Preventing Warping and Splitting of Hollow Wooden Furniture
Part of the Uniclectica Antiques and Collectibles Online Series "Caring For Your Antiques and Collectibles"
Almost everyone is familiar with a piece of wooden furniture that, in use, lasts many many years, even centuries. As soon as that piece becomes dis-used -- emptied, and put into storage -- it suddenly undergoes some dramatic changes, including warping and splitting. An understanding of why this is so can be applied to prevent damage to pieces of wooden furniture, particularly those stored in conditions with little or no environmental controls.
Splitting and warping in wooden objects is often attributed to an incorrect (too low), or fluctuating relative humidity (relative humidity is an expression of how much moisture the air is holding, compared to how much it could hold at a given temperature). This sounds reasonable, until we consider furniture -- we know, from experience, that even furniture in use may be exposed to long periods of very dry conditions (i.e. during a Canadian winter), followed by very moist conditions (i.e. in the summer). The shifts between these extremes can, in some climates, occur fairly quickly -- and yet, many armoires, dressers, and other hollow furniture exhibit no ill effects.
Studies done by researchers at the Canadian Conservation Institue in Ottawa indicate that the key is not in the fluctuating relative humitidy, but rather in how quickly it affects the furniture. Wood has some "plastic" or "visco-elastic" properties, meaning that it can "give" in response to stresses, provided that those stresses are gradual (imagine bending a stick; if done with a slow and gradual application of force, the stick will bend a fair bit before breaking. If bent suddenly, the stick will break sooner). In general, splitting of rigid, constrained organic materials (such as wood) becomes probable when the relative humidity drops 25% to 50%. Note, however, that if the drop occurs over several months, a piece of furniture may be able to survive a 40% drop without damage; if the drop in relative humidity occurs over one week, then damage may occur after only a 25% drop.
The response time of the wood to changes in relative humidity can be lengthened to help minimize damage. For a closed chest of drawers (in the test case, 5' high, 40" wide, 20" deep; wood thickness 3/8" average), response time is lengthened from 6 days for unfinished wood, to 8 days for wood coated with a light varnish. Most furniture, however, has already been finished; as well, a difference in response time of a mere 2 days is not sufficient to prevent damage.
The most dramatic difference in response time occurs when the furniture is kept full of textiles. The response time of the chest of drawers with a light coat of varnish is 8 days when empty, and 60 days full of textiles. Thus, the textiles act as a "buffer" to the changes in the relative humidity, allowing the wood to react over a longer period of time. This, as already stated, will prevent damages.
It is important that the textiles used be made of natural, absorbant fibres (such as cotton), and not be coloured (there is always the danger of colour transferring from the coth to the wood). Old, white terry-cloth towels are excellent for this. Do not put the textiles in plastic bags before storing them inside the furniture, as the plastic will prevent the "buffering" that you are after! As well, do not store the furniture with doors or drawers open, as this also decreases the effectiveness of the textile "buffering".
Simply storing your furniture full of textiles will help prevent splitting and warping -- damages caused by too-rapid changes in relative humidity.
The information presented here is a summary of the following conference presentation:
Related References in the Uniclectica Bookstore (click on the title for more information):Understanding Wood: A Craftsman's Guide to Wood Technology. By R. Bruce Hoadley. Published by Taunton Press, 1981. Hardcover, 256pgs. A valuable reference for anyone who deals with wood -- how and why wood does what it does. Clear, concise, wonderful illustrations and photos. Includes some basic wood identification information. Highly recommended.