Photographs and film

Photographic material is where preventive conservation has the largest effect and where neglect is most expensive, because some of it degrades in a way that accelerates itself and damages its neighbours.

Identify the process first

Almost everything else follows from what the item actually is, and the differences between processes are much larger than the differences between, say, two kinds of paper.

The broad groups that matter for storage are: nineteenth-century processes on paper, which vary enormously in stability; gelatine silver prints, which are relatively robust but sensitive to sulphur compounds and to high humidity; colour materials, whose dyes fade over decades even in the dark and for which cold storage is the only substantial intervention; cellulose nitrate film base, which degrades and presents a fire risk; cellulose acetate film base, which suffers vinegar syndrome; and polyester base, which is stable.

Where a collection has not been surveyed by process, that survey is the highest-value work available, because it identifies the nitrate and acetate that need isolating.

Vinegar syndrome and A-D strips

Cellulose acetate degrades by releasing acetic acid. The acid catalyses further degradation, so the process accelerates, and it affects neighbouring items in the same box. The film shrinks, buckles, becomes brittle and separates from its emulsion.

A-D indicator strips are the established monitoring tool. Placed in a sealed enclosure with the film for a period, they change colour according to the acidity present, giving a graded reading. They are far more reliable for this specific job than any general VOC sensor.

Once identified, degrading acetate should be isolated so that it does not accelerate its neighbours, and moved to cold storage, which slows the reaction substantially. There is no way to reverse it.

Cellulose nitrate

Nitrate film degrades to produce nitrogen oxides, becomes increasingly unstable, and is a genuine fire hazard in quantity. Many jurisdictions regulate its storage.

Identified nitrate should be separated from the rest of the collection immediately, and the regulatory position established with whoever is responsible locally rather than assumed. Duplication onto a stable base is the usual long-term answer.

Cold storage

For colour material and for unstable film bases, temperature is the dominant variable. Chemical reaction rates fall substantially with temperature, and cold storage extends usable life by a large factor rather than a marginal one.

Two practical requirements come with it. Humidity must be controlled at the same time, because cold damp storage is worse than cool dry storage. And items must be conditioned before removal, since taking a cold item into a warm room condenses moisture directly onto the emulsion. Sealed packaging that is allowed to reach room temperature before opening is the standard method, exactly as with transport crates.

Enclosures

Photographic material has its own test. The Photographic Activity Test assesses whether an enclosure material causes fading or staining of a photographic image under accelerated conditions, and enclosures intended for photographs should state a pass. The general acid-free specifications applied to paper storage do not answer this question.

Buffering is the other difference. Several photographic processes are damaged by alkaline conditions, so the buffered board that suits paper is wrong for them. Unbuffered, PAT-passed enclosures are the safe general choice for photographic material.

For plastics, polyester, polyethylene and polypropylene only. PVC sleeves are still sold for negatives and are actively harmful. Acid-free boxes and tissue covers the specification questions to put to a supplier.

Light

Many photographic processes are in the most responsive category for light, and some early processes are extraordinarily fugitive. Display of originals should be exceptional, brief and budgeted, with facsimiles used for anything on long-term view. Light exposure budgets covers the mechanism for deciding.