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WOOD MOVEMENT


The cause of many problems in antique furniture can be traced back to wood movement. We have all had the front door or the drawers stick when the humidity was too high. When the moisture in the environment changes, the moisture in the wood changes, the wood expanding and contracting as the humidity goes up or down. The problem comes when connecting wood too solidly to wood running 90o to it, restraining the wood on one or both members from movement. Hydraulic forces are some of our strongest forces so something has to give. The wood will split or warp and sometimes even bending the screw or nail!


Cross-grain construction is inevitable in furniture design. The rail of a chair attaches to the leg at a 90o angle. The joint is relatively small and the movement is not great. Even then the cyclical changes in the environment will eventually break down the glue line requiring regluing. Other construction methods are more extreme. An example of a problematic construction is the Queen Anne tea table pictured below.



































The round top has cross-grain supports screwed to the top that hold the top to the standard. The underside of the top is constrained by the supports. If the humidity goes down, the underside stays the same but the top of the wood will shrink a bit and try to deform making the top warp up on the edges. If the humidity goes back up the top may resume its original shape.  However, if the humidity goes up too much the wood will deform the other way and may compress to an extent that will be permanent. This is called compression set. When the humidity cycles through a dry spell again, the top becomes slightly but permanently smaller than it was when it was first made. Add to this shrinkage that the bottom supports are still trying to hold the wood in its original dimension and the top becomes permanently cupped. What you see below is the near and far side cupping up.

















If the environment is unchanging in temperature and humidity, there will be no movement and no problems. The constant humidity in England (and before the advent of central heating) has kept English furniture in great shape because there is no significant change in the moisture content of the wood—cross-grain construction is not much of an issue. Bring that same furniture from London to a modern apartment in New York City, in the wintertime, and the furniture will undergo spectacular, sometimes explosive changes!


Veneered furniture is often plagued by blisters and cracks where the veneer is running opposite to the direction of the substrate. This is especially prevalent in marquetry furniture where, for artistic purposes, the grain is in all different directions. Fortunately, having been glued with hide glue originally, reconstituting the glue line with thin glue and re-clamping can often repair this problem and allow for re-treatment of this same problem in the future. This is an important reason to use hide glue in some of these repair situations. See the GLUE section on this web site.


The best treatment for wood movement issues is preemptive—keep the environment as constant as possible. If you shoot for a middle ground of 50% relative humidity, you will keep the moisture content of the wood around 9%—that means air-conditioning in the summer and some humidification in the winter and not too hot!