Drying Wood

[GreenWood]

I gave the topic title a broad definition so as to leave anything open to discussion .

In this post I use it for ideas for making a vacuum kiln ... I follow on from a post I made here ( viewtopic.php?f=2&t=113992 ) for not wanting to take over that topic. There is a video of a simpler one there and some woodwork talk.

Anyway, looking to see if a thought up design idea that doesn't need a pump might be made, I found a video that explains what I meant better as well as of not nescessarily needing a pump.

https://m.youtube.com/watch?v=i0Frx_bPsT8


Or it can be understood via

https://www.sugarprocesstech.com/vapour-pressure-water/


Obviously a strong vacuum pump could also be used to prime, and the system would need purging a few times as air or volatile gases evaporated from the wood and lifted pressure. The aim is to vacuum dry a flute length piece of wood in a few weeks, yet another project for when time permits . If my design works I will share it here, and in the meantime the above gives some ideas on the topic. Just ideas, some in the previously linked discussions are quite secretive on design, which is fair enough but on the other hand if ideas and designs aren't shared then all moves forward much more slowly, and I have found many projects of others simply stalled without anything learned being available to others.

[paddler]

That video vacuum kiln video was well done! I have not built one quite like that, but have used a vacuum chamber as part of my system
for resin infusion ... a process that also requires a method for obtaining extremely dry wood. I won't go into a lot of details here about
precisely what I do, but I can share a few observations.

1. I think the crux issue for using a vacuum kiln to season green wood is how to keep the moisture from contaminating the vacuum pump oil.
Solving that problem is not that easy. In my approach I avoid that problem altogether by pre-seasoning my wood, using a variety of conventional
methods, ahead of time, and then doing a cooking phase to remove the last tiny amount of water before using the vacuum chamber for the
resin infusion stage.

2. It is fairly easy to achieve the required levels of vacuum, even with inexpensive pumps, but maintaining those levels seems to require the
pump to be continually running. If the pump is switched off the level of vacuum drops, perhaps through leaks in the pump itself, but also
perhaps just due to slow extraction of moist air from within the wood. When you leave the pump running this isn't a problem, and the pump
isn't working very hard either, but it does mean that moist air that comes out of the wood passes through the pump continually. This can
mess up the pump if the wood isn't already dry.

3. Heat is not necessarily the enemy of wood. For example, I can cook billets at over 200 F for several hours in a convection oven and have
virtually no cracking, so long as the wood is not too wet to start with and so long as it is preprocessed, for example, turned to cylinders, perhaps
with a small pilot bore. Cooking rectangular billets is much less successful. Similarly, microwaving billets in a sealed plastic bag repeatedly for
short periods of time can work. So, as a final stage of drying cooking can work quite well. One of the challenges is finding a convection oven
or microwave that has dimensions sufficient for your billets. Or making your flutes from billets that are sufficiently small.

4. I'm still not convinced that moisture removal is the only reason to season wood. I think it is important to process flute wood in a sequence
of stages, with sufficient time at every stage for the stresses in the wood to release. Generally, the acts of splitting a log in half, milling the
half logs to rectangular billets, and turning a rectangular billet to a cylinder, allow for stress relief in the wood. When left in a particular
form for a while the wood relaxes. I mention this to emphasize that it is not just about getting the water out in order to obtain stable wood.

5. I'm not convinced that zero moisture content is the goal for optimum flute making. As preparation for resin infusion it is important to get
that moisture out, but for general stability in a flute you want the moisture content of the wood to match that of its environment. Ultimately,
assuming non-stabilized wood, the moisture content will match that of the environment, and this is what leads to instability in the dimensions
of a flute. So, if you make a flute from totally desiccated wood and then ship it to a location with a damp climate, the flute will swell and warp.

Maybe much of the above is not directly relevant to what you are trying to do, but I think it is worth considering before committing too much to
a specific design direction.

