Drying Wood

[paddler]

I thought you might benefit from doing this cyclically for two reasons. The first being that the repeated steaming of the wood would be
an effective way to transfer more heat to the wood, hence accelerating the drying. And the second being that each time you reheat the
chamber you can establish a stronger vacuum than at the end of the previous cycle. Once you establish your vacuum (or whatever percentage
of vacuum this is able to obtain) the moist air trapped inside the wood will still be at higher pressure, but will start to slowly escape from
the wood and over time will weaken the vacuum in the chamber.

Have you seen this paper which discusses the state of the art in vacuum drying wood and also discusses a super-heated steam approach in
which heat is transferred to the wood using steam to accelerate the drying process.

https://link.springer.com/article/10.10 ... 016-0045-9

[waltsweet]

I drill a little undersize, turn a little oversize, use an EasyBake over for a week or two, then let it rest for several days in ambience.

[GreenWood]

Thanks Paddler, and that is a sensible approach as well. I have read that paper as I am sifting through all I am able to find online at the moment, and will present a list in a short while of the best of those, with their main take aways.

Where you say

"And the second being that each time you reheat the
chamber you can establish a stronger vacuum than at the end of the previous cycle."

the paper mentioned says

"There are two distinct phases in cyclic vacuum drying: an initial rapid drying and then a slowing down of drying as the pressure inside the material approaches the ambient pressure"

Which are both based on differential of pressure. I think the vacuum achieved is the same, it is just that as the wood cools it does not exhibit so much pressure vs vacuum as when just heated. That is to say the model isn't so much pressurising and depressurising the wood itself, but of heating it efficiently to a level where once a vacuum is applied that it basically boils up more pressure, until wood temperature drops. At that point it won't boil so easily or so much in that vacuum. As you say, it is fine details and depends what level vacuum is obtainable as well as how the wood behaves at different temperature and chamber pressure. So at first it will be trial and error, but my aim would be the simplest. I would be happier steaming to allow a vacuum to form once and leaving it for a month to dry than steaming it every day for ten days...but that is just own priority etc.

Anyway, there is much literature on drying wood but the one thing I am looking for in it all is very evasive. That would be how to end up with a natural dimensionally stable piece of wood that only changes dimension with humidity predictably (i.e. not curving or warping in the process). There is lots on de-stressing local stresses built up while drying and so on, but nowhere seems to offer a straight answer on how to naturally "set" a piece of wood so that afterwards it behaves. Steaming is one way that is sometimes used though, and not just as regular conditioning after drying. All that I will include in the list.



Many thanks Walt, that is really the sort of information I am looking for. Though it does not answer the question of possible warping (which I am not sure has an answer) it does answer as a way to dry wood in a reasonable space of time. When you say EasyBake , well after searching for a brand I came across

http://www.michaelbillecichairmaker.com ... oodworking

which was a nice site to land on. So I am assuming low heat cabinet powered by light bulbs. I will have to think that through. Pre-boring will help to avoid checking (and is easier on green wood) , but a main problem is that I am using branches that will quite often change shape. I suppose small pre-bore, then re-bore if no longer straight after drying, then turn. Gene Wengert from Woodweb says drying too slowly (i.e. air drying) often causes more warpage, so the oven would help overcome that. Possibly steam drying fresh wood down to FSP (as below from another forum) might help avoid checking also for very wet wood, but that starts to get a bit complex and I am trying to avoid high heating of the actual wood (for no particular reason, just that I feel happier with less altered in whatever way)

I will have to try both ideas now




"My understanding of steam drying is this:
It is not a final drying, but a lowering of moisture and oils content prior to shipping. It reduces the years often spent waiting for wood to become dry enough to put in an oast.
The steam atmosphere allows the wood to be heated up to near the boiling point of water, without creating an atmosphere where the outer surface is dryer than the inner core. Wood normally dries from the outside inward, thus creating stresses that show up as splits and checking. Since the atmosphere around the wood, in steam drying, is mostly water in the form of hot steam the wood does not split and check as much. The wood gets dryer since the water inside the wood is liquid and the water outside is steam, and the water moves to create equilibrium.The liquid water moves through the wood much faster through the softened and pliable wood fibers , thus avoiding splitting. When the steam is removed and the wood cools down, the amount of liquid water in the wood has been greatly decreased (steam is much less dense than liquid water).The wood at this point has reached its fiber saturation point (FSP). It still has water in the fiber and some inside the lignum cells, but the liquid water has been removed. The wood can then be kiln dried if a lower moisture content is needed .I believe this method is used on resinous and softer woods, like fir and pine.
Stacy"

https://www.bladeforums.com/threads/dry ... ly.506938/

[paddler]

