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The Home of Humidity Monitoring and Control
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PREVENTATIVE CONSERVATION
by Fergus Read
One of a museum's primary purpose is to ensure that its collections are available for future generations. A popular image is of white-coated conservators working in laboratories to stabilise and repair items that have suffered from damage or neglect. This is remedial conservation. It is, however, more efficient to prevent or reduce decay from occurring in the first place. This is preventive conservation.
Preventive conservation therefore is a process that seeks to prevent, reduce or mitigate the effect of all the factors that, every day, threaten an object's continued survival. It requires an all-embracing approach - a constant assessment of how collections are stored, handled, displayed and maintained. It should involve all people who work for a museum, not simply conservators. Good building repair and maintenance routines are as necessary as the more obvious and immediate issues of object handling, storage and security. Preventive conservation is a strategic enterprise driven by a museum's primary purpose, that cannot be introduced without planning, or its benefits demonstrated in other than the long term.
Deterioration is a continuous, natural process. It can, however, be slowed; indeed, science has suggested ways in which the natural lifespan of most museum objects can be extended. Many of these techniques are based on common-sense and good housekeeping. However, these must inevitably be reinforced by the results of current research, and access to specialist information is vital if an informed approach is to be taken. There is benefit in having formal systems in place, incorporating agreed standards (such as those set by the Museum & Galleries Commission's Standards in the Museum Care of ... Collections). Appropriate and continuing training, to ensure consistency in approach over time and between individuals, is also vital.
The table overleaf summarises the major threats to museum collections, how they are caused, and the action that can be taken to remove or reduce their impact.
WHERE TO BEGIN?
An intensive museum-wide study of the current situation - a preventive conservation audit -will enable a museum to:
- assess the degree of danger posed by the threats to that individual museum
- establish the priority areas for action
- demonstrate the benefits, in quantifiable terms, of any changes it introduces
The preventive conservation audit should work methodically, ideally gathering data and making consideration in this order:
- the building
- the collection
- the in-house procedures
Preventive Conservation - a summary of the threats (and their possible remedies)
| THREAT | SPECIFIC | DAMAGE | FREQUENT CAUSE | PREVENTIVE ACTION |
| People | Staff Visitors Intruders |
Breakage Abrasion Crushing Theft |
Unnecessary handling - open displays - badly organised stores - inadequate labelling Incorrect handling Inappropriate cleaning Insufficient supervision Inadequate physical or electronic protection |
Prevent visitors touching objects Re-organise the store Label stored objects clearly Handle only as required, using approved procedures Upgrade physical and electronic security Train & motivate staff See `In-house Procedures' for more detail |
| Relative Humidity |
Incorrect humidity level (high or low) Rapid fluctuation in RH |
High humidity causes - mould growth - corrosion Low humidity causes embrittlement Fluctuation causes - splitting - cockling - warping1 |
Weather changes Floods & leaks Damp visitor clothing Wet-cleaning of floors Building re-decoration Rising damp Poor building insulation (temperature and/or humidity transmission) Poor ventilation Inappropriate heating source2 or temperature control regulation |
Regularly measure & record RH Move collections to best environment Improve air circulation Introduce visitor cloakrooms Improve insulation Attempt to impose RH control (by humidifiers/dehumidifiers, air-conditioning or use of heating/temperature control3) See Appendix 1 for more detail |
| Temperature | Incorrect temperature (high or low) Rapid fluctuation in temperature |
Heat causes - increase in degradation4 - embrittle- ment Fluctuation causes - splitting - cockling - warping1 |
Weather changes Poor building insulation Poor regulation of heating Radiant heat from display lighting |
Regularly measure & record temperature Improve insulation Mount lights externally to display cases Control temperature (by air-conditioning or use of heating/humidity control) See Appendix 1 for more detail |
| Light | Exposure to excessive light especially - high intensity light - short wavelength light (ultraviolet) |