[david_h]

A looong time ago I was in a workshop where someone else did vacuum impregnation. My memory is that the pump didn't run continuously but cut in every now and again, presumably via a pressure switch. There was a water trap before the pump, which I think used dry ice. The samples had to be dry otherwise they disintegrated and I think they were dried over several hours in the same chamber but with not-so-low pressure.

I have often wondered with vacuum impregnation if the resin is soaking in all the time or if it mainly moves into now empty spaces as atmospheric pressure is restored.

I don't follow Greenwood's idea. Would a chip of wood in the 100% water vapour in the top of that jar do much drying? The boiling in the video is impressive but that's 'just' because there is a body of free liquid with nucleation points in it. Most means of drying something in air involve contriving the temperatures and/or pressures that water molecules leave the thing to be dried and are somehow taken away, In the jar they are coming out of the liquid in the bottom because others are condensing onto the colder lid. The drying cycle in a dishwasher (mine at least) does the same thing at atmospheric pressure - cold water is pumped inside the metal walls whilst the dishes are still hot.

[Terry McGee]

Paddler:
4. I'm still not convinced that moisture removal is the only reason to season wood. I think it is important to process flute wood in a sequence
of stages, with sufficient time at every stage for the stresses in the wood to release. Generally, the acts of splitting a log in half, milling the
half logs to rectangular billets, and turning a rectangular billet to a cylinder, allow for stress relief in the wood. When left in a particular
form for a while the wood relaxes. I mention this to emphasize that it is not just about getting the water out in order to obtain stable wood.

That's my feeling too, Paddler, and I draw attention to the word "season", which you've used and I've highlighted above. Certainly, when freshly cut, timber contains much more moisture than we want, unless perhaps you live in a swamp. But I don't think it's as simple as drying that moisture out. When timber dries out, stresses are built up, unless the timber is "seasoned". The ultimate example of this perhaps are boxwood flutes that, after finishing, start to bend. And bend, and bend. And then stay bent. Great for playing around corners....

A worse example of course is flutes that split. Look at all those flutes made in cold, dank 19th century London, with metal tuning slides pressed securely into heads and barrels. Brought out to India, the US and Australia, they split terribly. Not surprisingly. London has a humidity around 70%. The other places I mentioned around 35%. Bang!

The notion of "seasoning" - a term used by cabinet makers - is that the timber should be forced through a series of cycles - wet, dry, wet, dry, etc - to relieve any pent-up stresses it may harbour. In the simplest situation, this means leaving it outside under cover for a few seasons (that's where the term comes from!), before bringing it indoors to acclimatise with indoor humidity in their zone. But that was OK for the local cabinet maker making furniture for local use. That's not our situation. Imagine you are a flute maker working in say coastal Seattle, or West Clare, but making a flute for a customer in Arizona or Alaska. Dramatically different environments! The timber should at least be dried to approximately the customer's mean humidity levels. But, ideally it should be "seasoned" to that level. IE it should have been cycled to below and back above those levels several times. That may not always be practical, but it should be our aim.

[GreenWood]

I will pick up on the below Paddler

"4. I'm still not convinced that moisture removal is the only reason to season wood. I think it is important to process flute wood in a sequence
of stages, with sufficient time at every stage for the stresses in the wood to release. Generally, the acts of splitting a log in half, milling the
half logs to rectangular billets, and turning a rectangular billet to a cylinder, allow for stress relief in the wood. When left in a particular
form for a while the wood relaxes. I mention this to emphasize that it is not just about getting the water out in order to obtain stable wood."

as well, because it brings in a lot of what modern kiln drying is said to involve. I don't have much experience of drying wood, only some of how it behaves at different dryness levels. These are just some of the variables I have come across:

With planks during air and kiln drying, stacking reduces warpage.
Tension wood might continue to act even after seasoning
Kiln drying profesionally in ovens involves careful monitoring of moisture content and heat levels, timing.
The effect of heating the wood in a kiln changes the structure of the wood itself as water ruptures the matrix it is held in, vacuum might do similar (this from a definite scientific study).
Normal seasoning does not guarantee wood will not later move.
Objects dried by simple evaporation (i.e. heated in a kiln) change shape more than those vacuum dried (example given somewhere was fruit, shivelled for air dried, more as original when freeze/vacuum dried)


