Microclimate boxes help control the environment around humidity-sensitive objects like wood panel paintings. They aim to provide a stable climate and protect from pollutants and fluctuations that can cause damage like cracks and splits. Options include boxes with buffers like silica gel or salt solutions to control humidity, unbuffered boxes relying on the wood's natural behavior, or altering the box's gas content with nitrogen. While protecting the artwork, box design must also consider aesthetics and the viewing experience. Regular monitoring and stable temperature and humidity are alternatives to boxes for some situations.

1. Microclimate Boxes for Panel PaintingsAnna Drewes An article by Jorgen Wadum from the book The Structural Conservation of Panel Paintings

2. 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.

3. 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.

5. EMC=Equilibrium Moisture Content

6. Wood naturally seeks equilibrium between the internal moisture content and the surrounding atmosphere.

7. 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.

8. Seasoned wood is not immune to shrinking and swelling.

9. Type of wood, however, does effect the amount of moisture the wood absorbs. Hardwood has a higher moisture change than that of softwood.

12. 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.

16. Cracks can form parallel to the grain because of the stress imposed on them by the excessive movement of wood.

17. Tangential cut wood has very little contraction and expansion, but the materials will still want to move.

19. Panels will shrink in response to a drop in temperature.

21. Three Options A. A box containing a panel painting and buffer material B. A box containing only a panel painting C. A box containing a panel painting and an altered gaseous content.

23. Some examples:

24. Salt Solutions

25. Salt hydrate pairs (1933)

26. Magnesium nitrate hexahydrate (1963)

27. Silica Gel (1966)

28. The rate of air exchange between the case and the environment affects the buffering ability.

31. 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).

32. 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.

33. 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.

34. 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.

35. 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.

36. 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.

37. 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.

38. 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.

39. 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

41. 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.

43. Silica gel, Art-Sorb etc. may have an adverse influence on the EMC (equilibrium moisture content) of the enclosed object.

44. Microclimate boxes are beneficial to panel paintings being loaned to institutions with less than ideal environments.

46. Measuring and Monitoring Devices HOBO microclimate box Sling pyschrometer Humidity indicator card Thermohygrograph Hydrion strip Electric RH/temperature probe Basic hygrometer

47. Ducal Palace in Urbino. Sansepolcro Museum.

49. 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

50. 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

51. 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.

53. Thank You!