Environmental Guidelines for Paintings – Canadian Conservation Institute (CCI) Notes 10/4
The environment in which paintings are stored or displayed has a significant impact on their condition and long-term preservation. Environmental factors discussed in this Note are lighting, relative humidity (RH), temperature, pollution and biological activity (insect pests and fungal contamination).
The materials used in paintings—binding media, pigments and varnishes—vary in their sensitivities and reactions to visible light and ultraviolet (UV) radiation. While some pigments are “permanent” or lightfast, many pigments and dyes will fade or discolour in the presence of visible light. UV light will also promote chalking of certain paints and initiate yellowing, embrittlement and cracking of binders and varnishes. To reduce the overall risk to the collection, light levels should be kept as low as possible and the source of UV radiation should be filtered. As well, paintings could be glazed with UV-screening glass or acrylic installed in the frame. Light levels, both visible and UV, should be measured for all paintings on exhibit.
Light intensity, duration of exposure
The effect of light is cumulative and irreversible. To design a rational lighting scheme, an institution must decide how much change is acceptable over what period of time. The following information and guidelines may be useful.
Lower light levels
Light intensity is measured in lux. Damage by light is directly proportional to light intensity (lux) multiplied by exposure time. For example, exposure to 200 lux for 200 hours will result in the same damage as exposure to 100 lux for 400 hours or to 50 lux for 800 hours. Therefore, lowering light levels from 200 lux to 100 lux will reduce light damage by one-half over the same period of time.
Shortened duration of exposure to light
Alternatively, shortening the duration of exposure to light by half will have the same effect as lowering light levels by half.
Some response rates to visible light and UV radiation
At or even under the prescribed low-level light of 50 lux for works of art susceptible to light damage, highly sensitive lake pigments and aniline dyes will begin to fade noticeably within 1.5 to 20 years, and medium-sensitivity colours will begin to fade within two decades. Higher levels of light will accelerate colour change. In the case of highly sensitive pigments exposed to normal office light levels of 500 lux, noticeable fading can occur within a few months to two years. Medium-sensitivity paint will begin to fade when exposed to 500 lux at around two years. Exposure to average daylight levels of 30,000 lux will cause observable fading of highly sensitive paint in as little as a day to two weeks and of medium-sensitivity paints between two weeks and a year.
Medium-sensitivity paints can chalk and varnishes can crack within three years of exposure to UV radiation coming through unfiltered windows. Highly sensitive paints will exhibit UV damage from exposure to unfiltered daylight within several months.
Mitigating the effects of visible light and UV radiation
For more information on the response of artworks to light, refer to Agent of deterioration: light, ultraviolet and infrared.
The most vulnerable areas of paintings to light and UV radiation are thin washes or glazes of highly sensitive pigments. These can be any medium: watercolour and pastel are well known for their reaction to light and UV radiation, but oil and acrylic paints are also susceptible. Although oil or acrylic paint is not generally applied in thin washes or layers, like the average watercolour or pastel, museums must deal with the fact that many artists use or have used materials and techniques that make these paintings vulnerable. Common examples are 19th- and 20th-century portrait and figurative paintings in which the thin red glazes used for skin tones depended on highly sensitive pigments. Many of these pink tones are lost already, but some remain, by good luck, and need care if they are to pass to the next generation. The following suggestions will help paintings of all kinds last longer:
Reducing light levels
- The average person can perceive most colours reasonably well at 50 lux, but details in darker colours may require 150 lux. If you suspect that the painting contains a highly sensitive colour, regardless of the medium, reduce the light level, if possible, even though this may impede clear visibility of darker passages.
- The intensity of ambient light can be reduced by using a lower wattage bulb, by changing the angle of the light and/or by increasing the distance between the light and the painting.
- For complete control of light intensity, fixtures can be equipped with a dimmer switch.
- Tinted films adhered to windows can lower the level of visible light significantly.
- Drapes or blinds on windows will reduce both light and UV radiation exposure.
- Picture lamps attached to painting frames are rarely successful. They usually provide uneven illumination and sometimes overheat the painting, leading to blisters or cracks.
Reducing exposure time
- Exposure times can be reduced simply by raising light levels only when required and then dimming or turning all lights off when not needed.