I think that warping is influenced not so much by how the wood is dried/seasoned, but more by how the wood grew in the first place.
Wood does not grow uniformly. Its growth is influenced by various factors, such as the season, temperature, water availability etc. This
is why we have growth rings and why wood from a given species of tree grown in a dry, high altitude, area will tend to be harder and
finer grained than that from a tree of the same species grown in a warm, wet area.

But the main issue that seems to influence warping is gravity. The wood on the underside of a branch is structurally very different to that on
the topside. Similarly, wood from the trunk of a tree that is growing at an angle differs from one side of the trunk to the other. The best
wood for flute making is traditionally selected from heart wood of tree trunks that grow straight up. This wood will have the most uniform cell
structure and hence its response to changes in humidity it will be more uniform.

[GreenWood]

I agree completely on that. Though sawn lumber might warp because of distance across wood being equivalent to including different types of wood structure on one side vs the other, for branches it is the reaction wood and knots that act differently.

On one forum a writer had a three hundred year old plank of oak, and that still had enough inbuilt stress to curve it when cut. To put it differently, unless the moisture content is exactly the same as when turned, then some wood will be wanting to expand or contract more than other, causing warpage. Saturation techniques, good sealing, resinous wood, keeping flute at high steady moisture, will all bring stability of some kind. I was reading of one woodworker who would straighten planks by first steaming/heating them to be able to bend them straight, then he would brush on watery glue (or was it acrylic) on the sides that moved, which would soak in and set the plank.

Starting with evenly built wood is the main priority though. For branches that is not easy, even though I choose more vertical branches wherever possible and discard using where there is obvious reaction wood, as far as possible. There are always knots or remnants of those in a branch, and I appreciate those for the colour they bring to the flute. However they don't move with moisture like the rest of the wood.

So in a way it is partly an exercise in futility, however to make a flute that stays relatively straight out of difficult wood is a challenge, and especially without using saturation techniques . I should either be able to make the flute low moisture content and seal it well enough (as Geoffrey does), or at higher moisture content and keeping it humidified, and have it keep to its shape.

The other techniques such as setting with steam, would be a bonus if they have some permanent effect of setting the shape. That is to say, if a branch/flute is steamed to or at a shape would that set in in some way. There is not much if anything written on this, given almost all the literature focuses on setting the wood to projected (usually indoor) environment where conditions will not change much, and hence the wood will not move much either.

The other question is of if vacuum drying will act like steaming. Most wood seems set by higher heat, but I am not sure if this is due to pressures forming due to steam (in which case vacuum low temperature "steaming" might have similar effect), or if with higher heat there is some other kind of transformation that takes place, or is allowed to take place.

The effect of a long soak in linseed oil after drying very dry I might try also, but I am not sure this would not dull or change the natural resonance of the wood. I know dense resinous hardwood is much sought as material but I actually appreciate the tone of slightly lighter woods also, I find it nice or good for the whatever wood is being used to be heard.

[paddler]

It is for the reasons above that I thought you might be able to use your apparatus in a cyclical mode (at what ever frequency suits) to literally steam the wood at normal atmospheric pressure,
thereby not only transferring heat to the wood, but doing the equivalent of steam-bending to set it straight. Then close the tap and let the vacuum form as the chamber cools, kind of like
canning food. A proper steaming phase, or sequence of phases, might help accelerate the relief of stress in the wood.

It may be advantageous to clamp the wood while it is being steamed, or maybe it is better to just let it adopt the form it is most happy in.