My main aim would be to relieve stresses on any wood. Particularly, I would like to reduce warpage if using very fresh wood, by drying it some first. To now I usually leave green wood a few weeks to dry at least, then another several days after drilling pilot bore. By then it has usually started to warp a little, and I choose a mandrel for the lathe which fits the bore still (e.g. 10mm for now curved 12mm bore) ...and hope it doesn't curve much more. Sometimes it does. I wrote previously on choice of wood, and also have quite a large stock of wood now that is seasoning properly ... but it would be nice to be able to make a flute out of fresher wood with minimal warpage, and I figure if I dry say a 5 or 7 cm diameter billet by vacuum to say 15% before boring it would introduce most of the distortion beforehand ?

The platter/bowl presented in that first video was quite amazing as stable ... but it is not a flute , which will rehydrate and dehydrate quite a lot as it is used . So that is not a proof as far as flutemaking is concered, unless later resin/oil saturated, well sealed, or of resinous hardwood.

Apart from that though it is just fun, to see what is possible and how that plays out in practice. Definitely there is a lot more to it all than meets the eye though. I remember that discussion of hundred year seasoned boxwood ... it had me searching up antique furniture that might be made into a flute. There was a 19th century coat-stand on offer somewhere if I remember.

As Terry says, and as you say Terry, the cycling is important also, and I might even try something like that, or try clamping the wood as it dries and see how much it keeps to a certain form afterwards, and so on.

Definitely there is an art to preparing wood, a lot of skill and experience is needed in choice and method used, and in judging at what point any wood is suitable to purpose. On the other hand, if room is available for changes of dimension and so on, then there is not so much need to be so exacting. It depends on what is being made, as well as what finish is acceptable. Cracked flutes are not the idea, tuning slides aren't nescessary, warped flutes are not appreciated by some, re-reaming own made flute is not too hard, a bought flute which then goes (and stays) out of tune is not really acceptable to many... etc.

david_h

In that experiment the water that boils represents the wood. The top of the lid represents a cooled chamber where water condenses after evaporating from the wood.

[david_h]

david_h

In that experiment the water that boils represents the wood. The top of the lid represents a cooled chamber where water condenses after evaporating from the wood.

How do you set up the initial conditions with wood? I will await your design with interest.

[GreenWood]

david_h

In that experiment the water that boils represents the wood. The top of the lid represents a cooled chamber where water condenses after evaporating from the wood.

How do you set up the initial conditions with wood? I will await your design with interest.

Very roughly as so



Tap open, boil water till super-saturated atm .

Close tap, cool water.

If atm forms from air in wood, repeat.

Pressure and temp gauge lets you know if atm has formed, temp wood is etc.

The rest is just however long it takes the water to leave the wood, remembering it is going to be trying to boil off, i.e. hopefully relatively fast. Wood should be heated probably, various ways to do so...clear chamber then in sun is probably easiest.

This is an untested design, there are variables which are better answered by trial and error. No guarantee how well it will work, so go ahead anyone with the idea as they feel to, will be a while building and trying out my own probably. As usual, I waive rights and patent, offer free use of the idea as long as it does not cross any claims pre-existing this post 20/12/2022.

[Geoffrey Ellis]

I have often wondered with vacuum impregnation if the resin is soaking in all the time or if it mainly moves into now empty spaces as atmospheric pressure is restored.