- Safe retrieval and handling of paintings in storage requires bright “office-level” lighting (300–500 lux). Such lighting also reduces the need to move the artwork somewhere brighter for inspection, which can be risky. As long as such lighting is switched off most of the time (using timers for example), there is little danger of fading.
- Intermittent display and/or automatic or visitor-activated light switches will shorten a painting’s exposure to light.
Reducing UV exposure
- Ideally, paintings should not be exposed to any UV light from daylight or from unfiltered, UV-emitting fluorescent lamps. If it is not possible to completely block the UV radiation, levels should not exceed 75 microwatts/lumen (µW/lm).
- Clear, UV-absorbing films on window panes will reduce the amount of UV damage but will not mitigate the fading caused by visible light.
- A UV-absorbing acrylic glazing installed in front of the artwork in the frame will filter out certain percentages of UV radiation (depending on the acrylic). Glazing of all artworks is recommended, including artworks on paper, canvas and rigid supports. (For more information, consult CCI Note 10/8 Framing a Painting). Glazing will also buffer the work of art from short-term fluctuations of RH, especially if the framing includes a sealed backing board. Glazing will, additionally, protect against mechanical damage and the deposition of dirt.
- Low-UV-emitting tubes in fluorescent light fixtures or UV-absorbing sleeves that filter out the UV radiation from tubes can be used (for more information, consult CCI Note 2/1 Ultraviolet Filters).
Types of electric lighting
There are only two essential conservation recommendations regarding lighting: keep the exposure as low as possible to preserve colours for the future, and keep the colour rendering index (CRI) as high as possible in order to see the colours correctly. Opinions about the best lamp, colour temperatureFootnote 1, fixtures and location will vary depending on the display space and the type of collection. We recommend testing different lamps and fixtures in the display space before committing to a large purchase.
There are a number of different lamps available for overall or focused and directional lighting use, for example:
Traditional incandescent lamps have insignificant UV output and an excellent CRI. The higher the CRI, the better viewers can distinguish differences in colour. Incandescent lights have a CRI of 100, the maximum value for best colour rendering. However, any incandescent lighting system consumes more energy and will be about four times more expensive to operate than a fluorescent system.
Incandescent light may provide overall lighting or directional lighting from a distance. However, it should never be placed near an object or in a display case because these lamps are extremely hot. Incandescent floodlights positioned properly on a ceiling track system and equipped with dimmers are recommended for their versatility (consult CCI Note 2/3 Track Lighting and Agent of deterioration: light, ultraviolet and infrared for more details). The cooler, more diffused illumination from floodlights is preferable to the heat and more intense beam from spotlights.
Traditional incandescent lamps, which are not energy efficient, have been gradually phased out of the market in Canada since 2012. The more energy-efficient quartz halogen lamps (halogens) have replaced these as the only kind of incandescent lamp now available for purchase. The MR16 class of quartz-halogen lamps are often preferable for display lighting. They are small in size, operate at low voltage and have beams that are easy to focus and direct. Most MR16 lamps operate at 3000 K, but some models are available with colour temperatures that better simulate daylight (3500 K, 4100 K, 4700 K and 5000 K). Studies indicate that viewers prefer illumination in the 3500–4100 K range for paintings, given the normal light intensities in museums of 50–150 lux.
UV presents a dilemma with quartz-halogen lamps. The UV filters are expensive because they must be high-temperature resistant. However, the UV emitted by quartz-halogen lamps is only moderate, much less than that from daylight through glass and generally less than that from fluorescent lamps. The question is, are the filters necessary? In short-lived exhibitions, probably not. Control of light exposure down to recommended levels will mean that the rate of UV damage will be very slow. For long-term permanent displays, however, if quartz-halogen lamps and fixtures are selected, it is recommended to invest in glass UV filters that do not wear out. For a more detailed analysis of incandescent lighting, consult Agent of deterioration: light, ultraviolet and infrared and CCI Note 2/1 Ultraviolet Filters.