[GreenWood]

Steam setting apparently is relatively permanent, there are three polymers that set at different temperature/MC levels, lignin being the one in the range of steaming. Steaming twice isn't recommended for bending wood as it becomes dry and brittle after first go, but for drying to shape should be ok. However later high moisture is said to allow some springback, and temperatures over 50 centigrade might see the wood return to original shape, though there is also sometimes other structural change to the wood that might not allow that fully . On the other hand some have found it stable when reheated. Vacuum drying does not reach that temperature though, but movement temperature of lignin reduces with lower pressure, and if a piece of wood is held to shape it might just set firmer that way also under vacuum or EasyCook.

What I will do I think is just try different approaches and compare how reliable they seem, both part working a flute first and just billets or branch. I might try to straighten an existing flute also. For manual drilling and pole lathe turning green wood is easier to work, but allowing a part finished flute to dry near fully (in flute terms) is always part of the making of one, even if it is just a week or two interval of air drying before finer tuning.

Most makers will probably simply start with dry relatively stable wood, but even so all the various effects still play out to some lesser degree.


I will try your idea also, at least to provide an idea of how that goes.

Allow me a moment though to put down what I am reading up on... there is a whole mixture of info, with practical advice being mostly anecdotal, simply because the nature of wood is so variable.

[GreenWood]

Browsing all I am able to find online, and still I haven't come across a clear presentation on how wood will behave after drying. There are tons of technical papers, as well as very practical explanations elsewhere, on drying, on regimes to avoid stresses and warping while drying, how those occur.

Some go technically into residual stresses without giving further practical advice in terms of wood behaviour, others explain technically how changing moisture content after drying affects the wood. However there is no study I have found so far on how to dry a wood normaly or in a kiln to produce a final set result that is stable to changes in moisture, which is what flutes experience. By stable I mean that the wood behaves predictably, and not by warping predictably.

I expect a good choice of wood would go a long way towards that, but where that is mentioned is usually from a woodworker's advice, the various ways or kinds of wood structure that introduce stresses being more a "separate technical department". In other words, there is no guide that says using this kind of wood dried a certain way is going to be stable (beyond set low humidity household conditions). There is resin saturation, forms of steaming, forms of drying and stress relief and similar that aim to produce stable wood, but for normal use in a stable environment, with the rest of the advice seeming to be more anecdotal from woodworkers who simply share the methods that they find work in practice.

So the general advice seems to be to bring wood to a dryness that is compatible with modern indoor low humidity environment before making anything, because it is otherwise more likely to split while drying further than by expanding with more moisture. Changes in diameter for normal humidities experienced in a year might be 1 % or more . For a flute that might change the bore going on half a mm, which is relevant. Dense resinous hardwood is maybe slightly more stable in that respect. A link from "Swedish Wood" below does recommend start moisture values for exposed wood (outdoors) .

Low initial moisture might not apply to a flute, because as far as maintaining a flute is concerned, keeping one at a high moisture content (similar to regular playing) with a humidifier might be preferred for various reasons.

If it was not made at low enough moisture content allowing for full shrinkage, so it might crack if allowed to become very dry.

If allowed to dry and then the bore is wetted through playing, the inside might crack the outside of the flute by expansion.

If allowed to dry and then wet often it might adjust to a shape which cracks when dried again.

Pitch will be more stable if shape is roughly the same from start to finish of playing, tone will be more predictable, tuning also.

It won't go curvy by rewetting

As I'm more focused on the last of those, that is possibly more a question of drying to correct moisture content before making the flute. I expect correct moisture content would be one where when being played it did not increase much (if I remember 15% was roughly the figure) . Obviously different woods absorb water at different rates, and amount of playing, wood type and finish would change the level chosen also, so it is a variable. Personally I find the sound better when the wood is not too humid, but others prefer the wood quite saturated (not just the bore surface, which is normal) . Possibly some woods are better saturated, or they were voiced quite saturated and so sound better that way.


My own conundrum, which only applies to the occasional flute, is the following. Most wood used is part dry after being stored a month or more, and the drilling of the bore is usually over a day or two and allows faster drying without splitting. I usually leave the wood like that a few days. Then turning is over the space of a day or two and the flute is now drying fast. I leave it a few days and tune it over the space of a week, then fine tune it a week or more later. By the time I do the initial tuning the wood is already quite dry and does not move much more.

