~11 °C) requires either lowering the vestibule to 13 °C or fitting a vapor-tight vestibule with mechanical assist.
The IPI Dew Point Calculator and the eClimateNotebook subscription — both produced by the Image Permanence Institute at RIT — are the tools we use throughout. We have a copy of the calculator open on a second monitor at the design desk for every cold-storage project. We will not design a setpoint without it.
Mechanical approach
A typical 800–1,800 ft² cold-storage vault uses one of three mechanical approaches:
- Glycol-loop split system — outdoor condenser, indoor evaporator coil, glycol secondary loop, low-temperature ECM blower. Suitable for 5 °C / 30% RH setpoints in vaults of 800–4,000 ft². Less suitable below 0 °C.
- Two-stage cascade with desiccant assist — for −4 °C and below. The desiccant pre-conditions the air, the cascade chills it, the vault holds. We use Munters MX desiccant wheels paired with custom cascade racks built by the practice.
- Walk-in box with conservation-grade controls — for the smallest vaults, 80–800 ft². A modified commercial walk-in (Bally, Kolpak) with our controls and our sensor mesh substituted for the as-shipped thermostat. Cheap, fast, and surprisingly effective; we have nine of these in the field.
The vestibule is mandatory. The vapor barrier is mandatory. The condensation drain pan with a heat-traced drain line is mandatory. We have walked too many cold-storage retrofits where one of these three was missing.
- Vaults
- 11 active
- Coldest
- −4 °C (film vault)
- Smallest
- 112 ft² (Squirrel Hill)
- Largest
- 3,400 ft² (university)
- Equil
- 4–18 mo. typical
Equilibration
Bringing a collection from room temperature to 5 °C, on the boxes you are about to put on the shelves, is the work of months. We ramp at 0.5 to 1.0 °C per week, holding RH fixed within band at each step, watching the dew-point trace, and treating any out-of-band excursion as a stop-work event. Glass-plate negatives have a particular vulnerability to cooling-rate-induced fracture; for the north-side glass-plate case, the equilibration was 312 days and the temperature ramp was 0.4 °C per week.
We do not begin equilibration until the vault has held setpoint without a collection inside for at least four weeks. We do not consider the vault commissioned until it has held setpoint with the collection inside for a full annual cycle. The latter is what Gate M5 measures.
What it costs
A typical 1,200 ft² cold-storage vault at 5 °C / 30% RH, retrofit into an existing basement with reasonable structure, prices between $185,000 and $340,000 total construction, with engineering at 11–14% on top. A sub-zero film vault prices roughly double. Walk-in-box conversions for 200–400 ft² rooms can land as low as $42,000.
Standing-watch retainer (Gate M6) on a cold vault runs $14,000–$28,000 per year per vault, with a 30-minute alert response window any time of day or night.
Begin a cold-storage project
If your collection has a measurable percentage of color photography or motion-picture film and you are storing it at A-class room conditions, the conservation literature is clear about what should happen next. Write us. Begin a project. The first call is with Petra.