Fluorescent lighting (traditional tubes) is most commonly used to provide general, overall ambient lighting of rooms. If fluorescent lighting is used in a display area, it should incorporate a dimmer system to allow better control of the light intensity. It is important that the dimmer system, the ballast and the fluorescent tubes are all compatible with one another. Check with the manufacturers to confirm which components will work best together. T8 electronic fluorescent tubes, 1 in. (2.5 cm) in diameter, are recommended for museum lighting. Although the initial cost may be more, their energy consumption makes these tubes more economical than normal fluorescent tubes. A cooler, full-spectrum light with a colour temperature of 5000 K is a good choice for museum and gallery use. By comparison, daylight has a colour temperature rating of 6500 K and warm fluorescent tubes are in the range of 3000 K. The CRI rating of the T8 fluorescent tube at 75 CRI is not as good as that of incandescent lamps at 100 CRI. Some T8 tubes, however, use rare earth phosphors that increase the tube’s CRI rating to between 80 and 90.
Compact fluorescent tubes are now available that can be used at short distances from paintings or in display cases. Like traditional fluorescent tubes, the compact fluorescent lights have a good CRI (85), do not produce heat and do not need to be frequently replaced. Both traditional and compact tubes, however, emit UV radiation in the low-to-medium range (75–150 µW/lm) and will require UV filters.
White LED lamps
White light-emitting diode (LED) lamps are now available for museum use. The cost of the lamps is low, and they are available in the warm to cool colour temperatures ranging from 2700–6000 K. The CRI is fair to good (70 to 80). The UV output is very low (0–75 µW/lm). These lamps are energy efficient and can be used in display cases because they do not emit a lot of heat. They can also be used as directional lighting because they can be aimed.
Fibre-optic lighting has become very popular in museum lighting applications. Light from a specific illumination source travels in waves along a guide, either a cable of glass or acrylic fibres. Early fibre-optic systems used light produced by halogen or metal halide lamps. These have been largely superseded by LED light sources. LED fibre-optic lighting systems can offer high brightness, cool temperatures and very long life (50,000 hours as opposed to several thousand for quartz-halogen) at relatively low cost. Manufacturers have designed LED fibre-optic systems specifically for lighting displays in galleries and museums with correlated colour temperatures (CCTs) close to that of daylight (4000 to 6000 K) and a CRI of approximately 80. The brightness of these lights can also be adjusted through dimming. If replacing or upgrading metal halide or halogen-based fibre-optic lighting with LED fibre-optics, keep in mind that the colour temperature of these two light sources are sufficiently different (3000 to 4000 K for metal halide or halogen and 4000 to 6000 K for LED) and that using a mix of them will affect the appearance of the object under illumination.
The safest RH for paintings on canvas or wood is a stable level between 40 and 60%. National and international consensus has favoured a central value of 50% RH for the sake of uniformity between lending institutions.
Excessively low RH
During winter, interior heating without humidification will result in very low RH levels. The painting’s size, ground and paint layers contract to differing degrees in response to these conditions and become increasingly brittle. This introduces strains into the painting. These strains are exacerbated by actions, such as keying out, erroneously taken to correct slackness in the canvas that occurs at low RH levels. Keying out, which places the canvas under more tension, can result in the development of cracks in the brittle ground and paint layers. Cracking can then lead to lifting paint and paint loss. For paintings on canvas, handling during low RH conditions can also place the painting at greater risk of damage to brittle ground and paint layers.
As humidity levels fall, exposed, unpainted wood on the verso of panel paintings loses moisture more rapidly than the painted recto surface. The wood on the back of the panel contracts from this moisture loss and a warp develops. Dimensional changes to wood from contracting at low RH conditions are much greater than those of the paint and ground layers. When the wood under these layers begins to shrink, paint and ground break away from the wood and are forced up forming ridges or “tents.” In extreme dry conditions, seams in composite wood panels can crack or open up.
Excessively high RH
In summer, the RH in indoor environments is often high. At RH levels above 75%, some canvases will shrink at the same time that the wooden stretcher or strainer is expanding. This increases strain in the paint and ground layers. Often deformations in the canvas are naturally resolved during periods of higher humidity. For instance, corner draws caused by greater contraction at the centre of the canvas during periods of low humidity are pulled flat. If the stretcher has been keyed out to deal with such deformations prior to an increase in humidity levels, the additional contraction of the canvas at the higher RH can cause cracking to paint and ground layers. In the case of paintings on wood panels, the wood expands (across the grain) to a greater extent than either paint or ground layers in response to an increase in ambient RH. A distinctive, fine crack pattern is often noted on these works of art with cracks running perpendicular to the wood grain.
RH levels over 70–75% will encourage mould growth. These high RH levels can occur in areas of an exhibition or storage space that are cooler than the rest of the room (e.g. against exterior walls, near water pipes) despite good conditions in the centre of the rooms. Problems of fungal contamination are addressed below under Biological activity.