The trouble is that after boring and while turning there is quite a lot of drying going on. If I just turned between centres over a few days, then as any shape change occurred it would be turned out. However the wood would be more likely to split without being bored, and drilling the bore afterwards would have to be sure to be accurate or otherwise it would ruin the work done so far.

On the other hand, drilling the bore and then turning with the flute mounted on a spindle/mandrel, when I take it off it will sometimes curve.

Turning a bored flute without support of a mandrel is even more difficult, because as any curve occurs it tends to get amplified by any pressures on the flute, and while turning you end up taking off too much wood from the outside of the curve to catch up with the now reduced towards centre inside of the curve. That just weakens the flute and allows even more curving.

All of that might occur even starting with fully dried wood because of uneven tensions on opposite sides that aren't evenly distributed across the wood. Even steam set wood will have built in tensions I think.


So the drying method from green/part dry has to be gentle enough not to split the wood, fast enough to be worthwhile, and either not change the shape of a bored piece of wood or else leave an unbored wood stable enough during later working. So that is what I will be looking to achieve by trying different methods, from branch straight from tree through to fully turned supported flute, using different drying methods, and all using unstable wood.

Other makers are surely faced with other difficulties or choice of order, hopefully how this all goes will help answer some of that a little.


"Art (rather than science) is really important as the scientific analysis is too complex" Woodweb


On setting and heat setting, there isn't an obvious answer.

Wood dried to shape might still move unevenly due to changes in humidity if the flute is not from a very consistent piece of wood.

Drying while the wood is held to form might help introduce some set to it, and more so if dried with heat.

The paradox here is that anyone could first dry wood to a natural shape and then work it as relatively stable, or they could work it and try to get it to dry to that same shape.

Green wood is easier to work, so the temptation is to work it first, but that leaves the difficulties of dimensional change during making and drying. Equally, wood which is not worked but stable and dried, might change form as the balance of stresses is altered as it is turned.

So the question is open, and more so if we include purposefully setting shape.

Concensus seems to be that heat setting/shaping wood is best as it is drying towards 20% moisture content. It is not advisable to heat set it more than once. Later high moisture might undo the set some.

There is obviously a difference between steam bending wood compared to drying it to existing shape. Steaming to return to original shape is questionable also, here is a video where it does on softwood

https://m.youtube.com/watch?v=YJ05Z83oJC4

And in links there is a text that says set stable.

So there is a choice of how to balance all of that out, ranging from starting with working dry stable wood through to working green unstable wood and either allowing margin for further working after drying or trying to keep it in shape as it dries. I aim to keep temperatures low in whatever method used, but will try steamed or strongly heated just to note if there is great difference.

The margins between all is actually more blurred for various reasons, most obvious being that the wood in a flute is never fully dry but fluctuating.

I don't suggest anyone try to read through all the links below, especially the technical ones (which I generally only browse over for conclusions and graphs)

For those drifting elsewhere already...

Woodwork how to plane a round

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


Another way to form wood, probably not applicable to flutes

https://m.youtube.com/watch?v=Bv5opHk3_mo



Links


Drying rates
"Regarding drying rates and defects, generally everyone understands that “drying too fast” (that is, exceeding the safe drying rate) causes an increase in the risk of more drying defects especially splits, cracks, and checks. However, “drying too slow” (drying at a rate substantially below the safe rate) also causes a risk of drying defects. In this case, the risk is for an increase in warp, stain and uneven drying."

https://www.woodweb.com/knowledge_base/ ... umber.html



Vacuum kiln pros and cons

https://www.woodweb.com/knowledge_base/ ... _Cons.html

Choice of moisture content (structural wood)

https://www.swedishwood.com/wood-facts/ ... -moisture/


Effects of moisture content variation (structural wood)


https://www.swedishwood.com/wood-facts/ ... -movement/


Remaining stress in wood after drying

"Sadly, it's just as easy to dry wood badly either way. Also, there's built it stress in almost every log, and the way it's dried won't change that. You can introduce stress by drying it badly, but that's a function of not doing it well, not of how you do it. These built in stresses do tend to work themselves out slowly over time, but the key here seems to be time and moisture cycling rather than anything else. I've got a sample of air dried spruce that's somewhere near three hundred years old that still has some stress in it; it curves when you cut it. I'm sure it had a lot more when it was fresh. For stability, then you want OLD wood." (Spruce, not oak as I mentioned originally)