Fading Discolouration Embrittle- ment1 Destruction |
Natural & artificial light Too many windows Objects wrongly positioned Inappropriate artificial light sources Lack of filters or blinds |
Measure light intensity & ultraviolet (UV)
levels Filter to reduce UV light Reduce light intensity in display areas Reduce display times If appropriate, set annual lux hours exposure limit Block out all light from stores See Appendix 2 for more detail |
| Pests | Birds Rodents - Rats, Mice Insects - Moth, Beetles Mould Fungus |
Objects eaten Objects soiled5 |
Gaps in building shell Damp (high relative humidity) Unchecked new acquisitions/loans Poor cleaning Attractants & food sources used in displays (e.g. plants & display props) |
Regularly monitor with traps Inspect all new acquisitions Label & regularly inspect vulnerable items Isolate infested items immediately Keep store cool Maintain building shell Avoid damp or humid areas Regular & thorough cleaning of stores & display areas See Appendix 3 for more detail |
| Pollutants | Wind-borne gas & particle pollutants,
especially - oxidant & sulphiding gases - dirt & dust |
Degradation of materials | Close proximity to road Lack of air-filtration Poor door/window fitting Poor housekeeping Poor control of building/decorating works Inappropriate cleaning methods & agents |
Identify the type & sources of gaseous
& particulate pollution Determine the degree of risk Reduce ingress and/or circulation - create air/dust breaks - use mechanical air- filtration - repair the building shell - never use a duster Reduce impact on objects - box or wrap objects in store - use housekeeping regimes that remove dust & dirt See Appendix 4 for more detail |
| Storage & Display Materials |
Gaseous, chemical and particulate migration from materials used in construction of display cases, mounts & frames, storage racks, boxes & packaging | Corrosion Discolouration Physical Damage (e.g. staining) |
Use of inappropriate materials, with high
acidic & migratory characteristics especially - boards & papers - composite woods - many paints - many glues - some plastics - felt & other woollen material |
Ideally, use only those materials approved
& tested for a particular situation Certainly avoid use of known problem materials If possible test unknown materials before use; otherwise seal, cover or vent to mitigate possible effects See Appendix 4 for more detail |
| Disasters | Flood Fire |
Flood - staining - ink & dye run - mould growth - warping - swelling - disintegra- tion Fire - incineration - scorching - chemical deposit |
Flood - burst pipe - leaking roof - fire-fighting water Fire - electrical fault - arson - accident - lightning |
Formulate a comprehensive Disaster Plan, to
include checklists for all housekeeping routines and situations, including - contractors (special care to be taken during building works) - maintenance checks (internal & external) Enforce no smoking Train, motivate & test staff See `In-house Procedures' for more detail |
| 1 Especially organic based
materials (eg paper, leather, textiles) 2 Especially bottled gas (LPG) heaters 3 Heating a building above the external temperature will normally lower the humidity; lowering the temperature will have the opposite effect 4 Every 10 c increase, degradation rate doubles 5 Especially paper, textiles, wood & natural history specimens |
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THE BUILDING
The building is the first line of defence, and the audit should start here. A survey can establish how well the building cushions the indoor environment from that outdoors, and isolates the conditions within. If the building has poor features, yet is capable of satisfactory modification, its physical improvement should come before consideration of internal control equipment that will be expensive to install, and energy-inefficient to operate.
The environment within the building should be surveyed using 24-hour recording devices during a full calendar year. An external sensor, to provide contrasting readings, is essential to judge the efficiency of the building fabric. Such a survey may quickly identify the existence of differing environmental conditions within the building. As an immediate response, the collection may be moved, and the most sensitive items placed in the most stable and easily controlled zones - for example, in rooms near the centre of the building that are usually furthest from the effects of the weather. Any additional control measures needed can then be economically and effectively targeted on specific zones, rather than applied indiscriminately throughout the building. It may be that no area within the building can easily be brought to an adequate standard. In such cases, consideration should be given to relocating the museum.