Risks associated with fluctuations in RH
Short-term fluctuations (daily) in RH can be very damaging to vulnerable objects that respond quickly to changes in RH. Long-term (seasonal) fluctuations in RH, while not as critical to the immediate structural well-being of the painting as short-term fluctuations, are nevertheless damaging over time. Stable RH conditions are difficult to maintain year-round, especially in regions with extreme winter conditions. Mechanical control systems are expensive to build and to maintain and have a high energy consumption. Historic buildings can also be damaged by system and vapour barrier installation. Increasingly, museums and galleries ask: what risks are we avoiding by controlling RH?
CCI and the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) have summarized the risks to paintings on canvas and wood in relation to RH fluctuations using the following levels where “AA” describes best practice and “D” minimal control. Most museums with painting collections should aim for no less than Level B control.
- Level AA: precision control with no seasonal adjustments, short-term fluctuations within ±5% RH; no risk to most paintings
- Level A: precision control with no seasonal adjustments and short-term fluctuations within ±10% RH, or some seasonal adjustments but short-term fluctuations within ±5% RH; small risk to highly vulnerable paintings, no risk to most paintings
- Level B: precision control with seasonal adjustments of ±10% RH, short-term fluctuations within ±10% RH; moderate risk to highly vulnerable paintings, tiny risk to most paintings
- Level C: RH within a range of 25% to 75% year-round; high risk to highly vulnerable paintings, moderate risk to most paintings
- Level D: RH reliably below 75% at all times; mould growth is avoided, but high risk of sudden or cumulative mechanical damage to most paintings during very low humidity
Note: Paintings that have already been exposed routinely to a particular level of RH fluctuations, such as Level B, C or D, are at low risk of further damage at that same level. This is the notion of a “proofed fluctuation.” For more information on these levels of control, consult Environmental guidelines for museums and Agent of deterioration: incorrect relative humidity.
Acclimatization of objects
Rapid fluctuations can cause serious damage to RH-sensitive objects such as stretched paintings on canvas and thin wood panels. Bringing these objects back to an acceptable environment (between 40% and 60% RH), if this deviates significantly from the conditions under which the object has been stored, displayed or travelled, represents a risk. The objects should be loosely, but completely wrapped and partially sealed in plastic with an absorbent material such as blotting paper or cotton sheeting placed in the packaging. The absorbent material will help to regulate the speed of moisture uptake or loss as the object gradually acclimatizes, over several weeks, to the new environment.
RH control by glazing, framing and backing boards
The effects of RH fluctuations caused by the day-night cycle around individual paintings can be controlled by attaching backing boards to the back of the stretcher or strainer (described in CCI Note 10/10 Backing Boards for Paintings on Canvas). Placing glass or acrylic glazing in front of a painting to which a backing board has been attached will provide even better protection. If a reasonable seal can be made with the glazing and backing board, protection against even longer-term RH fluctuations is possible. For paintings that are particularly sensitive to environmental changes (e.g. those on wood or ivory), maximum protection can be achieved by enclosing them in a specially built or modified frame which incorporates buffering agents that will maintain ideal RH conditions (consult CCI Note 10/8 Framing a Painting). For further information on enclosed frame design, contact CCI.
Local control: portable humidifiers, dehumidifiers and fans
In the absence of the full environmental control provided by a properly designed building and a heating, ventilation and air-conditioning (HVAC) system, it is possible to achieve some protection by correctly using portable humidifiers, dehumidifiers and fans. An interior room (those with no walls connected to the exterior of the building) can be designed as a “humidified” zone during cold winter months with the installation of a humidifier. This room should be closed off; non-humidified spaces surrounding this room provide a buffer zone between this and exterior walls. In the summer months, the humidifier can be replaced with a dehumidifier if humidity levels exceed 60% RH. If RH levels are high, continual movement of air through the use of fans is a simple but effective control measure for the prevention of mould growth. It is important to note that localized humidification and dehumidification require an in-depth knowledge of seasonal levels, trends and variations within a room; meticulous attention to maintaining the equipment; and continuous monitoring of room conditions.
