So you have bought a nice new wooden chopping board or even a table or chairs; in fact, pretty much anything made from wood. Then you find the table starts to wobble or the chopping board doesn’t sit flat any more or that beautiful solid door just won’t close properly all of a sudden.
All these things are quite annoying and also quite normal; it’s what wood does.
The grain in timber is actually a whole bunch of tubes (like blood vessels in the human body) and they carry nutrients in the form of sap from the root system to the rest of the tree. These tubes are flexible in that they can expand and contract depending on how much pressure is in the tube; if it is full of moisture they expand and when they dry they contract.
So looking at a cross section of wood, if one side dries while the other side stays relatively moist the dryer side will contract and the moist side will remain as is. As those tubes that are losing moisture contract they pull the sides of the piece of timber together and it is this drying and contracting that will force the timber to curve or warp.
In a finished product this will happen typically when one side is exposed to air and the other side is enclosed or flush with another surface. Box bases and lids, table or cabinet tops, drawer fronts and chopping or serving boards will all, potentially, have problems.
Typically it is only the surface layers of any finished timber product that will be affected (therefore thick pieces suffer less than thin); it takes a while for moisture to permeate right through the complete thickness of a slab of solid wood. It is also worth noting that the moisture content of the air (humidity) will affect those surface layers and can cause warping. Something made in a dry climate (like Tasmania) may warp terribly if transported to a moist environment (like NSW and Queensland) and vice versa.
That seems quite straight forward; but wait, there’s more. If it was the same for all timber it would make life a lot easier but different woods warp more or less than other species and the type of cut (how the lumber has been sawn) will also have a big influence on the amount and direction of the warp.
Quarter sawn pieces of timber should, in theory, be more dimensionally stable than a rift or flat sawn board. Ideally, then, if all your timber was quarter sawn there would be no problem; however, I have seen perfect quarter sawn boards warp terribly.
Next you can add in the internal stresses in timber. If a tree grew perfectly straight and had no branches it would have minimal internal stresses; but they don’t – they twist and bend as they grow, they sprout branches, get torn and damaged by wind; it never ends for the poor tree. Felling trees in the Tasmanian bush is hazardous at the best of times; it is downright dangerous when you get one that twists and splits as you cut it – there is often no way to tell where the damn thing is going to fall. This is all due to those in-built stresses; some of those stresses get transferred to the timber when it’s sawn. Thankfully machining or dressing the timber will remove most of the warping caused by stresses but it can still magically appear right when you don’t want it to.
Wood will not shrink or expand very much longitudinally but the other two dimensions will change significantly as wood dries. Once the wood has dried and is machined and cut ready for use it will still change but not nearly as much as when ‘green’ wood dries.
So how do you prevent warping of timber in finished products; well, you can’t – not unless you are prepared to completely enclose the timber in something like epoxy or resin. The problem with this is the loss of the natural feel of timber; it becomes almost like plastic. There are things you can do to minimise warping; like, allowing air to all surfaces evenly (not always practical), keeping products out of the weather (not always practical), keeping indoor items out of direct sunlight (that is almost practical).
The important thing to keep in mind is that the warping of timber is natural, it has been happening since we first started making stuff from wood. Many fine antique timber products have warping and twisting of surfaces but this doesn’t make them any less desirable, it just proves the item is a genuine, hand-crafted classic.
What you shouldn’t do is try and fix warping as soon as you notice it; because, it may well change back to the way it was and if you have ‘fixed it’ then it won’t be too good then. Only consider doing something about a warped surface if the piece has fully acclimatised (which can take many months if not longer) and is not going to be moved to another location. Then get advice from a woodworker about the best way to go about correcting the problem; they may well advise you to just love it the way it is – which is what we do with the many antiques we have with ‘warping problems’.
Popular Woodworking magazine had a good article some years ago; it is directed at the woodworking audience but is still a good read: