Thread: Jarrah - properties of a fresh cut large slab

Most moisture meters are calibrated to give a MC reading for Oregon Pine usually, so normally when you read MC readings off the meter for species like Jarrah - you have to add (or subtract depending upon species) a correction factor from a table depending upon the reading you got...

So say 6% reading on the meter, for Jarrah - you might have to add say 4.5% or more etc - to get the actual reading...that table of correction factors usually should come with your moisture meter!.

5 inches thisck of Jarrah would take about 6years to dry down to ~ 18% EMC... unless you put it somewhere with extremely low Relative Humidity (desert, wheat belt etc) and under shelter (under the shearing shed floor) where it gets plenty of air circulation, and then you MIGHT get it down to MAYBE 16 EMC...

To get it below that usually takes a kiln to remove the last few % down to 12% or 10% EMC

Unless you get it down to that, it's not likely to remain stable.

Putting it thru the thicknesser while green won't achieve much other than to show you what it will look like when finished, also if it has a twist in it or cupping etc you have to take that out BEFORE you thickness it... otherwise you just end up with a smooth twisted piece of wood. (The jointer straightens, the thicknesser doesn't).

I would suggest it's not likely you will stabilize the slab while green - the reasons why I expounded on in this thread.

https://www.woodworkforums.com/f14/wh...opular-174348/

Others mileage varys - you get that!

Good luck whatever you do - rest assured you won't be the first or last to try it - whatever you do with slabs - someone will have done it before you....and someone will have got away with it, while someone else didn't. Fickle as wimmin's these Jarrah slabs.

Originally Posted by Timless Timber

Putting it thru the thicknesser while green won't achieve much other than to show you what it will look like when finished, also if it has a twist in it or cupping etc you have to take that out BEFORE you thickness it... otherwise you just end up with a smooth twisted piece of wood. (The jointer straightens, the thicknesser doesn't).

I would "mount" the Jarrah on a flat piece of thick ply before sending it through the thicky, where there is a will there is a way.

About twenty years ago before I even knew about letting wood aclimatise I grabbed some Jarrah from a place up the Swan Valley. Real rough stuff, been out in the elements for years, heaps of nails. I took it all home, de-nailed, sent it through my then small thicky and glued it all up. Turned it into a huge thick legged coffee table for my sister. She still has it, no cracks, no warping and strong as the day I made it. Maybe now I worry too much, a little knowledge is dangerous.

Originally Posted by Timless Timber

Most moisture meters are calibrated to give a MC reading for Oregon Pine usually, so normally when you read MC readings off the meter for species like Jarrah - you have to add (or subtract depending upon species) a correction factor from a table depending upon the reading you got...

So say 6% reading on the meter, for Jarrah - you might have to add say 4.5% or more etc - to get the actual reading...that table of correction factors usually should come with your moisture meter!.
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For an electrical resistance MC meter calibrated to Oregon the correction of a 6% MC meter reading correction for jarrah is nominally +1% MC, so a 6% meter reading is really 7% MC. In fact its pretty well +1% all the way between 6 and 24% MC - see PDF available from Measuring moisture content with confidence | CSIRO
This PDF provides several other species corrections that others might find useful.
There are differences between some meter calibrations (see below, some use a table and some use switches) but if your meter has no calibration you will be better off using the CSIRO corrections than using no corrections.

CSIRO has tested a number of electrical resistance moisture meters and found their calibrations could not be relied on across the range and some meters did not provide correction factors across a wide range.
Details can be seen in this PDF Testing Timber for Moisture Content - CSIRO

The table below shows the typical problem of inaccuracy specifically for Jarrah even after applying the correction factor.
Eg the Kett meter has a calibration that corrects 6 to 9% MC when the actual MC is 7%.

On top of all this all resistance MC will still have an absolute uncertainty of +/- 2% MC

Correct Bob,

As to be expected - your calcs are dead on.

However, in 20 years experience testing Jarrah (during and after kiln drying) for MC with a Tramex non invasive impedance moisture meter, and applying the correction factor, I've NEVER seen a reading as low as 6% - so i have my suspicions his batterys flat or he didn't test correctly...
At the outset we are told this is a 5 inch thick green slab... - then its only 6%...

Something just doesn't jive....(speaking from experience) with the info we are given and the data.

You might get 6% out of a heat bead charcoal for your Webber kettle - or a Jarrah ash after the woods been on a fire for a few hours, or it was found & tested on the surface of Mars by the Rover explorer, but not in the real world here on good ol Oz planet earth - where ~90% of our surface is covered in water!

In all those years I've dried timber down to 10% - maybe 9% for pieces on top near the electric heater and subjected to more air circulation.

6%?

Never.

And specially not on a green slab 5 inches thick straight off the log.

I don't know - where the error is, as I haven't seen the timber in question.

What would be an "interesting exercise".... is to calculate its volume (as accurately as possible) and also weight it (as accurately as possible) so we can determine its mass / specific gravity and compare that to the published figures for Jarrah green and dry.

Then we would have a clue as to what we are are REALLY dealing with here.

I didn't want to call the guy out on his readings was all... you CAN get variation in Jarrah - stuff grown in a open paddock situation, fertilized for 100+ years, with no competition for nutrients or light will often be less dense than the published figures for e.g.

All I know, is something doesn't add up with the initial (green slab) info and the figures he's obtained.

If its green I'd be expecting 30% - 40 % moisture readings and air dried for 5 or 6 years maybe 16% - 18%, evaporation kiln dried for another 3 months, ending at 50degrees Centigrade, 10% - 12%

I can't imagine what you'd have to do to the timber, to get it down to 6% in a slab fresh off the log, unless unbeknown to us he has some long microwaving kiln or something - but then if it's a new slab dropped to 6% in such a short time, it would have to have structural collapse... again unless hes developed a drying technique that no ones ever heard of before?.

I don't know whats "hinky" - but something is, based on my experience in this area.
It could possibly be as simple as a dead/dying battery in his moisture meter maybe?
Usually the simplest answer is the correct one.

I've been wrong before today - this won't be the first time.

Should I write something about drying schedules...and structural collapse, or has it been done to death before today?

One small addendum - those tables are for Regrowth Jarrah which CAN grow as much as double the rate of Old Growth Jarrah and as a result be less dense and thus dry quicker / easier. When I sold my business in 2005 I left my correction table (from csiro) folded up with the moisture meter, (coz the new owner would need it with the kiln & it was a walk in walk out sale) - the factors for old growth Jarrah (from memory) were closer to 3 or 4 % and on top of that it was temperature dependent as well, i.e. you add or subtract a certain amount for every 10 degrees C above an ambient of 25C or something...so the weather (temp and RH) on the day you take the reading factor into your correction as well.

Originally Posted by Timless Timber

At the outset we are told this is a 5 inch thick green slab... - then its only 6%...

S

I think you may be getting a little confused as there have been two posters (Ric and the original thread starter Michael) in this thread about jarrah.

Originally Posted by RicB

I have just picked up 7m of 200 x 50 Jarrah from a recycling place. i was hoping that their wood was under cover but unfortunately it is stored out in the open.

How long should I let the plank sit in the shed prior to dressing down? I stuck it with a moisture reader in the end grain and got 6%.

I ask because we have had a real wet few months here but 6% seems pretty good, any suggestions?

This was Ric to whom the current range of responses have been made.

That could explain it!