Control by avoidance
Measures, such as keeping doors and windows closed and avoiding display or storage near air vents, radiators or on the building’s perimeter walls will reduce exposure of works of art to sudden extremes of temperature and RH. For more advice on avoiding incorrect RH conditions, consult Agent of deterioration: incorrect relative humidity.
The only way to be certain of RH levels and fluctuations in an area is to measure them. A hygrothermograph or data-logger is recommended for continuously recording RH and temperature levels. These units are commercially available at reasonable cost and can record data on an on-going basis for daily, weekly and monthly outputs. Charts should be regularly checked to identify any problems with humidity and temperature levels. These charts should be preserved for comparison of fluctuations over time. One data-logger unit should be placed in each room where works of art are stored or displayed in order to provide an accurate picture of temperature and humidity throughout the building.
High temperatures (over 30°C) combined with high humidity can soften paint and varnish layers, allowing dirt to stick more easily to these surfaces. Exposure to high temperatures as a result of direct sunlight or high-intensity lamps placed too close to the surface of a painting can cause localized desiccation of paint, ground, size and canvas or, in a worst-case-scenario, blistering of paint and varnish.
Low temperatures cause ground and paint layers to become more brittle and susceptible to damage when the painting is moved or subjected to shock. Acrylic paintings are particularly susceptible to cracking under cold conditions because their glass transition temperature (the temperature at which a polymer changes from rubbery and flexible to glassy and brittle) is in the range of 5–10°C, considerably higher than that of oil paint (-5°C or lower). If paintings are to travel during winter, special precautions are required, such as using adequately insulated crates and heated vehicles. Exposing an unwrapped cold painting immediately to a warm, humidified environment can cause condensation to form on the cold surfaces of the painting.
If paintings are stored, transported and displayed at temperatures within the human comfort zone, they are not at risk from temperature-related problems. The recommended set point (as noted in the ASHRAE and CCI guidelines) is anywhere in the range of 16–25°C. In order to increase the RH in an unhumidified space during the winter, the temperature can be lowered. For example, lowering the thermostat from 22 to 18°C will raise the RH as much as 12%, as well as reduce heating costs.
Accumulated risk to paintings from fluctuations in RH and temperature
All of the above risk estimates due to fluctuations in RH and temperature are based on a very conservative assumption: that the paintings have never been exposed to fluctuations before. In reality, most paintings in permanent collections have a history of previous exposure. The concept of a “proofed fluctuation” or “proofed climate” states that the risk is extremely small for further cracking or delamination of the paint due to fluctuations that are less severe than those already experienced by the artwork. If a painting experiences more severe conditions from those to which it is acclimatized, the object will be further damaged and will now be “proofed” to this new climate. It is, therefore, important that significant fluctuations do not occur during display, storage and travel. As well, undamaged paintings, including contemporary works of art, must be protected from incorrect and fluctuating temperature and humidity.
Atmospheric pollution will damage painted surfaces. The main pollutants of concern are sulfur dioxide, hydrogen sulfide, nitrogen dioxide and ozone, as well as aerosol particulates and dirt. Some of the effects on painted surfaces can include:
- loss of gloss
- increased rates of oxidation
- darkening of lead-based pigments
- accumulation of fine particulate matter
Fortunately, most works of art are displayed indoors. As well, any museum or gallery that has an efficient HVAC system can filter out fine particulates and occasionally some gases. Problems caused by outside atmospheric pollutants can also be greatly reduced by:
- closing windows and doors
- sealing windows and doors with weather stripping
- caulking and sealing any cracks or other openings to the exterior of the building
- glazing paintings that have sensitive surfaces (described further in CCI Note 10/8 Framing a Painting)
Other harmful agents can also originate in indoor environments from common materials used for building, furnishing and finishing rooms and display cases. They can be controlled to some extent by using recommended materials (e.g. stable paints on walls and carpeting that does not emit harmful gases) and by strictly following non-smoking regulations.
Insects can leave very acidic materials (e.g. excrement such as fly specks) on a painting’s surface. Wood borers can virtually destroy a wood support.
All incoming objects should be closely examined for signs of insect activity such as wood dust, tiny flight holes and burrowing channels. If signs of insect activity are observed, isolate any affected objects and consult a conservator for advice.
The most effective safeguards against insect problems are meticulous cleanliness and regular, methodical inspection. Keep doors and windows closed or screened. Avoid using, storing or leaving beverages or foodstuffs in display and collection storage rooms (consult CCI Notes 3/1 Preventing Infestations: Control Strategies and Detection Methods and 3/2 Detecting Infestations: Facility Inspection Procedure and Checklist for more information on safeguarding against pests).
