Automatic Drying Cabinets presents controlled low humidity storage environments for various valuables from collections in museums to archival records, musical instruments and archeological samples.

Museum and Archives
Moisture is one of the main threats against books, paintings, furniture and other items in museums and archives. Change of relative humidity levels within the storage areas is another problem. It is well known that fairly low or very high relative humidity levels can have a disastrous effect on certain materials. To stop the damages which can even cause the loss of the valuable item, the condition that promotes the damage should be avoided.

If the relative humidity is kept within controlled limits, no micro-organism or bacteria activity will occur, no corrosion will form on metallics surfaces. Therefore, it is ideal from a conservation standpoint to maintain as constant a level of relative humidity as possible.
Extensive or rapid changes result in deformation, cracks, flaking, splitting, etc. Too high humidity results in mould growth and corrosion. Objects which are composed of cellulosic materials (wood, paper, cotton, jute and linen, etc.) experience great dimensional changes when the relative humidity changes. Other sensitive materials include protein substances, which are found in animal, bird, fish, and insect collections: also included in this category are materials such as silk, wool, parchment, leather, fur, feathers, horn, bone and ivory. Metals, although they do not take up moisture, can deteriorate when high humidity activates the presence of carbon dioxide, sulphur dioxide, chlorine, etc., and acids that evolve from wood. Ferrous metals rust at higher levels of relative humidity. In the presence of airborne chloride contaminants, copper and its alloys can develop certain conditions, such as bronze disease, which are activated at higher relative humidities. Excavated bronzes commonly suffer from this disease. Silver will form sulphide deposits at high humidities. Gold and platinum are theoretically inert, but the alloys of gold may not be. Some ancient glass objects also respond to some extent to moisture levels, by either weeping or crazing.

As long as the relative humidity is kept constant, slow and moderate changes in temperature, however, are generally of secondary importance,. Removal of cold-conditioned objects to warm areas should be done after a period of slow warm-up in order to avoid condensation.
It is difficult to select a relative humidity level which satisfies all the components of a multicomponent structure. Therefore different humidity settings of automatic drying cabinets offer a solution to protect these items. Selecting portions of the collection for special environmental control may be used to control the relative humidity of individual cases more effectively and economically. Many objects have already stabilized at some particular level of relative humidity, and if this level is maintained steadily, they will survive without additional stresses. Optimum levels should be determined for each type of collection.

Musical Instruments
To understand how to preserve musical instruments, you must first know what they are made of. You may find metal, wood, leather, plastics, textiles, even paper. And then there are the instrument coatings, both natural and synthetic. The instruments that will be easiest to care for are those made of one primary material, such as brass for trumpets or wood for violins. Most instruments, however, have components of several different materials. Conservators call items made of multiple materials composite objects. The care of these artifacts is much more difficult.
Several factors make the preservation of composite objects challenging. Sometimes the materials are not compatible. They may affect one another chemically. Or they may respond differently to changes in relative humidity. For example,wood and metal have different expansion rates so, when exposed to higher humidity, the wood on an instrument might split where it is restrained from expanding by a metal ring.
The skin material of choice for drumheads is untanned rawhide. Rawhide is prepared by removing decayable matter and hair from the hide and allowing it to dry into the desired shape. Rawhide is extremely susceptible to damage by insects, fungus and fluctuations in temperature and relative humidity
Wood can be found in many familiar forms in musical instruments – from drum bodies and sticks to the violin family and other stringed instruments. A brief word here about moisture content and relative humidity will serve to introduce some basic measures for preventative conservation. After harvesting, wood is dried before it is crafted into an instrument shape. During the drying process, the wood loses moisture until it is in equilibrium with its environment. It is then described as seasoned. Because of variations in ambient conditions, dryness is a relative term. An object made of wood that has been cut, dried,worked and stored in the arid countries will have a very different moisture content from an object made in the more temperate and humid region. Once an object is transferred to a new place, the key to its conservation is to prevent further loss or gain of moisture, preferably by maintaining as closely as possible the object’s original climatic conditions. (This is a basic principle for the preventative conservation of all organic-material objects.)
In practice, however, this is rarely feasible. In fact, in cases where an instrument has already adapted to another humidity level, it is no longer desirable to maintain the conditions of its manufacture. In addition, the presence of other materials on wooden musical instruments makes specific guidelines about moisture content and relative humidity difficult to establish. Most often, compromise climates are recommended. Wooden instruments do not have to be played regularly to maintain their tonal qualities; it is more important to maintain a consistent environment for them.
A wide range of metals and alloys are found in musical instruments, including brass, copper, iron, lead and “noble”metals such as gold and silver. Gold and silver are usually found as platings on baser metals like brass and copper. Metals are less vulnerable to breakage and the common deterioration factors that affect organic materials.The most prevalent problem experienced by the metal components of musical instruments is corrosion – the reaction of refined metal with water or water vapor, oxygen and mineral salts. Improper cleaning and polishing also can introduce corrosive chemicals such as ammonium hydroxide, which reacts with copper alloys such as brass. The rate at which corrosion occurs depends on temperature and relative humidity. Objects  corrode faster at higher relative humidity and temperature values.
When you familiarize with the materials used in musical instruments and the condition problems you may encounter with them, you are ready to address questions of how to handle and store the objects. Table offers an introductory guide to the long-term safekeeping and preservation of different material.Taking advice from those not trained in conservation techniques – dealers, collectors, traditional instrument makers, musicians – can result in irreversible and costly damage to a valuable object. Be proactive with the artifacts in your care. It’s always better to prevent deterioration than to repair damage done.
Humidity Requirements
The recommended humidity requirements outlined in Table below represent the most widely accepted standards. Safe environmental limits depend on the past history of the specimen, its structure and the conditions to which it has already become acclimatized. For example, if an object is fashioned of wood from an arid region (and has never left that region), its normal moisture content is much lower than wood from the northern regions of America or from Europe. It makes little sense to subject that 50% relative humidity required for northern wood, while 25-30% relative humidity might be the optimum humidity for wood from/in arid regions. Again, stability is the most important part of environmental control for the preservation of artefacts.

