Q & A
What is a Micro-Climate?
Why do we use Micro-Climate?
What is Relative Humidity?
How are Temperature and Relative Humidity related?
What is Active Micro-Climate Control?
What is Passive Micro-Climate Control?
How do Passive and Active compare?
How do Humidity Fluctuations Vary for Wooden Artifacts?
The dictionary definition of a micro-climate is: the climate of a small, specific place within an area as contrasted with the climate of the entire area. Here we create air tight cases that regulate the climate in a specific volume around an artifact.
A micro climate is generally the condition inside an enclosed space, insofar as it differs in temperature, humidity and air cleanliness from the surrounding ambient.
Some examples of micro-climates are:
a hospital operating room
a clean room
a green house
a museum display case or storage room
With regards to humidity, the required parameters for a museum display case are a constant, non-cycling level of relative humidity that is maintained within the conservator’s set parameters.
Micro-climates are necessary in the preservation of priceless irreplaceable artifacts in a more practical way than adjusting the climate in an entire gallery, room or building. These micro-climates keep artifacts in their own individual ideal climates in order to minimize their deterioration.
“Years of producing exhibits had shown that only by involving conservation early and systematically throughout the exhibit process can a museum ensure preservation-friendly exhibitions.”
from Preventive Conservation and the Exhibition Process: Development of Exhibit Guidelines and Standards For Conservation by Toby J. Raphael, Journal of the American Institute for Conservation
“The exhibition is, in fact, a compromise between the reason that you acquire and save each object and those conditions that will preserve your objects for the longest period of time” -Carolyn Rose
Relative humidity is the amount of moisture in the air compared to what the air can “hold” at that temperature. In other words it is the amount of water vapour in the air at a given temperature.
How are Temperature and Relative Humidity related?
The greater the temperature, the more moisture the air can hold. There is no single range of relative humidity that is ideal for all artifacts. Different materials have different ideal relative humidities and temperatures. It has been shown that lowering the relative humidity and temperature will increase the life of plastics and other organic materials. Relative humidity should never fluctuate rapidly. Many museums have their relative humidity at 45%. Lowering temperatures greatly increase the longevity of collections, but are hard on museum visitors.
Changing relative humidities cause stress on materials and damages a larger variety of materials than does temperature, especially to hygroscopic materials. Seasonal drifts in relative humidity are less harmful than abrupt changes. High relative humidity can cause mold and metal corrosion and low relative humidity can cause materials to become brittle. It is important to maintain a relative humidity between 25% and 65% as humidity above 65% will cause mold to grow, and lower than 25% will cause artifacts to lose structurally important water.
Temperature is a major factor in the speed of natural aging. Materials last longer when kept as low temperatures. High temperatures increase deterioration reaction rates and melt heat-susceptible materials. The rate of decay is unacceptably fast at temperatures that humans find comfortable.
To predict how an item may deteriorate, we must know the composition, the type of collection, how the artifact decays, and how it is stored or exhibited. Every artifact requires an ideal environment. Temperature and relative humidity affect artifacts in three decay processes: chemical, biological, and mechanical. Chemical deterioration is also called natural aging, and is when a chemical reaction occurs and damages the artifact. Plastic and organic materials chemically deteriorate spontaneously, whose rates are determined by temperature and relative humidity. Chemical reaction rates increase with higher temperature, increased concentration of reactants and increased pressure. Temperature affects chemical degradation the most. Biological deterioration is damage caused by living organisms. Mechanical deterioration is related to either the amount of water absorbed by an organic artifact or thermal expansion in inorganic materials. These things cause the object to change in size and shape rapidly, warping, cracking and splitting the artifact. These consequences are amplified with artifacts that are made out of mixed materials. Different materials react differently with increased temperature or humidity, which would cause more warping and the materials to split apart.
What is Active Micro-Climate Control?
Active micro-climate control is environmental control using mechanical devices rather than buffers to regulate the case climate. Mechanical controls are more effective and require less maintenance. Mechanical methods include the use of humidifiers, dehumidifiers, air cleaners and more. Active controls can be used in almost any enclosure and have many cost reductions in building energy use.
What is Passive Micro-Climate Control?
Passive micro-climate control is environmental control using buffers rather than mechanical devices. Buffers that can be used are saturated salt solutions, inorganic hygroscopic materials, or organic hygroscopic materials, they can even be absorbent materials used in the construction of the case. Making the decision to use a passive micro-climate control requires the consideration of a couple of factors:
- the primary material the artifact is made of
- the ambient conditions outside the micro-climate
- the size, design and leakage rate of the case
- the appropriate range and level of the humidity desired
- the type and amount of passive buffer needed
How do Passive and Active compare?
All passive control uses hydrophilic buffers to control the moisture in the air. The buffers absorb or release water through its pores. Active control uses specially designed devices to move and modify the air around the artifacts. Active control can regulate any size of case while passive can only be relied upon to maintain small, well-sealed cases. Buffers use diffusion to move moisture, so it would take a longer amount of time for the passive system to respond to sudden fluctuations in temperature than active controls. Active controls would also need much less maintenance than passive, seeing as if the buffer were to be left alone long enough, the case humidity and temperature would begin to match the gallery outside. Buffers must be replaced periodically and reconditioned to add or remove moisture. These processes can take from 12 hours to 7 days depending on the buffer and whether you are drying the buffer or hydrating it. Active control also have constant readouts telling the user of the humidity and temperature within the case.
How do Humidity Fluctuations Vary for Wooden Artifacts?
Very High Vulnerability
(coating over right angled grain joint or crack)
- 5% RH change, gradual fatigue fracture
- 10% RH change, fracture possible each cycle
- 20% RH change, fracture definite first cycle
High Vulnerability
(veneer over right angled grain joint, lacquer over knot-free wood)
- 5% RH change, zero fatigue fracture
- 10% RH change, gradual fatigue fracture or plastic deformation
- 20% RH change, fracture possible each cycle
- 40% RH change, fracture definite first cycle
Medium Vulnerability
(wood with little or no coating)
- 10% RH change, zero fatigue fracture
- 20% RH change, gradual fatigue fracture or plastic deformation
- 40% RH change, fracture possible each cycle
Low Vulnerability
(loose wood panels, single component tool handles)
- 40% RH change, possible accumulation of fatigue fracture or plastic deformation if the freedom to move or the coatings or the slowness of the fluctuation are less than perfect.