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General Info
Green Wood
If you are working with green wood you have many options for it's storage, preparation, and treatment. First though, why would we use freshly felled wood when we can buy seasoned blanks from the suppliers?
So you have acquired, by whatever means, a selection of logs to turn from. What now? Well the first decision is how you wish to store your logs. There are four basic options:
For each of the above options the method of storing to season is the same; the wood should be stored somewhere out of direct sunlight, shielded from the elements, and with good air flow around them. The general rule of thumb, which varies according to species, is that for every inch of thickness it will take a year to season. So a three inch board would take three years to season. If you rough turn your wet wood blanks this period is dramatically reduced, sometimes to months rather than years.
If we look at each option in turn you can decide which is most appropriate for you. You may find that you use a system comprising all options.
The majority of wood species are best stored horizontally. Before you stack them you should seal the endgrain with a proprietary endgrain sealer, wax, PVA, bitumen, or remnants on gloss paint left over from house painting. This reduces the rate at which moisture is lost through the endgrain and, hopefully, reduces end checking, or cracking. Your logs should be stacked horizontally, out of the weather, with plenty of air flow around them. A refinement on this option is to cut the log full length through the pith before stacking. This reduces the stresses associated with seasoning and balances moisture loss between end and side grain.
Once your logs are sealed and stacked you can use them as required. Once a log is selected you will cut it into blanks using either a chainsaw or bandsaw and prepare them for turning. Assuming the wood is still relatively wet and years haven't passed by, the blanks will need to be considered as green wood. Again you have several options for their treatment:
Thin-walled vessels such as hollowforms and bowls are often turned from green wood. If the piece is turned with walls and base of uniform thickness, or in the case, thinness, the stresses of drying out are dramatically reduced and splits and cracks should not occur. There will, however, be some shrinkage which inevitably leads to warping; a formally round bowl with turn ovoid, for instance. This process is very often part of the design process and can result in some interesting pieces.
You can also "rough turn" your pieces before setting aside to season. This process removes much of the bulk of the wood allowing for accelerated seasoning. Bowls, for instance, are turned with much thicker walls than are required in the finished piece. The mounting spigot is left in place to facilitate re-chucking later, and the bowl is then stacked, as previously detailed, to season. Ideally the bowl should be weighed, the weight recorded with the date, and re-weighed periodically throughout the drying phase. Once a period of, say, three weeks, has passed without weight loss it can be assumed that the EMC (equilibrium moisture content) has been reached and the piece is ready to re-turn. At this point to moisture content should be in the region of 10-15%.
Depending on the species and wall thickness this can take as little as three months. After this time the bowl would be remounted on the lathe and re-turned. The extra wall thickness allows us to turn out any distortions that have occurred during seasoning.
A further method of rough turning involves forced drying. In this case the rough turned object is dried using a microwave oven.
Once the piece is rough turned to an increased wall thickness the piece is weighed and recorded. The piece is then placed in a thick paper bag, or wrapped in several sheets of newspaper, or a microwave safe plastic bag, and placed in a microwave oven with a turn table. Using the DEFROST setting set the timer to between 1-3 minutes and start the microwave oven. After the cycle has ended the bowl is removed and taken from it's wrapping to cool. If the plastic bag method has been used the bag should be turned inside out - the released moisture will be evident on the bag. When the object is fully cool it is weighed again, recorded, and the process repeated. This process carries on until there is no weight loss between three cycles. The piece can then be turned to completion.
It should be noted that this process can be "hit and miss" depending on the wood species in question, actual wall thickness, oven power rating, and a number of other variables. Experimentation is the key factor. You should also be aware that trying to rush the process can, and inevitably does, result on the wood burning and SWMBO dragging you off to purchase a brand new, non-smelling, microwave oven. So beware!
You will be used to this by now...but there is a variation on the microwave process. That is boiling. Blanks can be immersed in a suitable container, submerged with a suitable weight, and boiled for a period of time. This can be anything from an hour to several hours, but for a ten inch bow with 1" walls one and a half hours should suffice. After boiling the piece is left to dry out for two to three days and then final turned.
Yet another variation is steaming...does it ever end?!
All the methods detailed here are intended as basic introductions to the techniques. More detailed instructions can be found in many places. The Internet is a fantastic resource and a search for "microwave seasoning" will result in many suitable results. Alternatively there are a number of very good woodturning books with detailed drying instructions. All
content © Andy
Coates 2005 - 2007 |