Material	Humidity tolerance		Dimensional response	Susceptibility to mould
	High (%)	Low (%)
Paper	60	45	Rapid dryness and freezing cause loss of Flexibility.	Extreme
	(45% optimum)
Stretched paper	60	45	Paper screens, drawings, pastels stretched on frames will tear from shrinkage in dry atmosphere.	Extreme
	(critical low limit)
Photographs, films	45	30	Rapid. Excessive humidity softens (sometimes dissolves) gelatine. Excessively dry, embrittlement of paper and gelatine.	Extreme
Parchment, vellum	Steady state		Extremely rapid. Dryness causes loss of Flexibility.	Moderate inherent alkalinity
	(55%)
Leather	60	45	Variable according to tanning process. Very susceptible to shrinkage after wetting.	Variable. Marked for fine leathers
Fabric (natural fibres)	60	45	Inverted. Because of twisted fibres, fabrics shrink when fibres swell, relax when they shrink. Silk and wool are more sensitive to moisture damage than cotton or linen. Painted textiles are most sensitive to moisture changes	Marked
Bone, ivory	60	45	Very slow, except in thin sheets. Ivory is more susceptible to moisture damage than bone or scrimshaw (outer bark left on scrimshaw providing buffer). Avoid hot lamps in cases for emphasis lighting	Negligible, except at very high relative humidity
Wood	60	45	Slow, varying with massiveness and moisture-barrier coatings. Affected by weekly cycles, especially by seasonal cycles	Negligible, except at very high relative humidity
	(critical)
Painted wood	60	45	Dryness which causes shrinkage is especially damaging to objects in which wood is the structural sqpport for other material, i.e. a painted wood panel. Wood sculpture, furniture, models, musical instruments, and decorative objects may also be coated with a gesso plaster, then painted or gilded. These rigid coatings are more or less unaffected by normal fluctuations in humidity but if the wood support shrinks, the coatings are compressed, causing them to buckle or blister and flake off. In extreme moisture conditions (flood, condensation, wetting of surface), gesso, glue joins, and some paints may soften and dissolve	Negligible, except at very high relative humidity
	(critical)
Manila, sisals feathers	60	45	In humidities below 30 per cent and 15 per cent relative humidity these materials become very stiff and brittle. They crush easily if handled. If left done until normal conditions return, they will absorb their normal moisture content and reassume their normal physical characteristics.	Moderate
Plastics			Not normally responsive to humidity changes. However, some do respond dimensionally to temperature changes	Some plastics will support fungus growth at high relative humidity
Metals (polished)	30,  preferably lower		No dimensional response to changes in humidity. Metals can react dimensionally to extreme changes in temperature. United States Navy Department tests find that no corrosion is present on steel surfaces at 15 per cent relative humidity or less. Corrosion does become evident after nine months in 30 per cent relative humidity. Corrosion is present after one day in 90 per cent relative humidity. Brasses and bronzes do not tarnish at 15 per cent relative humidity or lower
Stone, stoneware, porcelain, lead, pewter			Generally most resistant to changes in relative humidity and temperature. However, damage can occur in extreme heat and in extreme cold and humidity (freezing)
Glass	60	45	Generally resistant to normal environmental change. Rapid rates of change in relative humidity and temperature should be avoided. Crizzled glass can be damaged at very high and very low humidity conditions
Archaeological materials: bronze, stone, ceramics, plaster, terra cotta, low-fired earthenware	As dry as possible		Archaeological objects which have been long buried may have been infused with or corroded by salts which behave hydroscopically. ‘Bronze disease’ can be kept dormant in dry atmosphere
Source : The tabulation was prepared by William R. Leisher, Conservator, National Gallery of Art, Washington, D.C., adopted in part from R. Buck, A Specificafion for Museum Air-conditioning, (Museum News Technical Supplement No. 5.)

Humidity Requirements for Storage of Some Objects

Preventive Conservation