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Practical Humidity
At meaco we have encountered most applications before and have tried to put
some of that experience that we have built up into this website.
Please feel free to browse this section
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Dehumidifiers - a
beginners guide
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Dehumidifiers - low
temperature use
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Why Humidify?
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Humidifier
Maintenance
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Preventative
Conservation by Fergus Read
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Dataloggers
- a beginners guide
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Radio
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Using a Whirler
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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 |
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% |
|
|
| 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. |
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
Compiled by Fergus Read, former Assistant Director, EMMS, now Department
of Exhibits and Firearms, Imperial War Museum, London (E-mail fread@iwm.org.uk)
Publish by meaco, unit 62 smithbrook kilns, cranleigh, surrey,
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