https://www.acousticguitarforum.com/for ... p?t=412207


Some basic wood stresses that show while drying

https://ohioline.osu.edu/factsheet/F-72-11

PDF of schedules and variables during kiln drying

https://www.fs.usda.gov/research/treesearch/62261


Air drying basics

https://extension.missouri.edu/publications/g5550


PDF Air drying times of wood

https://www.fs.usda.gov/research/treesearch/9684

Solar kiln project. Here he graphs a month for 1" thick 30% to 15% moisture, roughly. Others I have seen give a few months to dry by solar .

https://www.instructables.com/Solar-kiln/

E.g. other drying results for solar kiln

https://www.appropedia.org/Solar_Lumber_Kiln


Wide and practical overview on drying hardwood lumber

https://www.esf.edu/wus/documents/Dryin ... gtr118.pdf



Very technical look at wood properties regarding drying

Drying of Wood: Principles and Practices
Perre & Keey

https://www.researchgate.net/publicatio ... _Practices


An overview of various vacuum drying technologies

Vacuum Drying of Wood—State of the Art
Omar Espinoza & Brian Bond 

https://link.springer.com/article/10.10 ... 016-0045-9


Humidity, Temperature, Wood Moisture Content, and Wood Movement
Basic technical information and an extended discussion of the way wood expands and contracts in response to changes in temperature and humidity. 

https://woodweb.com/knowledge_base/Humi ... sture.html


Warp in Drying
Causes and cures for warpage when drying lumber

https://woodweb.com/knowledge_base/Warp_in_Drying.html


Boiling Wood
Boiling wood before drying has various effects on the outcome

https://woodweb.com/knowledge_base/Boiling_Wood.html


Submerged Wood for Instrument Making
"Sinker logs" from the Great Lakes turn out to be ideal for making musical instruments (Add that they are talking string instruments)

https://woodweb.com/knowledge_base/Subm ... d_for.html


Lumber drying for musical instruments
Does the speed of the drying process affect a wood's musical quality?

https://woodweb.com/knowledge_base/Lumb ... ments.html



Wood Drying Conditions and Musical Instrument Sound Quality
Instrument makers, musicians, and sawing and drying pros discuss whether kiln-dried or air-dried wood affect the way a musical instrument sounds.

https://woodweb.com/knowledge_base/Wood ... sical.html


How to make an easybake kiln
http://chairnotes.blogspot.com/2010/04/ ... -kiln.html


Overview of bending wood with heat
https://www.woodworkingnetwork.com/maga ... ing-basics


"The short answer is "no", you shouldn't re-steam wood after it has already been steamed and bent."

https://www.finewoodworking.com/forum/b ... wood-twice


Will steamed wood return to it's previous size?

"Just about as long as people have been around wood has been bent for various uses... ship building being one good example and certainly furniture building is another good example.
I presently play mandolins with steam pressed tops and backs and they are close to 100 years old. In restoration I have steamed both of them apart and they went back together just fine.
Moisture in moderation is the instrument builders best friend. But if you want to use your instrument as a boat, you are on your own."

https://www.mandolincafe.com/forum/arch ... 82826.html

Densification of wood, relates to springback from setting possibly

https://www.researchgate.net/publicatio ... on_of_wood


Evaluating dimensional stability in solid wood: a review of current practice

https://jwoodscience.springeropen.com/a ... 019-1817-1


Steam bending instruction pdf

https://www.leevalley.com/en-gb/shop/to ... em=05F1501


Relaxation mechanism of residual stress inside logs by heat treatment

https://jwoodscience.springeropen.com/t ... 300004.pdf



To try to understand heat setting of wood :

The role of lignin in wood working processes using elevated temperatures: an abbreviated literature survey

https://link.springer.com/article/10.10 ... 20-01637-3

Effects of Moisture on Diffusion in Unmodified Wood Cell Walls: A Phenomenological Polymer Science Approach

https://www.researchgate.net/publicatio ... e_Approach

Critical Links Governing Performance of Self-binding and Natural Binders for Hot-pressed Reconstituted Lignocellulosic Board without

https://www.researchgate.net/publicatio ... e_A_Review