Drying Slabs in Tropical Conditions

Advice on drying uncommon lumber species in the jungles of Ecuador. August 15, 2011

I work with a startup community forestry company in the Choco Wet Forest of Ecuador. We have recently considered expanding into the slab market, and have had splitting issues with certain species.

We have little knowledge on many of our wood properties (we're compiling data as we work with unknown trees) and I was wondering about general tips to prevent cracking. We're slabbing the logs at 8'x8"x23/8" in the woods, and cabling the slabs to a drying facility. They will be unstickered, in the forest, from two weeks to a month. The area is humid, generally 70 - 90% all year.

I have read about wax sealants and Johnson paste - is this to be applied immediately, and do you cover the slab or only apply on the ends?

Forum Responses
(Sawing and Drying Forum)
From Professor Gene Wengert, forum technical advisor:
Where are the splits? We do have excellent commercial end coatings to prevent most checking and splitting (except for splits due to stress in the tree). If you are concerned about face checks and splits, then we have to slow down the drying rate so the wood does not dry so fast. This subject is covered in most texts. You can consult Drying Hardwood Lumber

From contributor H:
Anchorseal 2 is a water-wax emulsion made by UC Coatings (www.uccoatings.com). Paint a heavy coat on freshly cut ends. Bowl turners working with green wood often paint the entire bowl blank or partly turned bowl to make it dry more slowly. I paint it on the sides of boards where there is end grain due to a branch or other types of grain which look like it might split. If you've coated the ends of logs, then after sawing add a coat to all four faces of the boards about 2-3" in from each end. Sometimes one can get some more on as a second coat, sometimes not, but I would try in the case of your board sizes. If it rains before Anchorseal dries, then some or all of it will wash off. But not after it dries. You need to keep your boards out of the sun (the ends for sure) and as wet as possible, but not so wet that staining from mold, mildew, fungal growth, etc. happens in the 2-4 week period. This until you can get to their proper drying or resawing.

From contributor T:

With 70 - 90% RH, the end checking will be minimized a great deal once you seal the ends, and also make sure you keep them out of the sun. I can't imagine they're getting too much wind - probably not enough to prevent mold or mildew even - are you having a problem with that? Since they aren't stickered, are you dead-stacking them or just letting them lie on the forest floor individually? Is the ground still covered with vegetation or is your operation grinding the ground to bare earth?

What are the species giving you the most trouble? As the Doc said, you won't be able to do much about the stress cracking, but the checking due to rapid moisture loss is easy to control - except of course in some species. We have species up here that present drying challenges almost no matter what you do. Pecan and sweetgum come to mind.

Wood doesn't begin to shrink until it's reached the FSP (fiber saturation point), which for most species is roughly 30%. A month is a long time to leave them lying in the forest at 70% RH, and even at 90% if they aren't sealed at least at the ends and anywhere that figure, knots or other open grain characteristics exist, then the water will leave those areas much faster than the other parts of the wood and you'll see movement, i.e. warping/twisting/cupping/bowing/checks, etc.

From Professor Gene Wengert, forum technical advisor:
It is true that wood does not begin to shrink until it is at about 30% MC (many say 28% MC and for tropical timbers it can be 22% MC). This is in reference to an individual cell or fiber. So freshly cut wood that has the outer fibers dry to under 30% MC will begin to shrink even though the average MC for the entire piece is 65% MC (give or take). So, wood does shrink above 30% MC if that is an average value for the entire piece.

Regarding the original question, the sooner you can get end coating on, the better. In fact, applying Anchorseal or similar in the woods is not a bad idea, especially with your month long storage of the slabs. Otherwise, as soon as the slabs get out of the woods, coat the ends. Of course, if in transit, the slabs are exposed to sun, high wind, and/or low humidity, you can get some face checking that will only get worse as drying continues. Try to prevent end checks and face checks from developing in the first place. Such checks will often develop within 24 hours after the surface is exposed. Once checks form on the ends, end coating is of limited value.

From the original questioner:
The splits are generally on the ends. We have some wider slabs - 16" to 32" - that have cracks running all along the center, and some that have fully split out. The species we're currently sawing are endemic to the area - the hardwoods have no problem with mold, but the softer woods show signs of fungus and boring insects.

Appreciate the advice on the end coating. Not sure if Anchorseal is available in Ecuador, but I'll look for it or a similar product and apply it immediately after sawing, 2 to 3 inches in from the ends.

You would advise stickering the slabs/boards we process onsite? We are operating with a Peterson sawmill that we cabled in. The slabs are currently ripped by chainsaw, but we're considering the purchase of a slabber attachment. With the Peterson, it would be relatively easy to convert a low value tree to stickers.