Surveying a museum building: good & bad features
| FEATURE | GOOD | BAD |
| Site | Well-drained Sheltered (but not shaded) |
Low-lying Poorly drained Exposed Shaded |
| Building Materials | Damp-proof course Well-insulated Draught-proof Watertight Good ventilation |
Rising damp Poor insulation Draughty |
| Building Design (external) | Pitched roof Controllable ventilation Few windows Small glazed areas External rain water disposal Draught lobbies on external doors |
Flat roof Large glazed areas Large number of windows Many open doors & windows Internal pipe-runs & gutters |
| Building Design (internal) | Easy physical access Fire divisions Planned environmental zoning Separation of collection (storage & display) and non-collection areas Separation of public access & other areas |
Poor access Lack of internal physical & environmental barriers Mixed use of spaces, causing/requiring compromises |
| Building Environment | Stable and moderate RH & temperature,
suggesting - good buffering effect by shell of building from prevailing weather conditions, and/or - environmental services control systems that work |
Fluctuating or extreme RH & temperature,
suggesting - poor buffering effect of building fabric, and/or - environmental services control systems that do not work in desired way |
| Building Maintenance | Sound condition Preventive as well as repair maintenance Hot work procedures Disaster plan in place Regular checks (& system for fault logging/rapid repair) of roofs, gutters etc |
Poor repair No disaster plan No regular checks/reporting procedure No procedures governing contractor work |
THE COLLECTION
The suitability of the building for museum use depends largely on the nature of the collections that are kept within it. Ideally, surveys - either of the collections in their entirety, or of samples - should be undertaken to uncover and quantify the current condition of the collection, and suggest priorities for remedial treatment. At the very least, a visual inspection should be undertaken, and regularly repeated. While visual inspection is not ideal as the only form of check - severe damage may have already occurred before it is noticed - it is a start. Over time, it must include inspection of all items, not just those on display. To facilitate this, it is best that most boxed items in store are not wrapped, merely padded to prevent movement.
Different types of collections need different forms of care. However, it usually impractical
to create ideal conditions for each type. Compromise prevails in most situations, with only the most `difficult' material subject to specific provision. Some items, made of composite materials with widely differing requirements (e.g. wood & iron) pose particular problems, that may never be wholly resolved.
All museum workers need to be aware of the special requirements of museum collections. Light, temperature and humidity levels, pests, pollutants, and the use of inappropriate materials or inadequate handling, all pose threats.
The Basic Types of Museum Material
| Organic | Animal - Leather, ivory,
horn, bone, vellum, feathers, silk, wool, insect/animal specimens, some paint pigments Vegetable - Paper, parchment, cotton, wood, cork, canvas, botanical specimens, some paint pigments |
|
| Inorganic | Man-made - plastic, metal
(iron), metal (non-ferrous), enamel, ceramic, tile, glass Natural - stone, geology |
|
| Composite (organic) | Examples: Oil painting on canvas; painted carved-wood ethnographic mask | |
| Composite (inorganic) | Examples: Metal & enamel badge; Bakelite radio, with metal components | |
| Composite (mixed organic/inorganic) | Examples: Fork with bone handle; Iron tool with wooden handle; Leather sword scabbard with metal decoration | |
In-House Procedures
Comprehensive and effective in-house procedures are relatively easy to check. They should be written, followed, regularly updated, and feature in the induction of new staff.
| Staff training ensure all staff understand the principles and practice of preventive conservation, ideally as part of an induction process |
| Buildings cleaning, inspection &
reporting regimes ensure that comprehensive maintenance checklists are prepared and adhered to |
| Object movement and handling
practices avoid object handling wherever possible (e.g. within storage boxes padding rather than wrapping improves access and reduces need to handle) when unavoidable, use trained staff, the correct equipment, and fore-thought never move an object until a route has been cleared, & a space has been made for it to go to use cotton gloves for most objects permit no smoking, eating or drinking in the vicinity of collections |
| Object recording & storage
practices wherever possible, prevent stored items touching others box or cover items wherever possible, to protect from dust and light raise stored items/storage containers off the floor in storage areas, in case of flooding keep storage areas clean, tidy and uncluttered use acid-free boxes where possible, and always acid-free materials in immediate contact with item, whether wrapping or padding use `object removed cards', when objects have been temporarily removed from normal location in stores or display ensure that loans out are adequately recorded, monitored & safeguarded record conservation condition and treatments in the museum documentation system (preferably in an object history file, filed under accession number) |
| Contingency Planning ensure that damage or breakdown that threatens the safety of objects (e.g. to roof, or environmental control equipment) is repaired immediately; undertake temporary relocation of objects, or other preventive action, when rapid repair is not effected prepare a comprehensive emergency plan (e.g. The Museum & Record Office Emergency Manual - EmmS 1991) |
Emergency plans can be achieved for most museums by preparation of an emergency manual This should contain contact numbers for the many individuals and companies whose assistance may be required, either immediately or as salvage progresses. A manual might also contain master-copies of the museum's maintenance and procedural checklists, and also floor-plans, labelled to indicate features that may be vital in case of fire-fighting or salvage.
