Re: LDD and an Australian technique
- From: "George" <George@least>
- Date: Tue, 20 Sep 2005 11:26:15 -0400
"Tom Nie" <tomnie@xxxxxxxxxxx> wrote in message
news:htSXe.227$845.220@xxxxxxxxxxx
> If I can make this link work it might be of interest to see what some
> folks are doing. The lack of soap should be of interest considering prior
> formula discussions.
>
> http://www.woodturningtips.com/weeklytips_tip25.html
>
Interesting thought, but, as you know, the resins are not soluble in polar
solvents like water. Addition of Sodium Tetraborate, a surfactant
(detergent), in sufficient quantity may increase the solubility of resins
and other semi-soluble extractives in the wood, but these have nothing to do
with bound moisture. Loss of moisture bound by the cellulose and
hemicelluloses is what causes the wood to shrink. Where the forces of
shrinkage exceed the binding force of lignin, cracks appear.
The reason sawmills keep logs wet is - to keep them from uncontrolled
drying. If the wood were not kept fully hydrated where vulnerable to
evaporative forces, local fiber shrinkage could degrade the resulting timber
by causing checks, which can grow into cracks. What the turners want to do
is to get rid of the bound water at a rate which will keep the surface at
the fiber saturation point by allowing capillary draw of unbound water from
the interior. This is accomplished by control of the relative humidity
around the piece - immersion is 100%, so no bound water will be lost.
Relative humidity correlates directly to moisture content in wood, because
the rate of loss depends on the ability of the surrounding air to take on
water.
Which brings us to the realities of drying.
1) No shrinkage above the FSP. Free fall to ~30% moisture by weight,
regardless of how rapid, won't degrade the piece, as long as the rate of
loss from the surface is controlled.
2) Shrinkage is proportional and localized, depending on the orientation of
the early/late wood and the total section.
3) Water is lost from end grain at ten times or more the rate of face grain
loss.
We use these principles all the time. Spinning a piece to throw unbound
moisture will not result in end checks - rather the opposite, as the water
runs as a liquid from the surface. Gets us to the FSP much more rapidly.
Some even blast compressed air from the inside to get even more out than
centrifugal force can.
Cutting thinner allows more rapid drying with less visible distortion
because there is less section to distort, and less distance for the moisture
to reach open air. Eight percent of an inch is more than eight percent of a
half inch, and less contiguous section throughout means that there is also
less solid wood to pull against.
Weighing a piece as it dries will show the inapplicability of the "inch per
year" rule of thumb used by people for air drying planks. In my basement,
an inch thick hard maple bowl will reach equilibrium with a summer RH of
~70% within four to six weeks, because a bowl turned across the wood has no
place more than an inch and a half from open air along end grain.
So, no magic needed, just control of RH. If you want to develop a schedule
for drying like they use in a kiln, you will measure the same thing they
do - relative humidity.
Oh yes, soap is not a detergent, and the effect of LDD, minimal as it is, is
likely the result of the hygroscopic action of the glycol on the surface,
not the surfactant.
Good information on drying at
http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/fplgtr113.htm Chapters
two and three are especially pertinent.
.
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