Wood Drying

[Farrell]

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Hi Ron. I ran across the following - don't know the source:

Air Drying:=20
At MC lower than the fiber saturation point (usually 35%), moisture =
change takes place by diffusion within the wood. The standard diffusion =
equation may be written as=20
t =3D L=B2 / D=20
where=20
D =3D 1x10-6 cm=B2/s transverse and radial, 1x10-5 cm=B2/s lengthwise,=20
L is the length along the direction of diffusion.=20
t is the time to 1/e of the moisture change, that is to 63% of the =
equilibrium change.=20
So, if you have a piece of seasoned wood 2 cm thick that is at the 15% =
MC of typical outdoors storage here, and you want to estimate how fast =
it will come to equilibrium in your workshop at 30% RH (7% MC) if =
exposed to air both sides, L =3D 1 cm and the diffusion equation gives t =
=3D 1x106 s, 11 days. The equilibrium MC change required is 8%, so in 11 =
days you can expect 63% of 8% =3D 5% lower MC, that is 10% total MC. =
That leaves 3% to go, and you can expect 63% of that 3% to take place =
over the next 11 days, to 8% MC. So, 3 weeks should be enough time for 2 =
cm thick wood. If your wood is 4 cm thick, it will take 3 months, 6 cm =
thick, 7 months. According to the literature, most seasoned temperate =
woods change moisture at a rate within =B120% of this.=20

According to this, I should be able to dry my 15 mm thick spruce within =
a month from 40% MC down to 8% MC. We'll see what happens. Keep in mind, =
this wood is not green (about 140% MC) - it is about 40% to 45% MC - so =
I'm not drying from a green condition. I hope it will dry in a few =
weeks, as I need to get cranking on these boards ASAP!

I sealed the ends with a water-based polyurethane I had lying around. =
We'll see how that works.

I would love to see pictures of your hot box - let me know when it is on =
your web site.

I have pictures of cutting properly quarter sawn panel pieces from =
poorly quarter sawn boards and stacking and drying spruce for my first =
soundboard panel and ribs. I've got six relatively low resolution shots =
if you are interested.=20

You've got some good suggestions. I'll keep an eye on the edges of my =
boards - interesting, your comments about compression set in this area.

Wow, a lot of new experiences today!

Tomorrow, tuning spinets.  :-(

Terry Farrell

 =20
----- Original Message -----=20
From: "Ron Overs" <sec@overspianos.com.au>
To: "Pianotech" <pianotech@ptg.org>
Sent: Tuesday, March 25, 2003 4:49 PM
Subject: Re: Wood Drying


>Terry,
>
>Your most immediate danger is too rapid drying through the end grain.  =
THe
>dimensioinal change from differential drying through the end can create
>checking that will make large sections of wood unusable.  Don't waste a
>second getting some form of moisture-impervious sealant into the end of
>every piece.  Melted parafin wax works best, IMHO.

Or, if you have any wood glue (out of date Titebond or rubbish PVA)=20
lying about, that works very well also. When sourcing local logs, we=20
specify that the log is end-sealed as soon as it is cut.

Your idea of breaking the log down into 1/2" boards is a good one.=20
Such boards will be ready to use in one year with just air drying=20
(ie. for the panel, rib stock will take at least two years, unless=20
you plan to make laminated ribs out of the 1/2" boards). When we=20
break down a log for sound board material, after the large boards (2"=20
thick) have been cut out, we break down all the small triangular=20
flitchs into 1/2" wide boards, so as to obtain the maximum of useable=20
spruce from the log (you will need a resaw with a tilting table for=20
this purpose). The danger with drying is moisture gradients.=20
Therefore, the thinner cut boards will tend to have less of a=20
gradient problem, but they will be more prone to warpage. As soon as=20
you have cut your boards, they should be stacked with spacing sticks=20
(of say 25 mm by 25 mm section) between each layer of boards, spaced=20
at about 300 mm (1') for boards 1" or thicker. I would recommend that=20
the spacing sticks be set somewhat closer together for 1/2" boards to=20
reduce warpage. The entire stack should be covered with some kind of=20
roofing material and weighted on top (house bricks will do) to ensure=20
that the upper boards in the stack are held down to prevent them from=20
warping. Don't make the stack too high or the overall weight will=20
cause the lower boards to sustain crush damage adjacent to the spacer=20
sticks.

I'll photograph the drying stack I have at the house and send you a=20
jpeg. Looking at your location Terry (27 degress N, - I notice that=20
you have another Sydney a few miles east), I would recommend that you=20
commence the drying process during the winter months, since the=20
summer heat will be very risky for freshly sawn boards. It is the=20
initial drying phase during which degrade from moisture gradients is=20
more likely to occur. Furthermore, quarter cut boards will tend to=20
lose moisture faster through the edge of the board compared to the=20
quartered face. This will occur with a quarter cut board since the=20
medullary rays will be perpendicular to the edge and parallel to the=20
board face. We seal the edges of quarter cut boards as soon as they=20
come off the saw. This will greatly reduce the risk of stress damage=20
at the edge of a quarter cut board during the drying process. I have=20
noticed a tendency for belly wood to be more prone to compression set=20
damage adjacent to the board joints. While there may be a multitude=20
of reasons why this might be so, I have long suspected that the=20
tendency for quarter cut boards to lose moisture faster through the=20
edge, particularly during the initial drying phase, to be a factor=20
here.

By the way folks, our new sound board conditioning box is coming=20
along nicely (the new box is big enough to accommodate a B=F6sendorfer=20
Imperial board). If anyone is interested I'll be taking a photo of=20
the assembly prior to fixing the last side of the box, so that you=20
can view the layout of the air ducting. I'll put in on the web site=20
and let you know when it is up.

Best,
Ron O.
--=20
_______________________

OVERS PIANOS - SYDNEY
Grand Piano Manufacturers

Web: http://overspianos.com.au
mailto:info@overspianos.com.au
_______________________
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