Thread: Help Please With Drying Burl

Jeepers....

200- F - thats almost boiling point of water(212f) or 100C (A little under that).

After more than 10 years kiln drying WA Jarrah including some larger burl pieces... (4 tonne in one slabbed burl), I can tell you that after 3 - 4 months in the kiln my finishing temp was 50C... (So about half what he has his oven set at).

Kilns that dry softwood (eg pine P Radiata & Pinaster) will finish at 100C or much higher...and in just 3 or 4 days - but NOT for hardwoods like Jarrah.... EVER.

There's some examples here where Jarrah regrowth in a research kiln dried in a month (800 hours) displayed signs of surface checking -

http://oak.arch.utas.edu.au/publicat.../PN01.1302.pdf

(Pages 9 & 10 have the guts of what he needs to know)....

My own evaporative drying schedule started at ambient temp - and lasted, 3 - 4 months....(100 - 120 days) ending at 50C.... and I would get final MC of 11% i.e. way below the 30 day result in the research kiln depicted at the link above (20%) and checking wasn't an issue on Jarrah including burl wood.

It's like cooking a good roast - lower temp for longer.

100 days to go from say 25C ambient to 50C finish, so a rise in temp from ambient to 50 in ~1/4 degree C increments each day.... you increase temp when RH starts to drop and your water production decreases.

So you need to condense and collect and measure the water vapor you extract each day in order to know when to raise your temp.

Each day when you raise temp - the water production increases... - then as the timber gives up more and more water vapor (i.e. seasons / dries) - the water production drops - then you increase temp again.

Start studying terms like free moisture (sap), inter-cellular moisture, and intra-cellular moisture, structural collapse & checking in relation to kin drying schedules.

If you really want to know about seasoning timber then come see me (or PM me or email me etc) and I'll make us a coffee and give you the whole day to answer your question, if that's what you need.

Its too big a topic to cover in a response to a simple question on a forum post...

Cheers!

Honestly, I don't understand why he wants to cook it at all, it's just asking for trouble (checking). Patience is the key to burl and any other timber. Store in a cool, dry, ventilated room and wait.

Commercially, Time is Money.

I recall back when Pine was dry in 3 days in the Kilns down in Pemberton.

Price roughly doubles from green to dry.

So imagine 100% return on your investment in 3 days! A bit better than bank interest hey!
The pine kiln in Pemberton caught fire and nearly burnt the whole mill down.

Anyway - with Jarrah.. 3 months ~ 100 days for 100% increase in value... = 1% / day or 365% per year, which is a great return on investment.

Just by stickering out (strip stacking) enough sawn hard wood, under cover with air flow and cover to prevent sun checking, I figured I used to make $100 a week.... in increased value... just watching it dry!

I reckon I LOST $100 a week for every week that a trip of logs sat in the yard (under a sprinkler) for the checking that takes place before the timbers milled and strip stacked. If you stack it green sawn without strip sticks off the saw - I was losing $100 a week was how I looked at it...

Its slower when your milling alone.... to strip stack as you cut, BUT - your still losing $ every day that the timber sits there green stacked if its not air drying.

I'd cut a fork full of green then go strip it out - come back and cut a fork full and go strip it out, because from that moment on, I was making $ without lifting a finger as it air dries (and by nature I'm a lazy sod).

Every step of the process increases the value of timber...and leaving felled logs un-sawn decreases the end value of the timber if its surface checked... you can hear the logs popping and cracking in the stack when they are green waiting to be sawn.... that's the sound of lost $

So the problem for many (particularly wood turners who typically like larger sectioned pieces of wood) is that you get a mix often of radial and tangential grain in the one piece (back sawn and quarter sawn grain direction with differing rates of shrinkage and expansion) which creates internal stresses. Add in the desire to turn something from it in a hurry - and the incentive to "speed the process" of drying is understandable. In Burl wood particularly the changed grain direction leads to further increased internal stresses.

Removing the free moisture (sap) is the hardest part, it MUST be done SLOWLY (air drying is ideal).

The oldest way to remove sap was to lay boards in a stream with butt end the log board end facing up-stream... the water pressure of stream flow pushes the sap out of the board (and replaces it with water).

Once you remove the sap and then remove the board from the stream you then just have to remove the water slowly to prevent structural collapse.

Structural collapse.. occurs when there is a steep partial pressure gradient differential, created between the moisture inside the cellulose timber cells, and the spaces between the cells where the free moisture (sap) normally resides as it passes up the trunk to fuel evapo-transpiration & photo-synthesis in the leaves of the tree, as well as the passage back down the cambium layer to feed the root system.

The ONLY way for moisture to get OUT from inside the cells - without bursting the cell wall and causing structural collapse is to diffuse out thru minute openings in the cell walls (called lumens) as a gaseous (evaporated) form.

If you remove the free moisture (Sap) from between the cells too fast you create a vacuum (pressure gradient differential) of sufficient strength for the liquid water trapped inside the cell - to burst the cell wall - in order to move to the lower pressure area between the cells where the sap usually resides. This is called structural collapse - if you ever see it, the timber shrinks unevenly and has a dry rot dusty appearance...when dry. It loses its strength properties.

So the early part of drying hardwoods like Jarrah (removing the sap/ free moisture) is the crucial bit...it HAS to be S L O W. IN essence when you strip out and cover timber the free moisture sap wicks its way slowly out between the cells to the surface of the sawn plank & evaporates.

The "rule of thumb" is that this takes a period of 1 year air drying per inch thru the thinnest dimension PLUS a year on the swan coastal plain.

In a sawn 4 x 2 inch Jarrah Plank - the thinnest dimension is the 2 inch direction.... using the rule of thumb - the free moisture sap should be removed in 3 years (1 inch per year thru thinnest dimension = 2 years PLUS a further year = 3 years).

BUT - this is then only air dried and down to roughly 17% or 18% moisture content which is OK for say joinery frames...

If you want stable timber for say furniture then you need to kiln dry to get down to 11 or 12% EMC (Equilibrium moisture content) which is the stable EMC for Jarrah on the swan coastal plain.

The rule of thumb can leave out the +1 if you dry your jarrah - in a drier relative humidity environment than the swan coastal plain... like say the wheat belt. (East of the escarpment).

Removing the moisture inside the cells - requires kiln drying (Or many years stripped in a dry RH environment with passage of dry air e.g. stripped out under the floor of the shearing shed in the wheat belt).

In kiln drying, the application of increased air temperature and reduced relative humidity and passage of hot dry air thru the stripped stack - helps turn the water trapped within the cells into gaseous form where it can diffuse out thru the cell wall lumens into the spaces between the cells where the sap, 'free moisture' used to reside, and it then wicks its way to the surface of the board and evaporates.

Lastly the cellulose cells are formed from a double walled cell, (Imagine a balloon inflated inside another balloon) and this space between the cells inner and outer wall also contains moisture. Most of it remains behind in seasoned timber...BUT some of it has to come out for a EMC down around 12% & lower. And this moisture is the last to leave the cell...and also requires heat to turn it to gaseous form.

The "trick" with kiln drying is to start slow, and remove the free moisture sap as slow as possible... (Air drying is best for this step... its the cheapest way but not the quickest). By removing it slowly you avoid creating a big enough partial pressure differential gradient for the moisture trapped inside the cells - to burst thru the cell wall in order to fill the vacuum created by removing the free moisture 'sap' too fast!

When you put a piece of Jarrah burl like that depicted in this thread into an oven at almost boiling point of water - you draw out (wick) the free moisture (sap) from between the cells, so fast by turning it to a gaseous form - that the moisture trapped within the cells doesn't have time to turn to a gaseous form to escape thru the lumen's in the cell wall, without bursting it, & causing structural collapse and checking we see in the photo.

That technique (oven . microwave) might work in softwoods... but not so in hardwoods like Jarrah....

3 days to dry Pine in a kiln at well above 100C
versus
3 Months to kiln dry Jarrah at temp maxing at 50 C at the end.

To apply a technique that hobby wood turners and pen makers etc might use on very small section softwoods - to large section hardwoods - i.e. pushing the drying process in ovens, to FORCE moisture out - just doesn't work with Jarrah hardwoods unfortunately.

There's all manner of "research" to find a faster way - (because time is $)...

Unless you can change the properties of the wood or the properties of the moisture there seems to be some limitations.... that we just can't overcome yet.

One interesting program carried out 20 years back at the Harvey Wood Utilization Research Center - was to band saw mill Jarrah into thin 10mm strips... so the sectional density was low, and thus speed up the drying process to just weeks (in energy efficient solar kilns) and then machine and recombine the thin fast dried strips into a board product (30mm) called Valwood.

Wood-based Composite Building Materials

Valwood is a material developed in Western Australia and utilizes regrowth Jarrah thinnings from reclaimed farmland. The wood is used in such a way as to eliminate inherent stresses in young wood: small pieces of regrowth hardwood 10mm thick and 80mm wide are glue-laminated together. This enables quality sawn timber to be obtained from younger trees.

I seem to recall that the inherent stresses in the timber created a lot of problems joining the timber back together with glues on oil heated steel platens under pressure, the resulting composite laminated board didn't want to stay flat or glue joints would frequently 'pull' under the stresses.

Who knows where the research ended up these days.

In short putting green jarrah into an oven isn't a good idea UNLESS its fan forced, and you start around 25C and take 3 MONTHS to reach 50C and evacuate the moist air from the oven (Dry the air).

