Collapse and a DH kiln

[longtime lurker]

I'm hoping to tap into the knowledge base concerning timber collapse in DH kilns.

We're about to commence into a whack of timber of a collapse prone species: Turpentine (Syncarpia glomulifera) It's most often used here in the round but I'm eyeing off several years supply of logs and thinking flooring and framing. It's a good timber for both apart from the drying collapse issue.
Now the traditional method employed to deal with drying collapse as we all know is steam reconditioning. What I'm trying to find out is how one achieves that - if one can achieve that - with a DH kiln. Or (because Den will wander along here eventually ;D) a Vac Kiln. I'm going to have to install another kiln to handle the volumes and DH appeals on a number of levels, but I'm unsure if one would be suitable for the job.

Thoughts, ideas, experiences? All would be appreciated.

Many thanks
John

[Den Socling]

lurker,
I have never seen a North American species collapse the way some of the wood from "down under" does. Most people in North America would take a look at "wash board" wood and consider it junk. Few people up here are familiar with steam reconditioning, I would make that educated guess.

[GAB]

LL & Den:
Would either of you have pictures of before and after as I have no idea what you are talking about when you say - "timber collapse in DH kilns".
Gerald

[Ianab]

When it does this sort of thing


As well as the warping from the tight growth rings there is internal honeycomb cracks.  And this is from an air dried piece of wood. Kiln frying tends to be worse.  :-\

Buy adding high temp steam cycles during the drying process it softens the wood and allows some of that tension to release, instead of tearing the wood apart. 

But the question is how you would achieve this in a DH kiln?

Ian

[longtime lurker]

lurker,
I have never seen a North American species collapse the way some of the wood from "down under" does. Most people in North America would take a look at "wash board" wood and consider it junk. Few people up here are familiar with steam reconditioning, I would make that educated guess.

I've got a limited understanding of how a Vac Kiln works from what I've read along the way. My understanding is that when you pull a vacuum you get a decrease in the boiling point of water which allows the bound water to depart the timber easier. You heat either cyclically or through a water jacket because you can't carry heat in a vacuum. (Correct me if I'm wrong here please, because my knowledge is more osmosis learning then having really looked into it).

The drying stress that results in collapse is caused by (and again I'm a driver not a mechanic when it comes to wood drying - I can put it in gear and drive it, but that doesn't mean I fully understand what's happening underneath the hood) very dense timber that naturally forms a steep moisture gradient during drying acting against the high shrinkage rates of certain timber species. Basicly the dry externals of the timber are trying to shrink a lot over the core wood that has barely started to lose water. By introducing wet steam we can soften the lignin in the cell walls in a similar fashion to that used for steam bending of lumber... which allows the "deformed" cells on the outside to reshape, and lets the bound water from the core flow through the cell walls to bring the piece of timber back into a balance between core MC and wall MC. This gets rid of the washboard look, and done at the right point(s) during the drying cycle prevents the internal honeycombing like that shown in Ianab's picture by reducing the stress level before the fibres rupture.

To my mind there isn't a lot of difference between the two processes. You're dropping the pressure to "boil" the water off at at lower then atmospheric temps, but wouldn't that water jacket heat/ low pressure environment also soften the timber to let the water flow through the cell walls rapidly? Or I guess more properly my question should be: What's happening inside the wood when you're doing this that allows the bound water to migrate out so quickly. You can do in a few weeks what would take me a 4 or 5 month kiln cycle in the thicker cross sections.

[longtime lurker]

LL & Den:
Would either of you have pictures of before and after as I have no idea what you are talking about when you say - "timber collapse in DH kilns".
Gerald

Ianab's picture pretty much covers it, though if the wood is reconditioned using wet steam at the right point in the drying process then the internal cracking won't happen. The timber "washboards" because the inside is trying to shrink with drying while the inside of the piece is still full of water. Reconditioning lets it come back into M/C balance between the external surface and inside - basicly it looks "normal" agan

Page 118 of this link starts a chapter about reconditioning if you're at all interested.

http://www.utas.edu.au/__data/assets/pdf_file/0004/267817/best-practice-drying-part-2.pdf

[GAB]

Ianab:
Thanks for the education as I had no idea what you folks were talking about.
Thanks.
Gerald

[Den Socling]

Here's another example. 4/4 Black Walnut is easy to dry, right? Not if it's from Peru.



 

And I did that with a vac kiln. Very strange stuff. Some will dry normally and, in some, the cellular structure simply collapses.

[longtime lurker]

Here's another example. 4/4 Black Walnut is easy to dry, right? Not if it's from Peru.



 

And I did that with a vac kiln. Very strange stuff. Some will dry normally and, in some, the cellular structure simply collapses.

Well that answers that question. I was thinking that when you pulled your vacumm/ added heat that it would have reduced the softening point of the lignins as well. With steam reconditioning we air dry to around 20%MC, then apply wet steam at between 194 and 210 degrees F for a couple hours (time depending on thickness), then proceed to the kilns as per usual. The timber seems to pick up around 2% MC during the steaming process. With really collapse prone species it might be necessary to do another round of steam reconditioning at the end of the kiln cycle, in which case we'd take it 2% below target then steam it back up.

