Coping with stress
Wood is not inert - it moves! This fact should be uppermost when designing or making anything in wood.
End-grain, split and unpainted.
I do not intend to go into great technical detail concerning the physiological reasons for such movement, but rather to concentrate on the practical steps to take to avoid furniture-making disasters.
I am assuming that the timber purchased has been air - and kiln - dried, and that the finished piece is intended for use in a centrally heated house.
The movement I am addressing here is not that caused by bad making or unsuitable design, but by the inherent nature of the wood itself.
It's of two types - movement caused by the release of stresses in the fibres by machining and dimensioning, and that caused by changes in moisture content.
Air-drying
Timber in the round may contain half its weight in water, so the felled tree must first be cut into boards and left to dry out.
Once cut to thickness the boards are separated by sticks about 25mm (1in) thick which are stacked outside to dry.
The ends are painted or have a stick nailed to them to slow down the more rapid loss from the end-grain, so reducing splitting or checking.
During the drying the stresses released by cutting the fibres of wood, and those caused by the drying process itself, balance out. This takes about one year per inch thickness of the board.
The wood can then be said to be seasoned, air-dried and suitable for outside use. Very little general shrinkage takes place during air-drying.
I had been in the old workshop - a modern double garage, warm and dry but lacking adequate natural light - for ten years. Of course it wasn't big enough, but no workshop ever is.
I started my business there and the layout 'growed like Topsy' with me, and was due for a face lift.I wanted a nicer, safer, healthier, more efficient working environment, with more light, heat, ventilation, storage and working area and less dust, moisture and clutter, with pleasing decoration and proper fire, security, health and medical precautions.
Not much to ask.
Kiln-drying
The timber must now be dried further for inside use. It is placed in a sealed cabinet or kiln, and the humidity level artificially reduced to the required level. The wood loses more water until it is suitable for internal cabinet work.
End-grain, split and nailed.
The majority of the shrinkage occurs during this process.
I have given no figures, percentages of water by weight or relative humidity levels because these are difficult, expensive to measure and not constant.
Air-drying is to the average humidity level in that place at that time. Kiln-drying depends on the operator, and the level to which the wood is taken.
How long the wood has been out of the kiln, how it was stored before purchase, and how it is treated afterwards can also have a significant effect on the moisture content.
" Don't keep it in a damp garage with wet cars coming in and out, or even a dedicated timber store if it is not heated and damp-proofed "
Buying timber
A reputable timber merchant with whom a good relationship can be established is worth his weight in gold. Explain your requirements, ask when it was cut and how long it was air-dried, to what level it was kilned, and buy the best quality material you can afford.
Quarter-sawn oak end-grain table.
Observe the storage conditions, feel the weight and touch the surface.
I long ago realised that the selection, felling, conservation, drying and storage of timber is a very highly skilled task, one that is best left to experts. What I want to do is make furniture well out of materials I can trust. It is important, however, to take responsibility for the material when it comes into the maker's care, to understand the principles and apply them, and to put the final touches to the process.
Storing timber
Once timber leaves the kiln it starts to adjust its moisture content to its surroundings, so it must be stored in conditions as close as possible to those in which it is to live as a piece of furniture.
End-grain, painted.
Don't keep it in a damp garage with wet cars coming in and out, or even a dedicated timber store if it is not heated and damp-proofed.
Keep shocks of timber in the warmest, driest conditions possible - in my case the area between the beams in my insulated workshop roof.
If the storage temperature and moisture conditions are close to those of the end use, the timber can be stored and conditioned with sticks between the boards, allowing free air flow.
Make sure the sticks are lined up directly over each other so that the weight is borne through the sticks and the boards are not deformed.
Sheets of newspaper can be placed between the boards instead to suck some of the moisture out.
If the storage conditions are not as dry as the end use conditions store the boards flat on top each other, with no air gap, and cover with a waterproof sheet to minimise the moisture intake.
Moisture movement
The main movement takes place not in the length but across the grain. Movement per inch along the circumference of the growth ring is about twice that per inch between the growth rings.
From this understanding it should be apparent why quarter-or radially-sawn wood is more stable than tangentially sawn, see diagrams.
The key to understanding moisture-related movement in wood is in being aware that the major amount of movement occurs when the moisture content is reduced from air-dried to a level suitable for interior furniture use and vice versa.
Wood will always move if its moisture content changes, and it will always try to equilise is moisture content to its surrounding environment, absorbing moisture from damp air and losing it to dry air.
Tangentially-sawn elm end-grain table.
As it absorbs moisture it will swell and as it loses moisture it will shrink-no matter how old the wood.
The level of moisture in the air varies from summer to winter, warm summer air holding more water than that of cold winter. When this winter air is brought inside and centrally heated it can hold more water, which it takes out of its suroundings.