That's the short and long of it in my experience.

As always others mileage will vary.

Hey Timless Timber

Thanks for the info and the interesting read will pass the info on to my mate in the USA

Once again many thanks mate ;-)

Ian

Your welcome.

The Mechanical Properties of Wood 2

Hardwood Anatomy | The Wood Database

Besides the actual pore grouping, pores can also occur in other noteworthy patterns. For instance, Jarrah (Eucalyptus marginata) features pores that are exclusively solitary, but they are arranged indiagonal rows forming zigzag patterns. This classification includes pores that are in a more-or-less vertical (radial) orientation.

This is a VERY simplified diagram of cellulose (timber) cell walls depicting the 3 different types of moisture that needs to be removed from wood before it becomes stable via the seasoning (& kiln drying) process & shows a cell wall collapse where free moisture (sap) is removed too fast (with excess heat) and the inter-cellular moisture bursts thru the cell wall causing structural collapse.



I freely admit my paint PC drawing skills suck - & I did try to find a fancy electron microscope picture on google images to get a proper look at what the cellulose cell structure really looks like, but without success so my dodgy diagram will have to suffice unless someone smarter than I cares to come up with the real thing or do a better diagrammatic representation.

Also this is done off the top of my head from memory (and my experiences) of something I studied more than 20 years ago, so shoot me if I got some aspect of it 'wrong'.

By all means fell free to improve upon or add your own efforts.

Cheers

Originally Posted by Timless Timber

Your welcome

~

By all means fell free to improve upon or add your own efforts.

Cheers

Mate u are a champion many thanks for the info and all ur efforts in replying

Ian

Who knows Ian maybe someone else will find the contribution handy one day, if they have a similar question.

Once it's out there in the intranet my job is done...

Maybe not everything life taught me will be lost when I am gone.

Imagine if all of the wisdom of the ages, had been uploaded to the internet before the likes of Solomon & his wisdom had passed...Abraham and His faith....David and His Courage, Jesus and his love for mankind & his fellow man.

If only 10% of the collective wisdom of the ages that's been lost - were available today on the internet - what sort of quantum leap in education & knowledge would my kids and grand kids have available to them.... that we never did!

Originally Posted by Timless Timber

Who knows Ian maybe someone else will find the contribution handy one day, if they have a similar question.

Once it's out there in the intranet my job is done...

Maybe not everything life taught me will be lost when I am gone.

Imagine if all of the wisdom of the ages, had been uploaded to the internet before the likes of Solomon & his wisdom had passed...Abraham and His faith....David and His Courage, Jesus and his love for mankind & his fellow man.

If only 10% of the collective wisdom of the ages that's been lost - were available today on the internet - what sort of quantum leap in education & knowledge would my kids and grand kids have available to them.... that we never did!

Mate I am sure they will find it interesting as I have done ;-) and my mate in the USA

Yer mate true and thanks for passing it on much appreciated mate

Mate if that had happened that would be awesome to get all that knowledge in one place D

Ian

Originally Posted by Scott

Honestly, I don't understand why he wants to cook it at all, it's just asking for trouble (checking). Patience is the key to burl and any other timber. Store in a cool, dry, ventilated room and wait.

Given the density of burl timber I'm with what Scott has said. I was chatting to a supplier of burls at the Perth Woodworking show and he said one way to dry burls was to burry them in sawdust for at least 6 months. So that's what I've done.

The sawdust should be from dry wood tho...if your in a hurry to turn the wood.

Green sawdust will dry out slowly and help slow down the rate of moisture loss from the burl, which in turn helps prevent checking, but typically it will take a lot longer than 6 months to dry.

Sawdust from 12%EMC timber (i.e dry seasoned) is anhydrous and will help to draw out the moisture from the burl a little quicker.... in THEORY...

The problem is this.

The moisture wicks to the surface of the piece and then evaporates with the passage of warm dry air over the piece when air drying.

If buried in sawdust - there is no air movement, over the surface of the piece to evaporate the moisture that's wicked to the surface, away - instead you are relying on the dry sawdust to take up that moisture by its anhydrous nature.

The question thus then is - which is more effective at wicking the moisture out? - passage of dry relative humidity air across the surface, or the anhydrous effect of dry sawdust?

When it comes to anhydrous properties there are better products (kitty litter for e.g. or fine salt for e.g.) that dry sawdust.

There are many ways to skin the same cat & as long as you end up with a skun cat.... and are happy with your skun cat... pick whichever works for you.

I skun a feral cat once... nasty nasty bidness - they have a stink all their own!.

Another member from Morley!. If your ever stuck for a extra pair of hands Rod - just holler OK? (As long as its not to skin a cat!)