Sun, wind and low humidity are rarities. It takes three to seven days to air-dry my socks. Which brings me to another question - how long would you suggest pre-drying our large slabs at 8' long, with a depth of 2 3/8" and a width of between 16" and 50"?

Thanks for the advice and I'll keep bothering all of you as I encounter new problems.

From Professor Gene Wengert, forum technical advisor:
You will have to air dry for as long as it takes them to reach the desired moisture content. There is no way to predict drying time for species that are not used typically. In any case, you will likely find that the best air drying in such a humid condition is end racking (also called A-frame air drying with no stickers) with a small roof to prevent rain wetting.

Incidentally, many species do cause allergic reactions (skin and nasal oftentimes), so be careful with these less common species.

If Anchorseal is not available, you could use paraffin wax, but there is a high fire risk. Some non-latex paints provide limited protection, but for thick valuable pieces, paint is not a good answer.

The wide pieces with face cracks are most likely due to the close proximity to the pith (center of the log) and perhaps some quality loss due to bacterial activity in the living tree. If it is indeed bacterial, you will notice a foul odor as well as the weakening of the wood.

Fungal staining is controlled by fast drying (which you do not have) or by chemicals designed for fungal control in freshly sawn lumber. Likewise, new insects are controlled with the same chemicals, but existing insects cannot be controlled. Hence, it is important to avoid long storage in the woods, as the insects will enter the wood and then you cannot control them and they can do quite a bit of damage internally.

Questions on any of these items?

From the original questioner:
I've seen the A-frame stacking at furniture production shops in the area. Will board end contact with muddy soils present a problem?

We considered using a boro-borax solution for insect control. I've heard it easily washes off with rain. We can keep the wood covered, but it's likely to get rained on at some stage, in transit on the cables, etc. Do you have other suggestions for protecting fresh sawn lumber from insects and fungus?

Any suggestion for decreasing the chance of splitting when sawing slabs? Are staples or cleats effective in holding pieces together? As contributor T said, there will be some species that just won't work, but until we identify those I'd like to salvage what we can.

From Professor Gene Wengert, forum technical advisor:
With a-frame stacking, put something on the bottom to keep the ends out of the mud... usually a termite resistant species of wood.

Cleats have been used with limited success.

Borate does indeed wash off in heavy rain, as might other chemicals too.

You can dry any species, but some may require a special effort or technique (such as covered shed drying). We never had success with tropical walnut. Be aware that some species have silica and quickly dull tools.

From contributor D:
An interesting idea I recently came across is a vacuum kiln. It uses very low heat (about 110 deg) and a vacuum chamber. I have found this to be the best way to dry thick slabs with almost no checking. It dries 10 times faster than traditional kilns. It takes 2 days to dry a 16"x16"x16" chunk of walnut to 7%. Large industrial vacuum kilns are expensive but might be a good return if you have a good supply of slab wood.

From Professor Gene Wengert, forum technical advisor:
16" x 16" in 2 days? That is amazing. It must already be dried to 12% MC before it goes into the kiln. That time is not what I would expect for green material.

Can you find out how many kWh it uses per % MC removed?

I do wonder about the uniformity of incoming material... thick and thin, heartwood and sapwood, etc. Will a vacuum get all the material to a uniform MC? This was a problem with many kilns in the past decades.

From contributor S:
I agree with Gene. Drying a 16" x 16" x 16" chunk in 2 days must be a typo. Only a RF/V kiln could come close and, as Gene says, the MC would not be uniform and there would be honeycomb and surface checks. I wish we could dry a piece like that in 2 days!

From the original questioner:
What is the general consensus on vacuum kilns? It almost seems too good to be true. If it can do as promised, we could look into contract drying after building up our inventory with a wood buyer in Pennsylvania. We are a bit concerned about throwing high value slabs in with our current lack of knowledge on proper drying schedules. Seems a bit risky without a baseline.

The wood we're bringing down has bacteria and fungus. Is there something we should treat it with prior to putting it into pre-drying? Would planing it green be a good idea, or an unnecessary waste? We do not have a treatment pond - thinking of one for the future.

From contributor T:
Vacuum kiln drying is highly specialized - in the construction, in the software, and in the operation. Many companies have come and gone. I use Den Socling to dry thick and expensive slabs, and I can vouch for his ability to do what he says he can do, and not to promise anything his kilns can't deliver. There are other companies you can deal with, but you just can't go wrong dealing with Den based on my experience and that of others I personally know.

From contributor D:
Sorry - I was not trying to be misleading with my last post. I was typing too fast and left out that the (16x16 slab) was a bowl blank that was roughed out to about inch thick walls. I did not check the moisture level on this particular piece before I dried it, but water was slinging off it when I was turning it. It only takes 2 days at 25inches of pressure and a 100watt light bulb, which keeps the temp at 105 degrees.