The resin soaks in once the vacuum is released. When under vacuum, all of the air and moisture in the wood is being pulled out (though a physics buff explained to me that it is actually being "pushed" out, but I don't think that distinction is important ). So for as long as the wood is under vacuum the resin cannot effectively enter the wood's cell structure. My own practice (one that was recommended by the guy I get my resin from) is to release the vacuum and let the wood soak for a bit. I only use wood that is already very dry. I use the same method that paddler uses (with a minor difference, which is that the wood I use is already kiln-dried lumber in most cases). I then turn this into round billets and then bake it for 24 hours (or more) at 205F. Then I remove it, put it in a plastic bag to cool (where it can't reabsorb atmospheric humidity). Once cool, I drop it in the resin bath inside the vacuum chamber and put it under vacuum. Usually 12 hours is enough to pull all the air and any trace moisture out. When I can't see any tiny micro-bubbles coming out of the wood, I release the vacuum. Then I let it sit in the bath for 48 hours (or more). Then the heat cure.

[david_h]

Thanks Geoffrey Ellis. I don't think the physics is important once you have a successful process. Your physics buff would probably also complain about "releasing the vacuum". You are letting the air back in - to push the resin into the wood, though I guess surface tension is important too. I don't think these things matter much in conversation - I guess some physicists my quibble at the idea of wearing warm clothes 'to keep the cold out' but it doesn't matter until one starts to wonder how reflective 'space blankets' work.

I think the physics is important when inventing something and I have doubts about Greenwood's idea. Physics aside submitting the wood to 100% humidity at a higher temperature than Geoffrey Eliis or paddler mention for rapid drying seems as strange starting point. Also, I was told that one reason some samples disintegrate under vacuum drying is water vapour bubbles forming within the wet material.

[GreenWood]

Ten minutes of steaming is not going to do much to already saturated wood david, and even if it resaturated drier wood, the process of dessication will just continue that much longer that it takes for any added water to be condensed back into the cooled chamber. My questions are more along the lines of what vapour pressure differential is needed to be effective, and how much outgasing of non water vapour the wood will demonstrate, and over what timeframe. That is something practical trials will answer better I think.

[paddler]

What is the purpose of the water in your chamber greenwood? And what is the purpose of heating that water?
Are you trying to create a steam chamber, such as is used for bending wood. Is your idea to relieve wood stress
that way?

It does seem counter to the idea of drying though. I thought the idea of vacuum drying was to lower pressure to the
point where water in the wood could evaporate faster (boil even). If you have a pot of water in the same chamber,
surely that will evaporate and saturate the atmosphere, thereby making it less effective at drying the wood.

Basically, I don't understand how adding water will help dry the wood. But maybe your idea is to repeat cycles of
streaming and vacuum drying in order to ultimately both dry and destress the wood???

[GreenWood]

The water in the chamber is there to be boiled . The steam created will push out the air in the chamber.You are then left with an atmosphere of water vapour only, and the tap is closed. As the water chamber is cooled, it absorbs vapour by condensation. The chart linked above gives the pressure that closed system will settle at, at any given temperature, that is the vapour pressure. The colder the water chamber is the lower pressure in the main chamber will become, as the cooled water chamber absorbs and holds the water vapour. At one degree centigrade you will have a near total vacuum in the whole chamber, 0.00657 Bar

However, in the chamber you have a piece of wood. If the temperature of the wood (and so the water it holds) is higher, let us say thirty degrees centigrade, it wants vapour pressure to settle at 0.04241 bar . At that pressure it will stop steaming/boiling off water.

So it starts to steam/boil water as the pressure is pulled down towards 0.00657 bar by the cooled water chamber. All the time the cooled chamber is absorbing this new vapour though, pulling down pressure to below 0.04241 bar, and the wood will keep releasing water until all has collected via condensation in the cooled water chamber.

This is not diffusion, like dessication through air. With dessication water has a surface affinity to air, is collected by it and must slowly find its way to the dessicant so that the air might be recharged with new vapour. Instead here the water is continuously "pulled" out , basically low temperature steaming/boiling within and at surface of the wood .

This is a perpetual mechanism, which would only be interrupted by formation of a new atmosphere that did not condense into the cooled water chamber (by for example air stored in the wood) , so allowing pressure to rise to above that at which the water of the wood would steam/boil at.