" The desk shrank, took up all its allowance and cracked along the top. My client said that when they entered the room they had to hang out of the windows to breathe! "
Drying effect
Drawer casings in stick with a weight on top.
This is the drying effect which cause shrinkage in wood. The change of moisture content in the air - relative humidity - together with temperature and weather conditions, causes most solid wood furniture in a centrally heated environment to be in a constant state of movement from summer to winter.
If tends to swell in summer and shrink in winter, but fortunately wood reacts slowly to changes of humidity so the movement is gradual.
Moisture content change varies according to the variety of timber. I usually allow for about 3 mm (1/8 in) movement per 300mm (12in)of width of board across the grain direction in normal conditions, but adjustments must be made in the light of experience, and the end use conditions.
Laburaum oysters showing growth ring circumference and spacing.
If follows, then, that establishing the likely conditions in which the piece will end up is important when judging the allowance for movement.
A desk I made recently in best quality, kiln-dried, quarter-sawn oak (Quercus robur) was left in a room unoccupied for some weeks in mid winter, with the radiators left inadvertently at full chat with no thermostats!
The desk shrank, took up all its allowance and cracked a full 3mm (1/8 in) along the top. My client said that when they entered the room they had to hang out of the windows to breathe!
Fortunatelly he accepted responsibility and a repair was made to everyone's satisfaction.
Laburaum oysters showing growth ring circumference and spacing.
After six weeks near a radiator in Bowes Museum the drawers of the Apothecary's Chest in burr elm (Ulmus procera), simply wouldn't open. I was puzzled until I discovered that the museum has humidifiers to protect all the artefacts that don't like central heating, and the wood had swollen, not shrunk as I had expected.
A few days in my warm, dry workshop with the drawers out to allow good air circulation, and some small adjustments, rectified the problem and it is now ready to live in a house again.
Secondary machining
Left: Small amount of growth ring circumference included, movement fairly even and at right-angles to the faces, minimising distortion.
Right: Little movement in the thickness of the board - between the growth rings - but a lot of movement along the circumference, causing the board not only to shrink but to distort or 'cup'.
Right: Little movement in the thickness of the board - between the growth rings - but a lot of movement along the circumference, causing the board not only to shrink but to distort or 'cup'.
Wood is constructed of fibres all pulling in slightly different directions but balancing each other out. When the wood is sawn or planed some of the fibres are cut, releasing the pull they were exerting, and allowing the remaining fibres to pull the wood into another shape.
The released of stresses is usually a relatively short-term effect in seasoned timber, and once complete should stabilise. If the wood is clamped in position, some of the stresses will be taken out by streching the remaining fibres, thus minimising the distortion.
Home-conditioning cabinet or kiln.
Properly sessoned, kiln-dried timber is stable on starting a job, but dimensioning it on the planer or saw cuts other fibres, releasing other tensions.
To minimise the resultant distorsion, cut the pieces oversize and leave to settle overnight - or longer - on stickers, under a weight, in a warm dry place.
Finish to size and again store the pieces on stickers with a weight on the top until needed.
Workshop conditions
A warm dry workshop is a prerequisite to serious cabinet-making.
Look at the basic construction - aesthetics count for nothing if it is damp, or can't be heated.
HOME-KILNING
A KILN OR conditioning cabinet for home use for small amounts of timber can be made easily. Its box construction uses a home dehumidifier to remove the water from the air and from wood stacked inside.
Make the box as air-tight as possible and line it with polythene to prevent fresh damp air entering, or moisture permeating the walls.
The humidifier should be set to the manufacturer's recomended level for domestic interiors, and the water it removes from the inside air piped to the outside.
The wood should be stacked as for air-drying, with plenty of room to allow the air to circulate.
The wood must be air-dried and seasoned first to allow as much water as possible to be removed, the machining stresses to stabilise and the distribution of water in the wood to even out.
This secondary drying process in the home-made kiln should be gentle, with plenty of time allowed.
Assume thoroughly air-dried wood has a water content of 20% by weight. Weight a test piece and mark its weight on it. After about a month in hte 'kiln', re-weigh the test piece; when it has lost about 8% of its original weight it is about ready.
I would recommended that the dehumidifier is not left on all the time, but is switched off every fourth day or so to allow the water in the wood to distribute itself evenly.
I use it very successfully for burr elm (Ulmus sp), which I find difficult to get commercially kiln-dried.
The cabinet can also be used to condition klin-dried timber for a week or two as a precaution before making it up. Again, the timber is dimensioned slightly oversize and stacked on sticks in the cabinet for a few days.
The longer such timber has been out of the kiln, or in poor storage conditions, the more important this is.
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