Preventive Conservation - The Environment

Explaining to clients why the correct environment for their precious objects is important

Clients are advised how best to protect their objects from agents of deterioration. Understanding the effects of varying levels of humidity, temperature and light, the threat posed by pests and dirt and the ways in which these can damage textiles and other objects is essential for their preservation . Surveying the environment in which heirlooms and valued objects are stored and displayed and making changes where required is often a more cost-effective and holistic solution to preventing deterioration than simply carrying out remedial work on the objects themselves.

Light

Light is particularly damaging to organic materials: it causes textiles and many other organic materials to change colour, fade, become brittle and lose strength. Like some other damaging agents the deterioration caused by light is cumulative: the quantum exposure is measured in terms of illuminance, which is a product of the level of illumination, (measured in Lux), and the duration of exposure. The accepted exposure limit for textiles and other highly sensitive materials, such as parchment, leather, natural history specimens, graphic documents, photographs and albumen prints is 10,000 Lux hours per year. Light records of objects should therefore be logged (where possible) so the accumulated lux exposure can be estimated. Lux hours can be recorded by both high and low tech methods. Light sources emit a range of different colour temperatures and heat outputs: Tungsten, for example, is a 'warm' light with a high heat output; Halogen is a blue or 'cool' light. Heat and UV output from all light sources that may affect an object should be monitored. UV is invisible to the human eye: its high energy is undesirable and it should be blocked by the use of window films; (UV sleeves can be fitted over fluorescent tubes). Specialist UV recording equipment is recommended for periodic monitoring, which will confirm whether the installed film is still active.

Tips on Light:

  • in the home the general rule is to try to establish as low a level of illuminance as possible (less than 200 lux), block direct sunlight, draw curtains when the room is not in use, put UV film on the windows and avoid other high UV light sources
  • put vulnerable textiles in the dark corners of a room
  • rotate objects between locations to reduce light exposure
  • objects can be covered in rotation with custom made black out/dust covers - as the Victorians did!
  • Blinds that are thicker and darker are more effective at reducing light levels - shutters are best and if they are fitted use them! Remember that cream calico blinds cut out direct sunlight but do little to reduce light levels

Relative Humidity (RH)

RH is the ratio of water vapour present in the air to the maximum possible at a given temperature. A cubic meter of air at 20°C can hold up to 17 ml of water vapour, an RH of 50%. At 25°C the same volume of air can hold 23 ml of water an RH of 37%.It has been shown that deterioration of textiles and other vulnerable materials may be correlated to changes in RH. Air inside a room may be relatively dry in the winter because the RH of the cold air outside is often lower and when it is brought inside and warmed, with no water vapour added, its relative humidity decreases even further.

Temperature

Though of lesser importance, it is important not to ignore temperature as a damaging agent, as the following examples show:

1. The rate of deterioration of cellulose will increase two and a half times if the temperature is raised (at a constant RH) from 15° to 20°C.

2. Desiccation and irreversible dimensional changes may follow an increase in temperature unless the RH is kept constant due to the drying out of hygroscopic materials and the consequent loss of equilibrium moisture content (the amount of 'body' water hygroscopic materials contain whilst in a state of equilibrium).

3. Biological activity increases with a rise in temperature

4. A temperature rise of 5°C can speed up the evaporation process from the surface of an object leading to embrittlement.

Tips on Temperature & RH:

  • Temperature and humidity swings, if not carefully controlled, damage objects.
  • The optimum conditions for a general collection are 18 - 21°C and 49 - 61% RH. Some objects will need more specific parameters and objects in store can be kept at a cooler temperature of 10 - 12.7°C.
  • Avoid moving objects from cold to warm environments because surface condensation can occur. The colder object causes the warmer air on its surface to reach its dew point and cause precipitation. Enclosing an object in polythene prior to moving it can help to reduce the problem but it is best to condition objects gradually to a change in temperature.
  • Central heated rooms benefit from humidification because external RH in the winter months is low. Water in jugs and pots can help, but conditions should be monitoring so adjustments can be made if necessary.
  • Composite materials are more vulnerable to damage because different components move at different rates when conditions change and the objects cannot always accommodate the differential movements.

Dirt & Pollutants

Textiles may be damaged or discoloured by dirt, from handling for example, or by airborne deposits. Dirt such as salts, waxes, clay and soot may react unfavourably with the dyes in the textile causing colour changes (greying), may speed up photo-degradation or, in the presence of moisture, may raise or lower the pH to the detriment of the fibres. Some types of dirt, especially crystalline material, can cut or abrade fibres.

Tips

  • Specialist advice is essential before carrying out surface cleaning of textiles. More delicate objects should be cleaned only by a specialist using bespoke tools and equipment.
  • Bonded dirt can be removed by washing but this also requires specialist handling.
  • Ensure hanging objects are lined. A closely woven fabric that is not too heavy such as a cotton cambric is recommended for lining. Linings should be attached by a specialist because of the difficulty of handling objects that fluctuate in size according to atmospheric humidity. The lining and hanging method must not put undue strain on an object as this can cause damage.

Pest and moulds

Textiles may be attacked by certain pest species that find them an attractive source of food; they can also be damaged by moulds given the necessary environmental conditions.

Tips:

  • check textiles regularly for pest activity and moulds: the early signs of pests are fibre loss, frass and webbing debris.
  • Insects are attracted to dirt so keep the general environment clean.
  • A warmth and high RH encourage pest activity and moulds.
  • Be vigilant - pre-empt infestation by putting down a network of sticky traps. These blunder traps act as an early warning system and help to pin point an infestation; specimens caught can be examined for identification.
  • Pay particular attention to wool felt, which is a favourite food source for clothes moths.

See how it's done.. !


Using a hand held Lux and UV meter to spot check natural light levels on a chair.

Spot checks are a useful method of identifying problem areas. For survey work it is best to build up a whole year's worth of data so that Lux hours per annum can be calculated. The advantage of the sensor being on a cord and separate from the monitor is that readings can be taken in awkward places. Lux levels should be low, if possible 50 Lux (or higher if objects are displayed for shorter periods), and UV levels should be below 75 micro watts per lumen; this is easily achievable with modern UV film filters. Artificial lighting should be dimmed, objects moved or other light reducing methods implemented where possible.

Thermohygrometer in use to spot check temperature and humidity.

This battery run instrument gives a fairly instant reading on a liquid display. It needs calibrating on a regular basis (annually) and this can be done with a sling whirling hygrometer (wet-and-dry bulb type). The latter is accurate to ±2%. Insects are moisture sensitive and their presence can often indicate a problem with the environment in the early stages.

A healthy population of wood lice and silver fish is associated with damp conditions and a threat of mould growth.

Using a hand held Lux and UV meter

Using the Thermohygrometer

Damage by webbing clothes moth larvae

Carpet fragment showing damage by webbing clothes moth larvae.

This is the perfect breeding site for the webbing clothes moth - underneath a book case where dirt accumulated and insects were left undisturbed; the carpet had a high wool content and pigeons in the roof void adjacent to the office area may have provided an entry route for the insect pests. Infestation could have been prevented or minimised by good housekeeping a comprehensive survey of the building (to identify entry points and prevent the pigeons from returning) and regular monitoring. A chemical 'knock down' agent was used in this office areas and the carpets were disposed of.