Artworks are susceptible to fungal infestations when the humidity exceeds 70–75% RH. High humidity levels combined with warm temperatures and lack of air circulation virtually guarantee mould growth. Mould is often seen on artworks that have been stored in damp locations that have low air circulation (e.g. basements) or in storage and display areas that are exposed to flooding or leaking pipes. Mould can spread rapidly if the source of humidity is not located and the water stopped. From an aesthetic point of view, mould causes virtually irreversible surface staining. From a structural point of view, the acids released by the mould cause disintegration or breakdown of the affected material. Of greater concern, however, are the toxic volatile organic compounds released by mould (whether the mould is active or not). Any work of art that has been contaminated must be treated with special precautions, including:
- isolation from other artworks
- containment (wrapping and sealing)
- health and safety measures for anyone who plans to handle or examine the object (personal protective equipment [PPE] such as a disposable Tyvek coverall, gloves and face mask)
It cannot be overstated that fungal contamination of an object or of a collection should be avoided at all costs. All necessary measures should be put in place to prevent the conditions that will encourage mould growth. At the very least, storage areas that are below ground level should be avoided, especially if there is any risk of flooding. If in the vicinity of overhead pipes, artworks must be protected from these by draping storage racks with plastic and ensuring that nothing is in direct contact with the ground (consult Strang and Dawson 1991; Guild and MacDonald 2004). Emergency salvage plans must be in place to deal with water problems (consult CCI Note 10/5 Emergency Treatment of Water-damaged Paintings on Canvas).
By following the guidelines outlined in this Note, an institution can improve the preservation of its paintings. Even small efforts at controlling the environment where paintings are stored and displayed will have positive long-term effects.
Note: The following information is provided only to assist the reader. Inclusion of a company name in this list does not in any way imply endorsement by the Canadian Conservation Institute.
- UV-absorbing film:
Madico Window Film
Manufacturer of Madico products
- Filter sleeves (UVA7 Acrylic Tube Guard):
Mackenzie Associates (Canadian distributor for American Louver Company)
Lines ’n Curves Graphic Art & Archival Products
Curry’s Art Store Limited and Art Supplies
- Display lighting:
- Light meters:
- Hygrothermographs and data-loggers:
American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (ASHRAE). 2015 ASHRAE Handbook — Heating, Ventilating, and Air-Conditioning Applications. Atlanta, GA: 2015.
Getty Conservation Institute. Museum Lighting Research. Los Angeles, CA: J. Paul Getty Trust, 2012.
Guild, S. and M. MacDonald. Mould Prevention and Collection Recovery: Guidelines for Heritage Collections. Technical Bulletin 26. Ottawa, ON: Canadian Conservation Institute, 2004.
McKay, H. “A Sealed Frame-Case for a Painting.” Journal of the International Institute for Conservation – Canadian Group 15 (1990).
Michalski, S. “Damage to Museum Objects by Visible Radiation (Light) and Ultraviolet Radiation (UV).” In Lighting in Museums, Galleries and Historic Houses. London, UK: Museums Association, UKIC, and Group of Designers and Interpreters for Museums, 1987, pp. 3–16.
Michalski, S. “Time’s Effects on Paintings.” In Shared Responsibility: Proceedings of a Seminar for Curators and Conservators. Ottawa, ON: National Gallery of Canada, 1990, pp. 39–53.
Springer, S. “UV and Visible Light Filtering Window Films.” WAAC Newsletter 30,2 (May 2008), pp. 16–23.
Strang, T.J.K. and J.E. Dawson. Controlling Museum Fungal Problems. Technical Bulletin 12. Ottawa, ON: Canadian Conservation Institute, 1991.
Tailored Lighting Inc. SoLux: The World’s Most Trusted Source for Illuminating Art. Rochester, NY: Tailored Lighting Inc., 2007.
By Robert Arnold and Helen McKay
Revised by Robert Arnold, Wendy Baker, James Bourdeau, Debra Daly Hartin, Helen McKay, Stefan Michalski and Jean Tétreault in 2016
Originally published 1990
Également publié en version française.
© Government of Canada, Canadian Conservation Institute, 2017
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