But

How effective at different temperatures this would be is unknown for now. The wood might dry in an hour, a day, a week or a month for all I know.

How much new atmosphere emerges from the wood is also unknown, and that would nescessitate the chamber being purged of it by the process of boiling again.

[paddler]

Ah, I see. So the main point is to avoid having to use a vacuum pump, by repeatedly heating and saturating the air with the valve open
and the letting it cool with the valve closed, thereby causing the atmospheric pressure to drop. You could potentially do the same thing
without the water, but having the hot air saturated may help prevent the wood from cracking with the heat. This is similar to what you
do when you microwave wood in a plastic bag. In that case, it does seem to protect the wood a bit.

It will be interesting to see how many cycles, and over how long a period, it is necessary to do this in order to achieve significant drying,
and also whether the steaming actually helps with relieving stresses.

I agree that it would likely help to have the wood heated somewhat, but this could be achieved using a heating mat like the ones used
in the vacuum kiln video.

Basically, it is an approach that could be used to accelerate seasoning. It will require work, for the repeated <release valve, heat chamber,
close valve> cycle, and the amount of work you put in will influence how fast the wood dries. Faster may not always be better, so you could
tune your effort level to the circumstances.

[GreenWood]

No no, it isn't cyclical.

Maybe I should try to explain the physics better:

"This is not diffusion, like dessication through air. With dessication water has a surface affinity to air, is collected by it and must slowly find its way to the dessicant so that the air might be recharged with new vapour."

This was not well explained, because the visual model I was using was air as a medium, I still see it as one . It is heavier than water vapour (vapour rises through air, hence tap placed underneath for venting air) e.g.

https://www.theweatherprediction.com/habyhints/260/


So in a relatively static setting of a chamber the water vapour must travel through the air . I used the analogy of wicking, because I imagine water having to travel across the medium and I counted air and space between molecules as eq. to say a solid, say a wick. That is not absolutely correct though it is good enough to explain the difference between a "direct path" to the condenser and one through air.

So here is a fuller explanation

https://www2.atmos.umd.edu/~stevenb/vapor/


It explains that water vapour under normal conditions finds its way through the spaces in air. However the picture even there is confused. The water is so said to act like it is in a vacuum , with vapour rising to fill spaces in the air until vapour pressure equilibrium is reached, and that air is then considered saturated. There is no mention of how the air inhibits that occuring, the closest I have found so far is that when there is a weather high pressure system water evaporation is lower at earths surface. Alternatively if you try the experiment in the video above with air present in the space, it will not work.

So there are different ways to understand how the idea works:

It could be understood that air slows the pressure loss at condensation chamber transfering to wood evaporative surface.

It could be understood that air slows the movement of water vapour from wood to condenser.

It could be understood that air pressure slows evaporation from wood.

It could be understood that having only water vapour as atmosphere allows high rates of condensation at condenser.

And so on.


The main point is that with only water vapour as atmosphere, condensation at the condenser leads to such a pressure drop in the system that water in the wood would low temperature steam/boil out. That means much faster drying.


In theory , though the video experiment above was enough to confirm the idea as workable to me.

[GreenWood]

The other side to it all, as in what results different ways of drying bring, is interesting also, if only because there does not seem to be much concensus or else because information is not much published or available.

https://www.practicalmachinist.com/foru ... od.144564/

is a good start to that discussion because of the number of different opinions given. They go from three years of seasoning with trunk leaned against a wall and turned every day while being sprinkled with water, through to seal ends, debark and put in front of a heater...and just about any idea in between.

The one thing that seems true is that vacuum dried wood has a different structure to other types of drying, and not necessarily worse. I suppose I must try to find examples of how stable wood is when dried that way, I mean my only aim would be to bring moisture down to balanced with room level quite quickly without splitting the wood, and being left with a piece of wood that would stay more or less that shape afterwards, for flutes I am not sure I would want to use seasoned wood for. I guess most people would just go and buy their wood dried, but that is not how I go about it all.