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    Managing Moisture





    By Garrett Hack

    When I was still wet behind the ears as a furniture maker, I once built a tabletop that shrank so much against the breadboard ends that it cracked.

    I hadn't taken into account the amount of shrinkage the top would experience in the dry air of the first winter. As it tried to shrink against the pegs of the breadboard ends, two fine cracks opened up. It didn't matter how old the wood was or if it was kiln- or air-dried, it was doing what wood forever will do--shrink and swell as it loses or absorbs moisture from the atmosphere.

    Despite modern glues, well-engineered joinery, and sealing finishes, wood's moisture content demands attention if you want your work to last. Out of necessity, I've developed strategies for dealing with moisture content as I'm seasoning the stock, milling the wood, and building my furniture.

    Seasoning and milling the stock


     Store Vertically 

    Let new stock adjust to shop. When bringing wood into the shop, allow the moisture content
    to balance with shop conditions. The easiest way is to stack boards vertically or horizontally on racks in the shop for a few weeks.

    When lumber arrives in my shop, I stack it loosely on end against a wall or on horizontal racks with stickers between the layers so that air can move through the pile. Then I leave it alone. The simplest way to reduce potential moisture problems is to let the wood acclimate to the shop well in advance of starting a project.



    Measure moisture

    Measure stock moisture. A moisture meter can be used to check the seasoning process by comparing readings from newly arrived stock with an average
    reading from boards that have already acclimated to the shop.

    In the winter, softwoods can acclimate in a week, and dense hardwoods in a few weeks. If I need to speed the process, I rough-cut parts and stack them in the gentle warmth above my shop heater. Even in the summer, when the atmosphere is at its most humid, the wood dries further. The longer it adjusts, the better.

    For a long time I have relied on empirical methods that I'm sure past cabinetmakers also used. Dry wood feels warm, and produces shavings more crumbly than those that clump together. For thick stock, I drill into a waste section to see if the borings seem dry.

    Only recently have I also begun to use a moisture meter to know exactly how quickly the final drying is progressing. A simple meter that uses two pins inserted into the wood surface costs about $125.



     Mill in stages 

    Mill stock in stages. Leave extra material when ripping boards to width or surfacing them with the jointer and planer. Variations in moisture content within boards can cause them to warp after milling. Excess material allows room to correct this later.

    My milling strategy is aimed at producing stable parts with the same moisture content inside as on the surface. Wood not fully acclimated, especially thick stock, can be wetter inside than out. A few days of rain, however, and very high humidity, can leave boards drier inside than on the surface. If I have time, I cut parts to rough dimension first and let them acclimate further for a few days. Cutting up the pans exposes more surfaces to the atmosphere of my shop and alerts me to parts that have internal stresses, for they will warp right away. I also don't mill to final thickness in one go, but mill some from both sides evenly and let the parts acclimate again. Planing only one side can cause moisture-induced warp in an otherwise stable piece.

    Gauge and plan for seasonal movement

    Even after careful seasoning and milling, wood's moisture content will continue to balance itself with the relative humidity in the surrounding air, fluctuating with the seasons. This means that the same piece of wood can have slightly different dimensions at different times of year or if moved from the shop to an environment that is markedly wetter or drier. It's important to know how to estimate these changes ahead of time and plan for them when building furniture.

    Keep track of your shop's climate.

    Keep track of your shop's climate. To predict how furniture components will expand or contract in the future, it is important to know the working conditions in the shop. The author uses scraps to create a seasonal barometer of wood movement. By tracking and recording this moisture board's width throughout the year, he can measure the board on any given day and assess the seasonal humidity of the shop.

    The best way to predict how dimensions will change from season to season is to
    measure the wood movement in a board over the course of a year. For this purpose, I have a few "moisture boards" hanging in my shop. One is a wide, white-pine plank and another is a crosscut scrap from a cherry tabletop. I use white pine for drawer bottoms and case backs; its movement is similar to that
    of basswood and aspen, other woods I use. Cherry moves much like other hardwoods.

    I measure the boards' width throughout the year and mark the readings on the boards. Because they mirror what the rest of the wood in my shop is doing, as well as the moisture content highs and lows, I use them to know how tightly to fit drawers or panels in any season.

    Here's how it works: When I'm ready to fit drawers into a case piece, for example, I consult the moisture board. If the board measures 14 in. wide at the driest time of year and 15-5/32-in. at the wettest, and measures 14-1/32-in. today,I know to expect a slight amount of shrinkage and much more expansion
    during the coming year. If the drawers are 7 in. wide, or half as wide as my moisture board, I can calculate fairly accurately how much movement to expect (about half of what the board says, or 1/16-in. expansion and 1/64-in. shrinkage). In practical terms, I guess-and err on the side of a slightly larger gap.

     Predict wood movement. 

    Estimate wood movement. For tasks such as fitting a drawer, Hack consults the moisture board to predict how much the drawer front will expand and/ or contract and to decide the amount of gap to leave along the drawer opening.

    Work with dry wood and heat your shop

    I prefer to work with wood as dry as is practical, so that tabletops or case sides will expand slightly before any shrinkage occurs. If the parts expand first, they can work against any tight fasteners and create some slack for later contraction.

    On the other hand, on frame-and-panel work such as cabinet doors, I don't worry so much about a little excess moisture. I'd rather have the panels shrink a little instead of expanding and blowing out the joints.

    To help avoid dramatic changes in climate when moving a finished piece from my shop to its destination, I heat my shop as I do my house. This results in a moisture content somewhere between 7% in winter and 11% in summer. Also, a good finish will slow moisture transfer (but won't stop it). There are a variety of ways to outfit a shop with heat, but if this isn't practical, you could always store your lumber in the dining room-if you can get away with it.

    Photos by Steve Scott

    This article originally appeared in the November/ December 2003 issue of Fine Woodworking.


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