Responses to a Conservation Audit
If the audit is not to be wasted effort, adequate resources, both financial and human, should be provided to follow it through. Deficiencies highlighted by a conservation audit should be tackled in a logical and methodical manner. Likely responses (not all expensive) include:
| Environmental zoning, moving
collections to the spaces in the museum with the best inherent environmental
characteristics, thereby minimising the need for additional measures Building works, to improve performance of the museum `skin' (or, if more cost-effective, relocation) Installation of environmental control equipment and/or creation of micro-environments in museum display cases and within stores Introduction or extension of appropriate housekeeping routines and procedures, including a disaster plan A programme of remedial conservation, confident that priorities have been established and that an appropriate environment is ready to receive the treated items. |
The following self-assessment form will help in undertaking a speedy preventive conservation survey, and help grade your current museum stores.
Museum Stores Assessment Guide
| COLLECTIONS WELL MANAGED | ACCEPTABLE STANDARD | COLLECTIONS AT RISK | |||
A |
B |
C |
D |
E |
|
| Environment | Monitoring Air-conditioned Stable |
Monitoring Controllable heating Humidifiers/de- humidifiers Stable |
Monitoring Relatively stable within acceptable limits |
No monitoring No visible deterioration of objects |
Fluctuation problems egg leaks, damp |
| Light | No daylight Monitoring Control of individual lighting (egg dimmers, selective lights) Filters |
Some daylight Monitoring Control by other means (egg blinds, curtains) Filters |
Daylight No monitoring Control by other means (egg blinds, curtains) Filters |
Daylight No monitoring No control No filters |
Substantial daylight No monitoring No control No filters Sensitive items at risk |
| Cleanliness | Regular cleaning Dust prevention Programmed checking of items |
Regular cleaning Dust prevention (egg dust sheet, filter) |
Regular cleaning Anti-dust measures (egg floor covering) |
Dusty Poor dust resistance Inadequate floor covering |
Filthy Neglected |
| General Storage | Racks Boxes labelled by subject & item Handlists Access good Handling area |
Racks Boxes labelled by subject & item Small overspill to floor Access good |
Racks Boxes labelled by subject Large overspill to floor Reasonable access |
Partially racked Boxes Access poor Damage to objects may occur |
No racks No boxes Access very difficult Objects at risk |
| Individual Storage (assumes a collection for which boxed storage is appropriate) |
Acid-free boxes Acid-free packing Plenty of room Items individually wrapped/packed, compartment- alised & labelled |
Acid-free or suitable boxes & packing Plenty of room Items individually wrapped/ padded/packed to prevent damage |
Acid-free or suitable boxes Adequate wrapping/ padding/ packing |
Acidic boxes No packing, or inadequate/ inappropriate packing Movement will lead to damage |
Acidic boxes No packing Cramming or over- crowding, or free movement, risks/has led to damage |
From the `Collection Care Guide' in Collecting for the 21st Century (Yorkshire & Humberside Museums Council 1992)
APPENDIX 1 - Temperature & Humidity
Temperature and humidity are key agents of deterioration. Relative humidity (RH) is a ratio of water vapour in the air to the amount it could hold if fully saturated, and is expressed as a percentage. Low levels of relative humidity mean dry conditions since the air is then capable of taking up moisture. High values are recorded when the air is already humid or wet and unable to take up much additional moisture. Temperature is measured with a thermometer; RH with a hygrometer; a thermohygrometer measures both.
Extreme, or rapidly fluctuating, relative humidity poses a major threat, especially to organic materials. Their hygroscopic nature (they can rapidly absorb and release water) means that they quickly expand and contract, generating stresses which cause damage such as cracking. The goal for most types of museum collection is avoiding rapid RH change (no greater than +/- 3% in one hour, +/- 5% in 24 hours) while staying within the range of 40-65% RH.
If relative humidity is controlled, temperature control is generally less crucial. Ironically, however, since the human body is far more sensitive to heat and cold than to humidity, it is temperature control that is more often seen as the priority in public buildings. Public areas are usually kept between 17-19oC (63-66oF). For collections care, a range of 15-25oC is acceptable for most collections, while stores can be kept at lower temperatures (there is no lower limit, provided humidity is still controlled) to both save energy and reduce decay rates.
