Every woodturner uses wood but many have only a vague idea of where it comes from and why it can be so unstable. This evening’s presentation by club members Bob Hope, Alan Sturgess and Dave Gibbard shed some light on the subject.
Bob described how trees take in their nutrients dissolved in water via their roots by osmosis and how this passes up the trunk in the sapwood. So wood contains water, which, as a % of the dry weight, is called the moisture content. For a living tree the m.c. can be over 100% (i.e. more than half the total weight). Once a log is cut it will lose water and eventually reach equilibrium with the atmosphere. Even in a “dry” house, at the equilibrium point the wood will still have a m.c. of about 12%. In a damper atmosphere and it will re-absorb water and will reach a higher m.c.
Wood is composed of cells. Free water between the cells is lost quite quickly but that contained in the cells takes longer.
The problem is that wood shrinks as it dries and the shrinkage is not uniform. Shrinkage along the log is negligible but radially it is typically 4% and circumferentially it can be 8%. This means if you leave a log to dry the stresses build up and it will surely split.
There are several ways of dealing with this:-
Cutting
Alan showed the various cuts a sawmill could make to allow the cut wood to dry without splitting. The cheapest option is to cut “through and through” into parallel planks. However, depending on the position in the log, some distortion will occur, producing what some wag in the audience described as B&Q planks. A better method is to “quarter saw” so that the grain is mostly through the planks. However this is laborious as the log has to be turned 90o for each cut and is wasteful of timber.
Water replacement
The water in the wood can be slowly replaced by PEG (poly ethylene glycol). This is how the recovered timbers of the Mary Rose were treated to stabilise them. In that case spraying took several years but small items can be immersed in hot PEG solutions and the process can be reduced to a month or two. Few of us have experience of this process and it is not widely used by turners because of the complexity. It can also change the appearance and texture of the wood.
Rough Turn
Dave showed some bowls that had been rough turned when wet and allowed to dry. Most of the wet wood is removed to give a rough bowl shape with a wall thickness roughly proportional to the bowl diameter (say 1” per 10”). This will dry much more quickly than a thick disc blank and is the only practical way of making a deep bowl. The drying process is often slowed down by sealing the exposed surfaces. This allows the wood to accommodate the stresses produced by drying. Sealing can be done by dipping in molten wax, painting with PVA or wrapping in paper or cling film.
After drying the rough bowl will have distorted and is re-mounted on the lathe and turned to its final form.
Wet Turning
An alternative to drying before turning is to turn wet. It is necessary to turn to a thin wall to allow the wood to dry quickly without splitting. Some distortion will occur. Decorative items, often with the bark retained, can be very attractive when produced in this way. progression from cut log to finished item can be as short as a day or two. Dave showed several examples turned with grain parallel with the turning axis and at right angles to it.
After tea, Bob and Alan demonstrated the process of cutting a log and rough turning it.
Normally the cutting would be done with a chain saw but for the demo a safer but slower reciprocating saw was used. A cylinder was cut and split to produce 2 “D” section pieces. The corners were cut off to reduce the subsequent tuning and the work piece was mounted on the lathe with the flat face of the D facing the head-stock. It was turned to a rough bowl shape with a spigot on the bottom. Reversing to mount the spigot in a chuck, the centre was hollowed out. As an unintended part of the demonstration, the spigot split off and had to be re-turned. This was because the spigot was cut from a very soft part of the log near the surface. It is a good idea to sacrifice a little more wood to ensure the spigot is from harder material.
After all that excitement Jean Turner gave a critique of the gallery items.
It was good to see a new face, especially that rare phenomenon of a female one in this role and to hear some thoughtful opinions from a fresh viewpoint. Nice gallery too.
Finally let’s not overlook the excellent display of work donated by our members to the Wessex Heartbeat charity.
Next month is the last chance to make a donation before the presentation at the Heartbeat Dinner where our efforts will be auctioned.
