If the air at a particular temperature contains half the water vapor it can hold at that temperature, the relative humidity is 50%.
During transient phases of sorption (absorption or desorption) in response to a change in RH, the water-in the form of bound water and vapor in the cell cavities-diffuses from the region of high moisture concentration to one of lower moisture concentration.
So, with air conditioning, the RH will rise and with a heated environment, the RH will fall.
Key outdoor pollutants include ozone and oxides of nitrogen from atmospheric reactions or combustion of fossil fuels. These acts as oxidizing agents or cause acid hydrolysis. Other pollutants can come from within the frame for example, from the wood or the sealants emitting volatile organic compounds including aldehydes and organic acids.
A computer controlled HVAC (Heating, Ventilating, Air-Conditioning) system maintains 70° temperature/50% humidity environmental condition year round in the galleries and storage areas.
Microclimate Boxes for Panel PaintingsAnna Drewes An article by Jorgen Wadum from the book The Structural Conservation of Panel Paintings
What is a Microclimate? Controls the moisture content of humidity-sensitive objects such as wood, fabric and paper. Some objects benefit from a different climatological environment than the museum. Without a microclimate, objects may suffer from swelling/shrinkage, cracks, splits and cleavage of the support or between the support and image layers. Internal and external pollutants Key outdoor pollutants include ozone and oxides of nitrogen from atmospheric reactions or combustion of fossil fuels. These acts as oxidizing agents or cause acid hydrolysis. Other pollutants can come from within the frame for example, from the wood or the sealants emitting volatile organic compounds including aldehydes and organic acids.
Relative Humidity RH= Absolute Humidity Saturation level The degree of humidity greatly influences almost all the technological characteristics of wood Dimensional variations Mechanical strength Elasticity Propensity to decay from fungi attack And strongly conditions the conservation of wood objects. MC=moisture content based upon the relative humidity and temperature of surrounding air.
Wood naturally seeks equilibrium between the internal moisture content and the surrounding atmosphere.
When surrounded by moist air, there will be continuous evaporation of water molecules from the wood into the surrounding air, and simultaneously, continuous ‘capture’ of water vapor molecules by the cell wall.
Seasoned wood is not immune to shrinking and swelling.
Type of wood, however, does effect the amount of moisture the wood absorbs. Hardwood has a higher moisture change than that of softwood.
Also the thickness of the wood also makes a difference when responding to moisture.
Extractives are saps and resins that are expelled from the tree.
Dimensional changes in wood caused by variations in moisture content are anisotropic.
This means that the wood changes in the transverse, radial and tangential directions.
Shrinking and swelling always accompany the variation of the moisture content. External resistance depends on air temperature, humidity, speed and type of flow as well as wood surface characteristics such as roughness, MC and density.
The rate of air exchange between the case and the environment affects the buffering ability.
Buffering material should be avoided in small microclimate boxes.
Salt Solutions Salt-hydrate pairs When air enters through the box, it passes over the salt-hydrate pairs. One salt may absorb moisture while the other will release moisture to balance the humidity within. Magnesium Nitrate Hexahydrate Drawbacks: The surface of the solution has to be covered by a membrane(usually made of silicone rubber), impenetrable for the solution, in order to prevent contact between the solution and the exhibit and the inevitable creep of the salts outside of the container Transport of water in the solution is very slow, which results in dilution of the surface solution and, consequently, an increase in the RH over it; hence the solution has to be stirred to avoid the dilution.
Silica gel found in medicine bottles and shoe boxes are not the same as those used to preserve artwork. That product only has a one way transfer of moisture. They are designed to remove the moisture from the air.
Silica gel made for artwork has a two-way transfer. It takes care of adsorbing moisture (taking in moisture when it is too humid) and desorbing it (putting moisture back into air when it is too dry).
The amount of silica gel placed in the microclimate box should be double the weight of the enclosed material.
The response of the silica gel to temperature is negligible.
Art Sorb values are up to 6 times that of ordinary silica, equating to moisture buffering capacities. Art Sorb consists of an aqueous solution of lithium chloride embedded in silica gel pores. Art Sorb® Bead Type: Recommended for all other applications including custom designed displays and containers. Most economical and more RH levels available. Cassette Type: Recommended for display or shipping crate applications. Sheet Type: Ideal for small volumes such as framing systems and can be cut to fit any size. Constructed of non-woven PE/PP fibers impregnated with Art-Sorb particles.
Microclimate Boxes without Added Buffers Wood panels rely on their own hygroscopic behavior and maintain their own internal moisture equilibrium at changing temperatures. A well sealed case made completely of metal and glass or plastic is essential. A wooden case in and of itself can be considered a buffer. The wood of the case is left uncoated so that it could play its part in absorbing and giving off moisture. The case should then be exposed to the subsequent environment to reach equilibrium.