Evidence of deterioration due to temperature and/or relative humidity problems
| MATERIAL | INDICATOR | POSSIBLE CAUSE |
| Metals | Fresh corrosion products Tarnish on polished surface |
RH too high (polished metals, e.g. brasses & bronzes, do not tarnish at 15%RH or less) |
| Glass (unstable) | `Weeping' - wet surface `Crizzling' - fine cracks Glass becoming opaque |
RH incorrect or fluctuating too much |
| Glass (stable) Ceramics Tile Unfired Clay Stone |
Flaking glazes Powdering fabric Efflorescence - salts coming out |
RH too high and/or fluctuating too much |
| Cracking/Shattering (outdoor location) | Temperature too low (effect of freeze-thaw action) | |
| Fossils Minerals |
Pyrite Decay (Sulphide oxidation) Salt Efflorescence |
RH too high (accelerates deterioration) |
| Cracking - sub-fossil bone & shale matrix around specimens | RH too low | |
| Natural History | Mould & Fungus Distortion of specimens |
RH too high |
| `Spring' in insects | RH fluctuating too much | |
| Wood Textiles Bone Ivory Leather Paper |
Mould & Fungus | RH too high |
| Cracks, warping, flaking | RH fluctuating too much | |
| Embrittlement Shrinkage Drying out & breakdown of adhesives |
RH too low | |
| Plastic | Warping | RH incorrect or fluctuating too much |
| Electrostatic/excess dust | RH too low |
Types of Museum Object requiring exceptional RH conditions
| MATERIAL | OPTIMUM RH (50% unless stated) |
ACCEPTABLE RH BAND (40%-65% unless stated) |
SENSITIVITY ( indicates need for tighter control of RH than +/-3% per hour, +/-5% per 24 hours) | NOTES |
| Excavated Metals (non-ferrous) | 35% (less if possible) | 15-55% | ||
| Excavated Metals (ferrous) | 15% (less if possible) | 0-40% | ||
| Coins & Medals | 15% | 15-40% | Depends on corrosion products, oxides and patina formation, and their degree of stability | |
| Ceramics, Tiles, Stone | 20% | 20-60% | Depends on the activity of embedded salts, and if corrosion products are present | |
| Geology (general) | 45-55% | |||
| Geology - Pyrites & Marcasite (& fossils containing these minerals) | 30% | 30-50% | Should never exceed 50%RH | |
| Geology - Sub-fossil bone, tusks & teeth; fossils with shale or clay matrix | Should never be less than 40%RH | |||
| Paper | 45% | 40-55% | Some authorities recommend less | |
| Photographs - B & W prints - B & W negatives - glass negatives - colour prints - colour slides / colour negatives |
40% 35% 30% 40% 25% |
30-50% 30-40% 20-50% 30-50% 25-30% |
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| Costumes, Textiles | 30-50% | Silk & wool are more sensitive to moisture damage than cotton or linen. | ||
| Glass (crizzled) | Grizzled glass needs a narrow band of controlled RH to prevent advance of this condition. | |||
| Furniture (inlaid) | Inlay work needs particular stability; exact sensitivity varies with wood type, adhesive used, & the condition of surface or barrier coating | |||
| Lacquer ware | 50-60% | Japanese authorities recommend higher levels (to 70%) | ||
| Paper (stretched) | 45-55% | Paper screens, drawings on stretched frames etc. need narrow band | ||
| Ivory, bone (carved) | 50-60% | Carved items require more control than anatomical collections (although less than sub-fossil material). Dimensional responses very slow, except when in thin sheets egg miniatures on ivory | ||
| Leather, skins, binding | 45-60% | Variable according to the tanning process used | ||
| Paintings (on canvas) |
40-55% | (see notes) |
Unlined paintings, or paintings lined with hygroscopic adhesives, are more reactive than those lined with wax or synthetic materials. NB Some sources suggest that temperature variations (even short-term and slight) pose a greater risk than RH fluctuations, due to varied thermal expansion of the paint layers | |
| Paintings (on wood) |
45-60% | Depends on type, grain & thickness of wood, the ground and the method of jointing sections. Some need narrow RH levels to minimise warping | ||
| Wood (painted & varnished) | 45-60% | Includes musical instruments, models | ||
| Plastic | 40% | 30-50% | In general, plastic materials have slight humidity responses, but do warp when in thin sheets & exposed to varying conditions. Low RH causes electrostatic properties, encouraging dust accumulation | |
| Parchment, Vellum | 50-60% | Narrow control required because of great hygroscopicity | ||
| Metalwork (historic) |
35% | 15-55% | Depends on condition of metal, and oxide formation. Other components (egg wooden handles) may restrict ability to go lower than 50% RH |
APPENDIX 2 - Light
Light can create serious irreversible damage to museum collections. Light is a form of energy and can cause fading, and deterioration, in the materials from which an object is made. Most objects are affected by light, although metals and ceramics not to the same extent as other materials. Light is measured in lux (or lumen per square metre. 10 lux equals the amount of light produced by 1 candle at a distance of one foot (therefore 10 lux = 1 foot candle).