We have the same issue with variable collapse. Same species off a different location and it might collapse more or less.... or in some cases not at all. You can usually pick collapse prone stands by the simple fact that they are largely intact - but individual trees in a stand can vary considerably too and I've never met anyone who could tell the good ones from the bad on the stump.

I'm just going to have to invest in a steam generator.

[Den Socling]

Since vacuum drying is "cold", I thought the lignin would be strong enough to withstand hydraulic forces.

[GeneWengert-WoodDoc]

The historical definition of collapse includes the concept that the liquid water cannot get out of the wood cell.  Sometimes this is due to bacteria blocking the cell.  A "normal" cell when freshly cut will have both liquid water and an "air" bubble with water vapor.  As the vapor moves out of the cell, through a narrow capillary opening, it creates tension related to surface tension, etc.  This tension is so large that it pulls the opening in the cell closed, which is what we call collapse.

Collapse can almost 100% recovered by steaming at the end of drying.  If not recovered, a water-based finish or even high humidity will result in some recovery and uneven surfaces.

[Den Socling]

Suppose those cell walls are weakened by bacteria. Do you think vacuum drying could create a pressure differential strong enough to collapse the cell walls?

[GeneWengert-WoodDoc]

Cause?  No.
Increase slightly the extent of collapse?  Maybe

The theory discusses the force from vapor tension in small capillaries, but in a vacuum we have mostly mass flow at higher MCs, so I do wonder if vapor tension is much of a consideration in Vacuum drying.

[mesquite buckeye]

Some of the red gum eucalyptus I have cut can collapse by as much as half. I've never seen it pop back from getting wet. Steaming maybe, I guess that is what they do with it in Australia. I'd like to see how much they can recover and actually see photos of it. It seems like the collapsed pieces have lower initial density than the ones that shrink less. I can feel it when I run them through the jointer.

Gene, with american woods that collapse, do we see lower density boards having more problems?

I have had elm and persimmon with this problem also, but maybe not as bad. :(

[longtime lurker]

Some of the red gum eucalyptus I have cut can collapse by as much as half. I've never seen it pop back from getting wet. Steaming maybe, I guess that is what they do with it in Australia. I'd like to see how much they can recover and actually see photos of it. It seems like the collapsed pieces have lower initial density than the ones that shrink less. I can feel it when I run them through the jointer

Figures 30, 32 and 33... down around page 40 or so.... have before and after pictures from a trial several years back.

http://www.crcforestry.com.au/publications/downloads/TR200-Harwood.pdf

I'm fortunate in that the really collapse prone eucalypts all grow well south of here. I hit two species that collapse, but not to the extent of the southern stuff. I've played around a bit in house... backsawn and quartersawn patterns, reducing the rate of initial drying, throwing boards in a creek to return moisture etc.
Generally speaking I've found the following:
Both backsawn and Q/S will collapse, however Q/S will recover following reconditioning far better. This is in line with the accepted practice for the high collapse eucalypts of southern Australia.
Slower initial drying reduces the amount of collapse, and the slower the better. I keep a wet burlap screen across in front of packs of freshly sawn timber, and keep the wettest packs to the back behind that screen. I've not done a measured trial or anything but I think it makes a difference to the degree of collapse. Collapse is still happening though.
Badly collapsed boards that I threw in a creek for a few months recovered. Then collapsed again during the next attempt at drying. Lets call that a resounding fail. :D

I always wondered when I went through the west coast and southwest why of all the eucalypt species that they could have planted, they chose to plant species that were difficult to dry. For the plantings in northern California I get it... colder climate and all. But for down around LA and points east of there... they should have just stuck with Forest red gum or river red gum - not that high collapse stuff.

[mesquite buckeye]

Thanks LL. I only have experience with Eucalyptus camaldulensis, but that one, at least as it grows as an urban tree, is extremely tough to dry and to keep it from turning into cracked pretzels. I had to tarp and mist mine for a couple of weeks just to slow down the drying to control the cracking. Then, when I broke down the stack, some of my 1" boards turned into 1/2" washboards. If I were to do it again, I would mill in the winter and cut the lumber extra thick. My thicker material had less problems than the 1" stuff, and even if they collapse there is still something there to work with. Too bad, as the lumber has gorgeous figure. :-\

[JustinW_NZ]

I mill a lot of eucalyptus here for flooring.

Small stuff is the most annoying, and stating the drying over the winter months does help.
Otherwise we quartersaw everything and leave no sap wood.
If the air dry stacks and placed out of direct sunlight that seems to help a lot as well.
I kind of think you just have to use ALL the best practises and then your recovery should be good.

We edge and grade everything after drying too. (down to 15% moisture then it gets kilned and finished)

Cheers
Justin