#AskAConservator: cold storage for photos
CAPC is participating in #AskAConservator Day! Andrée-Anne of l'Université du Québec en Abitibi-Témiscamingue asks:
For the conservation of our post-1970 photos (black and white negatives, black and white photos, color negatives and color photos), we were informed that it is best to keep our photos at 0 to 2 degrees Celsius. As we are a small archive center, we want to buy a refrigerator. I can't find any information on what to do with the containers if we have to put the photos in Ziploc's a bit like the freezing method. Can you give us some information?
Photograph conservator Chloé Lucas says:
Low temperature storage or freezing is recommended for monochrome (silver gelatin) and color (chromogenic) photographs on cellulose nitrate, cellulose acetate and polyester supports, as well as for chromogenic color prints on baryta or resin coated (RC) paper.
The temperature - relative humidity (RH) pairs for each type of photograph are indicated in the table below. The environment must be as stable as possible with daily variations of ± 2°C and ± 5% RH.
Table - Recommended temperature and relative humidity for the conservation of monochrome and color photographs, dating from after 1970. Sources: ISO 18911:2010 and ISO 18920:2011
Monochrome photographs | Silver gelatin | On cellulose acetate | 2°C and 50 % RH 5°C and 40 % RH 7°C and 30 % RH |
On polyester | 21°C 50 % RH | ||
On baryta or RC paper | 16°C and 50 % RH | ||
Colour photographs | Chromogenic | On cellulose acetate or polyester | -10°C and 50 % RH -3°C and 40 % RH 2°C and 30 % RH |
On baryta or RC paper | -3°C and 50 % RH 2°C and 40 % RH | ||
Dye-destruction (Cibachrome®️, Ilfochrome®️) | 20°C and 30-50 % RH | ||
Instant dye diffusion (Polaroid®️-type) | 15-21°C and 30-50 % RH | ||
For low-temperature storage and freezing, it is necessary to condition the photographs using the Critical Moisture Indicator method (explained in this answer) to protect the documents from relative humidity variations:
- during frost/defrost cycles inside the refrigerator/freezer,
- when taking the boxes out of the refrigerator/freezer, to avoid condensation that would damage the photographs. It is recommended to allow the boxes to acclimatize for 24 hours before opening them.