Light produces damage in proportion to its intensity and the exposure time of the object. A light of 500 lux will theoretically cause the same amount of damage in one year as a light one-tenth the strength (50 lux) will produce over ten years; or, if two 100 watt lamps are put in place of one, then the same amount of damage will occur in half the time. Thus short exposure to a high lux (eg 2500 lux caused by photographic or laboratory lights, or 200 lux for short exhibitions) need not cause undue damage over the total life of an object, provided this high exposure is compensated by a proportionate period of time in a lower than normal illuminance, or in total darkness.
Hence, while it is usual to reduce light-levels in museum displays, this need not be the only response - reducing the time during which an item is exposed to light is equally legitimate. Fitting display lights with a timer, or, if continuous lighting while on display is required, limiting the overall time an object is on exhibition in a given year, is appropriate. For the most
light-sensitive objects, it may be useful to establish annual lux-hours exposure limits. Lux-hours are a measure of exposure (illuminance x time).
Calculating current exposure in annual lux-hours
| Lux of display light source(s) = 50 lux Number of hours on display in a year = 1,920 (8 hour day x 5 day week x 48 week year) Annual lux hours exposure = 96,000 lux hours per year This example assumes that no light falls on the object when its 8 hour display day is over (probably only accurate when the lux of the light source is constant, and a strict regime of covering a display case out of hours is followed; in reality: if any natural light is involved, its illuminance will vary with the time of day, weather and season; the precise illuminance of many artificial light sources declines over time, as the filament ages (and dust settles!); most museum exhibits are illuminated well before (and often after) public admission times, as lights are turned on, blinds raised and curtains drawn back, to facilitate gallery cleaning, out-of-hours events etc. |
Recommended light exposures for museum collections on display
| MATERIAL | RECOMMENDED MAXIMUM VISIBLE LIGHT LEVEL (lumen per m2, or lux) | RECOMMENDED MAXIMUM ANNUAL
LUX HOURS EXPOSURE (illuminance x time)B |
| Costume, textiles Watercolours, prints, drawings Paper items (including wallpapers, manuscripts) Photographic prints (colour) Transparencies |
50 luxA | 96,000 |
| Natural history (most) Ethnography (most) |
50 luxA | 96,000 |
| Minerals (light-sensitive examples, including argentine, celestite, chalcocite, fluorite, lepidolite, pyrostilpnite) | 50 - 200 lux | 96,000 - 384,000 |
| Furniture (inlaid or
with grain/surface feature) Plastic (especially Bakelite, Ebonite & polythene) |
100 - 200 lux | 192,000 - 384,000 |
| Paintings (oil &
tempera) Undyed leather, wood, horn, bone, ivory Lacquer ware Furniture Photographic Prints (black & white) |
200 lux | 384,000 |
| Metals Stone Glass Ceramics Geology (except some minerals, as above) |
300 lux (material would not be unduly harmed by higher, but a maximum level at this reduces the eye adaption difficulties for visitors where other collections [as above] are displayed in darker illumination; for similar reasons a maximum illuminance of 400 lux in the remaining public spaces in a museum might be recommended) | 576,000+ |
| A 50 lux is most often cited in
the literature as the acceptable light level for this category, although Japanese
authorities have chosen 100 lux as the lower threshold. Museum designers and curators
generally prefer this higher value for reasons of colour rendition and visitor perception.
At all low levels of illumination the use of an artificial source such as tungsten light
is preferable, as it has a warmer feel than daylight of the same intensity. B There is no published source for these recommendations; they are calculated assuming an exposure to the recommended maximum lux for 8 hours a day, five days a week, 48 weeks a year; in many situations museums will be doing well if they restrict exposure to no more than double these amounts. |
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The effect of any measures to reduce visible light levels is to darken a room, a change all the more apparent to the human eye if made too rapidly. Measures such as the gradual reduction of lighting levels in preceding spaces, giving visitors' eyes time to adjust, can usually compensate. Neutral-density window film reduces the amount of light entering, but still allows the visitor to look out, while from the outside the window appears darkened. Such films can offer an effective alternative to blocking or blinding a window.