Altering Gaseous Content Interest arose from the need to reduce the deteriorating effects of oxygen. Amazing history In 1893, the first microclimate box was made for a J.M. William Turner painting using a vacuum technique and removing the air from the box. Up until the time this article was published (1995), this box has still not been opened. This was the first attempt at making a microclimate box and also to alter the gaseous content within the microclimate box. Paper containing calcium carbonate can be used to absorb acid gases caused by air pollution.
Zeolite Used in the microclimate box for the Mona Lisa in 1975 in Tokyo. It can be used to absorb H20, sulphur dioxide, hydrogen sulphide, ammonia, carbon dioxide and formaldehyde. Microporous, aluminosilicate minerals commonly used as commercial absorbents. Commonly used in laundry detergents, cat litter, asphalt concrete, medical and nuclear technology.
Using Nitrogen Gas Pumping nitrogen into an air-tight case to remove oxygen and moisture. Nitrogen gas has to be added every few years due to a loss in pressure. Recharging is done by first bubbling the gas through a water bath. After filling the case, an oxygen scavenger can be placed inside, extending the life span of the sealed case. A slight color change in cinnabar, litharge, and sienna has been observed on cases filled with nitrogen.
Ageless Oxygen Scavenger Prevents the growth of microorganisms It consists of a finely divided iron oxide powder, potassium or sodium chloride, and a zeolite containing water, packaged in an oxygen-permeable plastic packet. Absorbing packets range in capacities from 100cc to 2000cc of oxygen. They do release water, so a relative humidity buffer will be needed as well. Hazardous to place in case containing air because of the heat produced and also risk of implosion when the oxygen (20% of air) is removed.
Aesthetics A. A box containing a buffer and panel without framing B. A box encapsulating a framed panel and buffer C. A framed box containing a panel and buffer D. A framed box containing only a panel
Microclimate Boxes: Barriers between the spectator and the artwork Can be a heated dialogue between curators and conservators Goal: distracting the spectator as little as possible Plexiglas (Perspex) is highly reflective. Use a coated and low-reflection glass. Low-reflection glass of low iron content takes the green out of normal glass and limits the amount of disturbance.
Jorgen Wadum makes it clear that man-made buffers should not be used unless there is a large air to object volume ratio.
Silica gel, Art-Sorb etc. may have an adverse influence on the EMC (equilibrium moisture content) of the enclosed object.
Microclimate boxes are beneficial to panel paintings being loaned to institutions with less than ideal environments.
Therefore, the conservator should concentrate on keeping the moisture content of the wood constant and thus ensure dimensional stability of the panel.
Alternatives Museums/owners of a panel painting should see that the following measures are taken to prevent damage. Keep within thermohygrometry parameters which include air temperature and especially Relative Humidity. Temperature should stay within 68°-72° F and 45%-55% RH Install measuring and monitoring devices. Do not place painting near an AC unit, heating vent, near lamps or windows with direct sunlight.
Wadum, Jorgen. “Microclimate Boxes for Panel Paintings." The Structural Conservation of Panel Paintings, Proceedings of a Symposium at the J. Paul Getty Museum, April 1995. Kathleen Dardes and Andrea Rothe: Los Angeles, The J. Paul Getty Trust. 1998. 497-524. Print.
Uzielli, Luca and Fioravanti, Marco. “Physical and mechanical behavior of wood used for panel paintings”. Panel Painting: Technique and Conservation of Wood Supports. Ed. By Marco Ciatti, CiroCastelli, and Andrea Santacesaria. Trans. Diane Kunzelman. EDIFIR, Florence. 2006
Richard, Mervin. “The Benefits and Disadvantages of Adding Silica Gel to Microclimate Packages for Panel Paintings.” Web. 25 March 2010. http://www.natmus.dk/graphics/konferencer_mm/microclimates/pdf/richard.pdf
Allegretti, Ottaviano and Raffaelli, Francesca. “Barrier Effect to Water Vapour of Early European Painting Materials on Wooden Panels.” Studies in Conservation. Volume 53, Number 3 2008. 187-197.
Glaznev, Ivan and Alekseev, Vladimir. “ARTIC-1: A New Humidity Buffer for Showcases.” Studies in Conservation. Volume 54 Number 3, 2009.
Further Research Propaint website http://propaint.nilu.no The Getty Panel Paintings Initiative http://www.getty.edu/conservation/education/panelpaintings/index.html Supplies and Materials for Museums http://188.8.131.52/files/supplies_cdn.pdf