[Woodcarver] drying a 10" log

Mon Jun 21 20:29:07 EDT 2004

Hi Ray.  there is no simple way ( if drying the wood ) to prevent splitting
without involving an environment with a partial vacuum in it. We carve large
( and small )green wood regularly and anything you do that accelerates air
flow past it will cause cracking where the air crosses the surface. (
aromatic cedar and a few imported oil laden woods are some of the only
exception ). drilling holes will help if naturally drying, but hurt if you
force air with the vacuum.

 Making a vacuum chamber for a log is not as difficult as it seems.Large
schedule 80 pvc ( the kind you will see around public sewer projects ) will
work in a pinch, but if it is something you might want to repeat later, a
section of iron pipe that the log will fit inside of will do it. I got mine
from a local scrap metal place and had enough left over for the " shop"
smoker/bar-b-q). Welding a plate over one end and fashioning a removable
plate for the other end will work for the vacuum. remember a vacuum is self
sealing and getting an airtight seal is more important than fasteners or
bolts etc. It is also much easier to do. If your project allows, remove any
surface of the wood that you can before drying, but keep the fresh cuts wet
until you are putting it in the tube.
The fluids inside the log will gasify at near room temperature in a 25"
vacuum. If you are lucky enough to score a good vacuum pump, it will work at
room temperature.
As I am sure you know, wood splits when drying as air removes moisture from
one location more than the rest. Usually this is the exterior of the wood.
The resulting shrinkage from the loss of moisture is unable to occur because
the interior wood still contains it moisture and it's original size. When
using a true ( partial) vacuum to remove moisture, the wood is not drying
from the outside in. As the pressure drops the moisture, in a pretty uniform
sequence, aspirates the moisture into gas which escapes the wood.

This process is accelerated by heated wood. This is the only difficult part.
There are expensive heating blanket made specifically for vacuum kilns, but
they represent a significant invest in the make-up of the chamber and the
heater itself. Any heat source has to be contact only. Remember that a
vacuum insulates and normal convection does not convey heat. Radiant heat
( light in various forms ) heats only the exterior and may cause uneven
drying. All we do is pre-heat it just like an oven and try to get the vacuum
pulled down while there is still some heat in the log. It will not cool down
much after the vacuum has removed the air.( think about a thermos jug ) When
you heat it up you have to let it remain heated for several hours to insure
that the interior of the log is the same temperature as the exterior.

You can get serviceable vacuum pumps off eBay for $50 bucks if you are only
going to do it a time or two. The actual original purpose of the vacuum pump
is inconsequential, only that it will pull a 22" or better vacuum and run as
required for several days. the drying time will depend on species, size, and
too many other factors to name. But the thumbnail measuring method of choice
is to weigh your entire contraption. If your log is of any size there will
be a drastic weight change ( usually a hundred or two pounds) as it loses
moisture. When your assembly stops losing weight, your done. At lest as far
as you can go without mortgaging the house. Commercial kilns dry 8/4 white
oak for about 48 hours at 22" vacuum. Almost zero checking /splitting and a
very uniform 6% moisture content.

Woodmizer ( the same folks that make the saw mill) used to sell some light
commercial vacuum kilns and are a great source for technical info.

All that being said, if you can find it, aromatic cedar is real forgiving
the same day it is cut. We are in the middle of a 2000 piece run of
"spirits" about 20" tall and 4" thick through the nose area. We produce
about 50 a day out of aromatic cedar that is often sawmilled on the same day
we mount it on the machines. We have an occasional loss from internal
checks, but they seldom to appear to be from drying in the wind of the
router motors, more like internal stress cracks with some resins visible.

I also have a friend that is about to try Vacuum bagging a log just like you
were gluing laminate and putting to in a hot box in the sun. I will be
curious to see how it comes out. In theory it should work the same.

And as said in the previous post, if you can stress relive the back, that is
always going to give you a more long term stable product.

Sorry for the oration, I get asked this from time to time and just hadn't
had a forum to share it on.

Best of luck and let us know how it goes.

Phill Pittman
digicarve at verizon.net
www.masterwerkes.com

----- Original Message ----- 
From: "RAY MIGHELLS" <raymighells at earthlink.net>
To: "[Woodcarver]" <woodcarver at six.pairlist.net>
Sent: Monday, June 21, 2004 6:31 PM
Subject: Re: [Woodcarver] drying a 10" log

> To make a DONATION to the Mailing List using PayPal OR regular mail, click
this link: http://wwwoodcarver.com/WWWList/WWWList.html
>
> Hi Ray;  What kind of wood?   How dry is it now?  What are you going to
> carve on it?  Many woods are easier to carve "green".  Keep it wet with a
> spritz bottle of water, cover it with something wet/damp, and wrap in
> plastic when not carving.  Depending on the depth of your carving,  you
may
> have to hollow it out from the back.  There's a lot of grain tensions in
> different degrees of stress,  so even if it is dry when you are finished,
> the seasonal changes can have distastrous effects if the tensions are not
> somewhat compensated by relieving the back.  A PEG treatment will help
some.
> Happy carving and good luck  Ray Mighells  6760 Rt 417 Killbuck NY 716 945
> 0098 Please view my work at:  http://www.picturetrail.com/razaxnstuff
> ----- Original Message ----- 
> From: "Fishley, Ray (IS - Municipal Engineering)"
> <Ray.Fishley at City.Saskatoon.Sk.CA>
> To: <woodcarver at six.pairlist.net>
> Sent: Monday, June 21, 2004 5:27 PM
> Subject: [Woodcarver] drying a 10" log
>
>
> > To make a DONATION to the Mailing List using PayPal OR regular mail,
click
> this link: http://wwwoodcarver.com/WWWList/WWWList.html
> >
> > I would like to carve a 10" x 12 foot log.
> > I am looking for away to dry it so that it does not crack or check.
> > I was thinking of drilling a 1" hole through the center of it and
> connecting
> > a vacuum to it for a few weeks.
> > Does anyone know how I might go about drilling the hole or does anyone
> have
> > a better idea?
> >
> > Thanks in advance
> > Ray F
> > _______________________________________________
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