Most light sources, natural and artificial, also emit some ultraviolet (UV) radiation. This is light beyond the limit of human vision, at the violet end of the spectrum, and is also damaging to most museum objects. Since UV is not needed to view objects, and it can readily be reduced through the use of filters, elimination should be the target. A meter is used to measure the proportion of UV in the light source in microwatts per lumen (µW/lumen) - a maximum acceptable reading is 75µW/lumen, although filters should be able to reduce this to less than 10µW/lumen. A periodic check is needed to test the continuing efficiency of UV filters (especially of window coatings and film), as this declines over time.
Reducing damage by light - a summary
| Reducing the amount of time an object
is illuminated: reduce display exposure time (screens or covers to display cases; timer switches to room/case lights reduce overall exposure time (curtain room when closed; rotate exhibits; set an annual lux-hours limit; store in dark) |
| Reducing the intensity (lux) of the
illumination: move displays further from light source (ie. window) reduce number of light sources (eg block windows, less lights) reduce intensity of light source (eg partially blind windows, use neutral density film, reduce wattage of bulbs and/or use dimmer switches) |
| Reduce the proportion of ultra-violet
(UV) light striking an object: place UV filtering adhesive film or varnish to external glazing replace window, case or picture glass with UV filtering product - laminated glass (with UV absorbing interlayer), or special acrylic or polycarbonate sheets (eg Perspex VE or VA; Plexiglas 201 or 209; Lexan 9034. NB Ordinary plexiglas or perspex does not filter UV) fit UV filtering diffusers, sleeves or filters to electric lighting sources ensure light illuminating an object is first reflected off a UV absorbing material (eg white-painted wall) |
The radiant heat from lights, especially in enclosed cases, often poses as much of a problem as the quantity and quality of the light that they issue. Low-voltage bulbs often emit great heat. Fluorescent tubes are generally the coolest, and are much used inside enclosed cases; even so, they need diffusers or sleeves to counter relatively-high UV emissions.
APPENDIX 3 - Pests
All museum spaces should be regularly set with traps to detect the presence of pests, hopefully before their numbers are a problem. Simple `window' traps placed along skirting boards (the usual insect run) can be very effective, and are cheap and long-lasting. They are most likely to detect pests in May - July, the months when most adult forms migrate to breed. By carefully noting which traps have caught what insects, the source of the problem can be identified. It will frequently be a hole in the building fabric, permitting ingress; alternatively, an item already infested with eggs or larvae may have been introduced to the store or display.
Measures to prevent pest infestation, and to mitigate their effects:
| Set monitoring traps widely; inspect regularly Ensure gaps in building fabric are closed (close pest entry routes) Keep temperatures low (most practical in stores) Inspect collections regularly, especially the most `at risk' items (which might be labelled as such) Isolate or cover new acquisitions, to contain any emerging adults, if in doubt about their state Remove external accretions of pigeon droppings (carpet beetle larvae love this); try to deny roosts on ledges Have good housekeeping regimes, that remove dust (curved skirting boards reduce it collecting) and, in particular, clean light fittings (warm places, where insect corpses can gather, especially in fluorescent light diffusers) |
APPENDIX 4 - Pollutants
Pollutants in the museum atmosphere require identification and measurement, and, if they pose a threat to the collection, exclusion. External sources for both gas and particle pollution include vehicle and factory emissions, and burning fossil fuels. Within the museum, pollutants may derive from building works, items already in the collection (eg cellulose nitrate film, or collections in formaldehyde), cleaning agents, display cases or storage materials.
Of particular concern are materials used to construct museum displays. In the confined atmosphere of a display case, levels of harmful pollutants can cause chemical changes, even without physical contact. For example, hydrogen sulphide, exuded by wool, leather, parchment and certain adhesives, will tarnish silver, polished copper and photographic prints and negatives; sulphur dioxide will harm paper, leather and some types of stone, while organic acids (eg acetic acid), contained naturally in wood, stimulate corrosion, particularly of lead.
