Controlling the environment (climate, pollutants, dust, light)
Damage to photographs can be chemical, physical or biological.
Too high a temperature, too high or too low a humidity, too large variations in these parameters, such as the presence of pollutants, accelerate the degradation process of photographs. Damage will be chemical (hydrolysis, oxidation), physical (deformation, detachment, breaks, tears) or biological (mould). Wherever possible, a dry, cool and stable space should be found.
Issues addressed:
- Why control the relative humidity rate and variation?
- In which cases should low temperatures be preferred for preservation?
- How to store at low temperature?
- Can photographs kept at low temperature be consulted at room temperature?
- How to store cellulose nitrate media?
- Why and how to limit pollutants?
- Why and how to protect items from dust?
- Why and how to protect photographs from light?
Why control the relative humidity rate and variation?
The photographs require conservation conditions that vary according to the media, but all require a rather dry atmosphere without too high variations (see table below). High humidity promotes the hydrolysis of organic materials and the development of mould. Cellulose acetate, for example, hydrolyses by releasing acetic acid (known as vinegar syndrome because of the smell it gives off) and plasticizers, which causes the substrate to retract and fold on the image layer. This process has a catalytic effect because the acid released will initiate the degradation of objects in the vicinity. Confinement is an aggravating factor.
On gelatin-silver photographs, the humidity and pollutants cause the migration of the silver of the image layer which is deposited on the surface in the form of a blue metallic veil called «silver mirror».
Too much moisture (above 65%) and lack of ventilation also promote the development of molds, which feed on gelatin and destroy the image. Insects and rodents feed on paper cellulose, glues and gelatin. Variations in ambient humidity finally make the materials play, which generates embrittlement, detachments and cracks.
In which cases should low temperatures be preferred for preservation?
Some photographic media may be kept at room temperature, but for others, such as color media, nitrates or cellulose acetate, only a low temperature storage can slow down the degradation process and ensure a very long shelf life (more than 100 years). Studies show, for example, that conservation at 20°C and 50% RH makes it possible to envisage conservation over a hundred years of cellulose acetate supports, when conservation at 7°C makes it possible to envisage conservation over 600 years.
But when storing at low temperature, photographs cannot be manipulated or left at room temperature too frequently. This mode of storage is therefore rather reserved for funds that are used little, digitized funds for example (this mode of conservation at very low temperature is contraindicated for amateur Polaroid and magnetic tapes).
How to store at low temperature?
We do not improvise conservation at low temperatures. It is necessary to define beforehand the conditioning and operating protocol to limit the risk of condensation on the object during the exit and excess moisture in the pockets (marvelseal©type pouches, absorbent buffer materials, set-up time).
Can photographs kept at low temperature be consulted at room temperature?
Low temperature storage is a static solution. It must be reserved for objects that do not need to be handled at room temperature for a long time. Studies show that after 30 days of storage at room temperature, the benefit of low temperature storage, which is by definition expensive and energy-intensive, is lost.
How to store cellulose nitrate media?
Cellulose nitrate negatives are unstable objects, difficult to preserve and for which we lack feedback.
Cellulose nitrate, like cellulose acetate, is destined to degrade rapidly at room temperature. The rate of deterioration depends on the production batch and the temperature and humidity conditions to which the supports are subjected. It increases when the latter are high. As this medium degrades, it generates aggressive fumes and strong acids.
The declaration in the prefecture (mandatory above 50 kilograms) does not make it possible to obtain in return recommendations on the conditions of conservation or on the health and safety rules that must apply for the personnel who handle nitrate media. So the precautionary principle prevails:
- Separation, if possible, of films on nitrate and acetate support (mention «flame film» for nitrates and mention «safety film» for acetates);
- Storage at low temperature;
- Individual pouch with alkaline reserve (nitrates and acetates);
- Separation of media according to their state of alteration (excellent, medium very degraded). A precautionary approach should limit the rapid spread of degradation to supports still in good stay1;
- Ventilation/air renewal of the storage room (in this case it becomes more difficult to regulate the climate) or air filtering if the air is recycled;
- Protection of personnel handling nitrate media. The photo restoration workshop of the City of Paris (ARCP) has set up work sessions of up to three hours per day with masks, gowns and gloves, even if the supports are not very altered.
Photographs on cellulose nitrate media can be packaged in individual pockets and in storage boxes.
Why and how to limit pollutants?
The silver that forms the photographic image is sensitive to the pollutants present in the atmosphere, produced by the surrounding materials or by inappropriate packaging. Pollutant attacks are manifested by image weakening (oxidation) or yellowing (sulphation). The replacement of inappropriate packaging helps to limit sources of pollutants and effectively protect against environmental pollution. The replacement of inadequate packaging, such as the installation of a chemical air filtration system, helps to limit pollutant concentrations.
Why and how to protect items from dust?
Dust, whether it comes from the outside or the inside (house dust), consists of mineral, organic and biological contaminants. It may contain inert or potentially active particles (pollen, microorganisms, mould spores, insect eggs, bacteria). The main types of airborne particles or pollutants are clay, cement, coal and carbon dust.
It is important to protect objects from dust because it is abrasive, it promotes certain reactions such as corrosion, it attracts insects and nourishes mould. The best way to protect items is to prefer packaging in pouches and boxes. It is also possible to isolate the premises to limit exchanges with the outside and filter the air using particulate filters.
Why and how to protect photographs from light?
Natural or artificial light sources: daylight/solar, fluorescent tubes, halogen lamps or light-emitting diodes (LEDs), emit a very large amount of radiation, in the field of visible and often invisible (ultra-violet, infrared). Photographs are particularly sensitive to this radiation and degrade each time they are exposed to it. Ultraviolet rays are very energetic and cause photochemical reactions (change of color, browning of paper and mechanical weakening). Infrared heats surfaces and can have an indirect effect (drying out and accelerating chemical degradation). These impairments are cumulative and irreversible. It is therefore necessary to eliminate this undesirable radiation (with dichroic filters) and to monitor the annual dose of illumination received by the photographs.
The illumination of an object is measured in lux. The total illumination dose is expressed in lux.heure. The annual acceptable dose varies depending on the sensitivity of each photograph, which depends on the technique used but also on the treatment variables, which are more difficult to know. This sensitivity can be fine-tuned using a micro-discolouration test. In the absence of this test, we recommend an illumination of around 12,000 lux.hours per year for the most fragile photographs (19th century photographs, colour photographs with chromogenic development, instant development photographs, Dye-Transfer). This corresponds to an illumination of 50 lux for 8 hours per day over a period of 30 days, hence current practices that adopt an exposure of 50 lux for three months, once every three years. We then recommend 42000 lux.hours per year for sensitive photographs (cibachromes, black and white photographs on RC paper) and 84000 lux.hours per year for the least sensitive photographs (black and white photographs on barium paper, monochrome or colour pigment photographs). These thresholds are indications because some images that are poorly fixed, have residual stains during treatment or are exposed to pollutants can still be altered at doses below these thresholds.