In Winter when the outside temperatures fall below inside temperatures the cold, moist air entering the warmer building becomes hot, dry air. In the same way that moisture in the air will be absorbed by materials in a building, this hot, dry air pulls moisture from everything it comes in contact with, trying to reach “equilibrium” (the point at which the material no longer loses or gains moisture). Moisture always travels from wet to dry.
When outside air at 20°F and 65% RH is drawn inside and warmed to 70°F without humidification, its relative humidity drops to about 10%. Air travels from hot to cold.
If you take wood from outdoors that has equalized with the outside air moisture content, into a heated building with a lower moisture content, the wood will begin to give up its moisture to the dry air in the building. As the wood loses moisture it will shrink and begin to crack and check. If the wood doesn’t crack, then warping is the result. Cracking and warping is just the beginning of dry air problems in a woodworking environment. Dry wood will suck the solvent from glues before they can properly cure, leaving only the glue residue in a very weak glue joint. Shrinkage of the wood in the glue joint or warping on either side can then literally pull the joined wood apart. Dry wood also sucks solvents from paints and finishes, often leaving a rather dull finish that won’t last very long.
Cutting and sanding dry wood results in a much higher production of dust due to the dry wood fibers breaking off during the process. Moist wood fibers cut more easily, rather than breaking. Cutting dry wood increases the sharpening frequency required with many blades.
The sound of wooden musical instruments will change as the moisture content changes. , Electric, acoustic and Spanish guitars should be built in humidity controlled environments to insure the rich sound musicians expect. Most insurance companies require proper storage of valuable instruments, such as a Stradivarius violin, to be in humidity controlled environments. Mahogany sounding boards used in pianos require proper humidity control or they will dry out changing the sound quality. In addition, when the pegboards dry out the pegs become loose. Dry pianos require more frequent tuning.
In the case of a museum, where expensive paintings, sculptures and other hygroscopic materials are kept, repeated, rapid changes in the relative humidity can be devastating and eventually destroy the artifacts.
The key to protection of wood and wooden artifacts is the stability of the environment where they are kept. It is detrimental to permit the relative humidity to vary rapidly or widely throughout the year where precious wooden articles are stored. The expansion and contraction as the wood takes up and gives off moisture will pop joints, crack paint finishes and fade finishes.
Dry air results in as much as 50% more dust in the air. Humidity levels below 35% relative to the temperature will allow electrical static charge buildup inside non-grounded exhaust ducts and other metal surfaces. Humidity levels above 40% in a confined space promotes dust particles in the air to agglomerate (attach to one another), become heavier and fall to the floor more rapidly resulting in less dust to breathe. Larger dust particles are also more easily captured by filters in the air handling system. Viruses will also stick to the larger dust particles that fall faster. That results in a reduction of air borne contagious agents. The flu season really means the dry, winter season when small flu viruses stay dispersed in the air longer after someone sneezes.
In most woodworking, painting and printing environments, a Carel® humidification system will pay for itself in reduction of scrap and quality improvement in less than one year. Woodworkers should also recommend to their distributors and retailers that their high quality furniture, instruments and cabinets be stored and displayed in a humidity controlled environment, and that the owner do the same.
Want to learn more about furniture and humidity? Go to http://aic.stanford.edu/library/online/brochures/furn.html