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Timber Drying Faults, by Richard Jones © 2008Drying wood, whether by air drying or in a kiln, sometimes results in stresses building up in the timber making it unsuitable for its planned purpose. The main stages of the drying process and some of the faults that might occur are depicted in the graphic below. To maintain reasonable crispness and sharpness of this image I had to save it as a quite large resolution file, about 250 kb, which may mean the download time might be quite long for internet users with slower connection speeds; please bear with it.
Photographs below the graphic illustrate a selection of drying faults.
The Fork or Prong Test.
This is one of the tests you can undertake quite simply to determine if your wood is free of stress. This test indicates if the wood has transverse case-hardening. To conduct the test, move in from the end of the plank by at least 300 mm (12") and cut a full width and thickness section out about 40-50 mm (1-9/16" to 2") long- see sketch at right. Turn the end grain of this short length down on to the table of a bandsaw and cut a pair of prongs about 6- 10 mm (1/4" to 3/8") thick by removing the wood in the centre. If the plank is thick, three or four prongs are possible. The forks will soon tell you if the the wood is stress free.
Sometimes, if there is stress, the forks move instantly, and at others it might take an hour or so for the forks to move. If they remain parallel to each other, even if they are not straight as in the top image in the above photograph, or at 3, right, then it is fair bet that the wood is unstressed: you can reasonably confidently proceed to undertake that deep ripping operation you had in mind to create a book matched panel, see sketch below right. If the forks move towards each other the wood is case-hardened, 4 right; and at 5 right, where the forks spring apart, this is an indication of the rare condition of reverse case-hardening.
In the same way that stress reveals itself using the prong or fork test, just described above, deep ripping, aka resawing, a plank will show if it is stressed longitudinally. The sketch at right indicates either longitudinal case hardening-- or possibly longitudinal growing stresses-- through the concavity formed by the faces of the ripped board. Both case-hardening and growing stresses may cause the concavities, ie, cupping across the width and/or bowing in the length. When the wood warps like this, or the opposite of bowing and cupping away from the cut, whatever the worker had in mind after the deep ripping (aka resawing) may not succeed because the two resultant planks might be too warped and unstable to use for that purpose, eg, as a bookmatched panel.
Good timber merchants dislike selling stressed wood and will generally rectify the problem without question. Less reputable timber merchants, of which there are thankfully few, generally won't argue for long that the wood they sold you is "fit for purpose" if the fork or prong test reveals case hardening or reverse case hardening, especially if you show them the bent forks. You can also suggest they do the test for themselves with the wood you purchased on their bandsaw whilst you watch-- at this point the merchant will probably guess you know a bit about wood and that you won't easily be fobbed off.
Honeycombing. The first cross cut on a radial arm saw of this American black walnut revealed the fact that aggressive drying had caused
End splitting in English oak boards dried too rapidly. Even though the ends were painted this did not prevent end splitting in the pile of air dried and stickered planks. Although painting the ends of boards to slow down loss of moisture during initial drying is often recommended, especially in woodworking forums and the like, research by many reputable timber technologists suggests most paints are ineffective. An emulsified sealing wax designed for the purpose works much better, eg, End Seal in the UK or AnchorSeal in North America. Roofing tar (bitumen or pitch) can serve as an effective substitute; and some people, especially small woodworking operations and amateur woodworkers that dry their own wood sometimes use PVA thinned by about 20% or 30% with water. I have been told by those that have used, and continue to used PVA for this purpose, that it works reasonably well.
Surface checking, right, in a European oak board caused by over-rapid drying. They penetrated quite deeply and planing off somewhere between 4 and 6 mm didn't get rid of all of them completely.
Sticker Stain, right. Blue or black stripes of stain running perpendicularly across planks are sticker stain. Sometimes the staining is not so obvious and prominent as in these examples of 'sticker shadow' in hard maple; the staining was originally much more evident, but planing reduced their intensity . Maple and sycamore are two species particularly prone to this fault. The cause of sticker stain is mould that develops due to moisture trapped between the sticker and the drying boards. Sticker stained planks are generally unusable for show parts of furniture and sell at much reduced prices to the upholstered furniture market.
Preventing sticker stain. Fairly new on the market are extruded plastic stickers. These are chemically neutral, flat on two opposing faces, and V shaped on the other two opposing sides- bowtie shaped in section. When used with the opposing V’s top and bottom they let air pass over the wood and reduce the likelihood of sticker stain occurring. The yard where I took this picture in 2007 told me that at that time they had invested approximately £30,000 in these plastic stickers costing something like £2 each, a substantial sum even for a large operation.
A traditional method for preventing sticker stain in maples is 'end rearing' the planks as they come off the saw, see right. Sawdust and any other debris is swept off the plank after it is cut and then it is reared on end against a wall, post, or anything else convenient. Next a spacer is set against the face of the first reared board, usually at both the top and bottom and the next plank reared up against these spacers. This lets air move freely between the planks which swiftly evaporates surface moisture. Quickly removing the surface moisture is a significant key to reducing the chance of sticker stain developing. With only a spacer at the top and bottom any mould that develops should do so only at the board ends. After a few days, or perhaps about a week, the boards must be properly stacked horizontally on stickers to reduce other drying faults that can develop, eg, warping, surface checking, case hardening, etc.
© 2016 Richard Jones