Wood Carving Illustrated
Wood Carving Illustrated
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 Woodcarving and Design 1936 book Lynn Miller VG HC 1st Edition Illustrated  US $67.00
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 WOOD CARVING ILLUSTRATED MAGAZINE 96 PAGES BACK ISSUE SUMMER 2000 ISSUE 11 US $4.70
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WATER AND WOOD
Moisture changes in wood can cause problems for a piece of furniture, some merely annoying, others quite serious. A freshly cut log can contain water equal to twice its dry weight; made into a piece of furniture, it can turn stone diy. This capacity to hold different amounts of moisture under different conditions causes wood to swell and contract. If this property is not considered by the cabinetmaker, a drawer that opens smoothly in the dead of winter can swell and jam shut in the humidity of summer. A perfectly square carcase cabinet can pull itself apart as humidity levels change from season to season.
The amount of water in a piece of wood is often expressed as a percentage of its oven-dry or water-free weight. For example, if a 50-pound block of wood drops to 30 pounds after ovendrying, the weight of the shed water-20 pounds-divided by the wood's dry weight-30 pounds-is the moisture content of the original piece: in this case, 66 percent.
Wood holds moisture in two ways: as free water in cell cavities and as bound water in cellwalls. As wood dries, free water is exoelled first. When this is all discharged, the wood reaches what is termed its fiber saturation point (FSP). At this point, the cellcavities are empty, but the bound water remairts, Dermeating the cell walls. For most woods, the FSP occurs between 23 percent and 30 percent moisture content depending on the species, with 28 percent the average. The kev ooint to remember is that at the fiber saiuration ooint. there is no dimensional change in wood from its freshly cnt size. It simply weighs less. However, if the wood dries further, falling below the FSB it loses bound water from its cell walls. The cells shrink and so does the wood. As the illustration on page 80 shows, the more bound water a board loses the more it shrinks.
The only way to prevent wood from shrinking is to treat it with a chemical such as PEG-1400. (PEG is an abbreviation of polyethylene glycol; 1400 is the chemical's molecular weight.) PEG- 1400 diffuses into the wood and replaces the bound water, keeping the cell-walls fully swollen. The treatment is suitable only for green wood, however, and is most popular for use with turning and carving blocks.
Wood gains and loses moisture as the relative humidity in the air around it changes. If the relative humidity rose to 100 percent, a piece of wood would reach its fiber saturation point and be at the same size as when it was milled. If relative humidity fell to 0 percent the wood's moisture content would drop to 0 percent. Because relative humidity falls between those extremes only a portion of the bound water is lost. Realistically, the moisture content range of most stock is 5 to 20 percent.
From season to season, the relative humidity in a given location can vary 80 percent or more. This is because relative humidity and temperature are closely intertwined. Warm air can hold more moisture than cold air. As a result, when cold winter air is heated, as it is in homes and workshops, its ability to hold moisture increases dramatically. If here is no added moisture available, the relative humidity plummets to extremely low levels. In contrast, hot summer air can hold a large amount of moisture. But when cooled indoors, it can hold much less. The result can be fairly high relative humidity. Both extremes cause changes in the moisture level of wood and in its size.
You can take several precautions to counteract the effects of changing humidity levels. If you store lumbei indoors, try to keep the relative humidity fairly constant, using a dehumidier, for example, when ttre levels get too high. And although you may not be able to control the environment where your furniture will end up, you should build the piece to compensate for wood movement. When cutting a panel for a frame, for instance, leave a 1/4-inch gap in the grooves thatwillhouse thepanel. The extra space will allow the panel to expand and contract as humidity levels rise and fall.
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