Abbi Senior: What is cold storage?

Vanessa Torres: When museums or cultural institutions consider storing their collections, there are four temperature categories that they can use: room, cool, cold and frozen.

The room category ranges around 20C, cool around 12C, cold around 4C and frozen is 0C and below. While the temperature ranges are specific within these categories, the relative humidity would ideally be between 30% and 50%. We have procedures in place to make sure there are no sudden changes between these values—for example, when objects are moved from one space to another.

Cold storage is an area, like a storeroom, that is kept at around 4C, with a stable relative humidity. Cold storage is typically used to store cellulose-based materials, such as photographic film.

Abbi: And why do we use cold storage in the museum?

Vanessa: As a conservator working with photographic collections, I know that the right environment is the single most important factor to dramatically increase the lifespan of these objects.

In our use of cold storage, a big consideration has been finding the balance between object preservation and minimising our carbon emissions. With this in mind, we have chosen to prioritise cellulose nitrate and cellulose acetate film for cold storage.

Small strips of film spread out on a light box
Examples of different cellulose nitrate film stocks,

This decision was based on research showing that the lower the temperature, the more an object’s lifespan will be extended. For example, by moving some of our degraded cellulose acetate film from room to frozen storage—reducing the temperature from 18C to -18C—we were able to increase their lifespan from 10 years to 5,540 years!

Screenshot of a web 'storage calculator for acetate', next to an image of the conservation freezer.
An example of the storage calculator Vanessa uses to plan the storage conditions for acetate film.

Abbi: Can you tell us about some objects we’ve used this method on?

Vanessa: In 2021, the museum purchased three lab-spec freezers to store some of the cellulose-based film from the Oxford Publishing Company rail print collection.

A tall thin freezer with the door open, showing shelves full of black boxes
One of our conservation freezers, full of objects packed to ensure preservation.

The Oxford Publishing Company rail print collection (OPC) had recently been moved to Bradford and covers a wide range of subjects related to railways from the 1890s to 1980s. It is composed of many different photographic and archival materials, from paper and glass to plastic supports.

The archival boxes containing all cellulose nitrate film, alongside the more decayed cellulose acetate film, were prioritised for cold storage in the new freezers.

An important thing to note is that before these objects could be stored, they needed to be repacked. This is because freezers undergo several fluctuations in relative humidity throughout the day, which would diminish the quality of the objects’ preservation. to help with this, we use a packing method involving archival boxes, static shielding bags, thick poly bags, several layers of self-adhesive tape, filler material and humidity cards. The humidity cards help me monitor the environment inside each sealed package, so I can ensure that each microclimate hasn’t been compromised.

Vanessa wrapping a box in silver-coloured plastic on a white workbench
Vanessa packs up an object for cold storage.

It is a time-consuming undertaking, but worth it!

A black box wrapped in clear plastic with a white card showing a colour humidity scale
A box ready to go into storage with its humidity card

Abbi: How many objects do we have in cold storage?

Vanessa: I’d estimate that we currently have around 3,900 objects currently in cold storage.

Abbi: It sounds like you collect a lot of information throughout the cold storage process. How do you use that information?

Vanessa: Obviously, the main priority for us is to use the information we collect from this process to continually learn and develop our approach to cold storage and to ensure that we are giving these objects the best possible preservation environment.

For me, another important aspect is sharing these learnings with other people so we can all learn from each other. For example, I have given talks to internal and external conservators and colleagues to explain my understanding of the overall performance of the freezers and the packing method.

I also think it is important to share these processes with the public so they can have a better understanding of how we are working to preserve their cultural heritage. Blog posts like these contribute to that, too.

Abbi: Tell us about your favourite object you’ve worked on in cold storage.

Vanessa: I assessed and packed four rare cellulose nitrate reels of early motion picture films by William Friese- Greene for cold storage.

A small reel of film which has turned brown with decay
One of the films by William Friese-Greene on nitrate film.

Friese-Greene was an English inventor, photographer and pioneer of motion pictures. He patented an early two-colour process, in which sequential frames of black and white positive film were tinted in green and red.

Abbi: Lastly, out of curiosity, what is the oldest object we have in cold storage?

Vanessa: The Oxford Publishing Company rail print collection spans from 1890s to 1980s. It is likely some of the negatives in frozen storage are as early as 1890.

Otherwise, I can say for sure that the four early motion picture films by William Friese-Greene are presumed to have been made circa 1920.

Read more blogs from the basement and find out what our collections services team get up to behind the scenes.