Materials for display and storage
| MATERIAL | PROBLEMS | GENERAL RULES | BEST | WORST |
| Wood & Wood Panel Products |
Acids are released, as vapour or liquid, from wood; acidic vapours arise from formaldehyde- based resins used as the bonding adhesive There is little control over the wood species used in wood products |
No wood or wood product is perfect Consider substitutes (eg acrylic sheet, glass) If used in a display case, cover the side facing inside with an impermeable material1 to improve the seal around the wood or wood product Wherever possible seal wood and wood products (especially cut edges) with suitable paint, varnish or lacquer |
Aged/Seasoned Dry Certain species (eg beech; birch, mahogany) Wood products for marine or exterior use Overlaid plywood (eg high or medium density overlaid plywood) Plastic-laminated panel (eg Formica, Melamine) Blockboard |
Green Unseasoned Knots Certain species (eg red cedar, oak, Douglas fir, sweet chestnut) Chipboard Hardboard Plywood (interior grade) |
| Plastics & Rubber, Foams & Foam-boards |
Some plastics release harmful degradation
products and additives Rubber is unstable & perishes under most conditions over time |
Choose acid-free (or acid- reduced) products Avoid Polyurethane or rubber-based materials |
Polyethylene (PE) Polypropylene (PP) Polyester Polyethylene (PE) foam (eg plastazote) |
Poly vinyl chloride (PVC) Rubber with sulphur vulcanising agents Polystyrene Polyurethane foam board PVC foam board |
| Paints, Varnishes & Stains | Release organic acid vapours, peroxides etc at
different rates Products have variable efficiency as a barrier to volatiles No coating is a complete barrier |
Wait at least one month before putting object
in a closed system, to allow for evaporation of solvents Cover coating on side facing inside a case with an impermeable material1 to improve the seal around the coating |
Water-based paints | Corrosion-resistant paints Oil-based paints Oil-modified polyurethane varnish |
| Adhesives | Release corrosive or otherwise damaging gases
during curing or on ageing Solvents & adhesives may be transferred to objects Adhesives may embrittle, ooze, yellow, become acidic |
Use alternatives if possible (eg screws for
joints, brass staples for attaching lining fabrics) Wait at least one month before putting object in a closed system, to allow for solvent evaporation Never use adhesives to secure objects or their labels (substitute with perspex mounts, brass pins etc) |
Acrylic contact cements Certain acrylics & 2-part epoxies Certain poly vinyl acetate (PVA) emulsions & ethylene/vinyl acetate copolymers (EVA) Transparent acrylic adhesive on polyester tape Pure acrylic tape without carrier |
Polysulphides Most poly vinyl acetate (PVA) emulsions & solids Natural & synthetic rubber cements (most contact cements) `Blu-tack' (& similar) `Sellotape' (& similar) `Plasticine' (& similar) Masking tape |
| Textiles | May have finishing treatments that compromise their inherent good qualities | Avoid wool products (includes standard felts) Check dyes are colourfast and sulphur-free Wash fabrics before using Use undyed, unbleached fabrics for storage (eg dust covers) |
Unbleached cellulose-based materials - cotton - linen - hessian Polyester |
Wool (tarnishes silver) Fire retardant treatments Carpet material (accumulates dust; especially avoid rubber-based backings) |
| 1 Impermeable barriers can be provided by polyester sheet or metallised plastic barrier film (eg `moistop') | ||||
| The opinions expressed herein should not be relied on in substitute for specific professional advice - © 1994 East Midlands Museums Service, Courtyard Buildings, Wollaton Park, Nottingham NG8 2AE Telephone: Nottingham (0115) 985 4534. Compiled by Fergus Read, former Assistant Director EMMS, now Deputy Director, North West Museums Service (Tel: 01254 670211; Fax 01254 681995; e-mail:- nwms @ nwms.demon.co.uk) |
The opinions expressed herein should not be relied on in substitute for specific professional advice - © 1994 East Midlands Museums Service, Courtyard Buildings, Wollaton Park, Nottingham NG8 2AE Telephone: Nottingham (0115) 985 4534.
Compiled by Fergus Read, former Assistant Director, EMMS, now Deputy Director, North West Museums Service (Tel: 01254 670211; Fax 01254 681995)
Publish by meaco, unit 62 smithbrook kilns, cranleigh, surrey, england gu6 8jj
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