Why wood moves and understanding humidity

[Dan Bombliss]

In another post there was a question about flat sawn blocks that ultimately led me to looking into my wood information to accurately share the details with humidity and wood movement. If you understand this concept of humidity, you are better equip to decide for yourself what grain orientations you are willing to use for certain applications. Otherwise you'll at least understand why everyone's always harping on about 40-50% humidity.

I apologize if this post gets a bit "ranty", it's late and hard to focus, but the info is there. Take it as you will, and I am by no means saying anyone is wrong. What I do believe is that this is science, and I trust science better than I trust people. Also granted, the exact details of this science could vary from source to source, but ultimately it's the same thing.

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Wood is hygroscopic, which means it takes on and gives off excess humidity. That is the reason why it's important to give wood time to acclimate to your shop before working with it.

RH is Relative Humidity, which is the level of humidity in the air, aka the environment the wood is in.
MC is moisture content, which is the level of moisture in the wood.
The relationship between the two is the EMC, the equilibrium moisture content. The EMC is the point to where the wood's MC matches the RH.

When RH goes up, the MC goes up and the wood swells. When RH goes down, MC goes down, and wood shrinks. If you put a dry board in a very humid room, it will take some time for moisture content to raise to match the higher RH. When the moisture content of the wood does equal out to the same moisture of the RH, that is what is called the equilibrium moisture content. It's the balance between the two.

A chart showing examples of the relationship between RH and MC read:
RH MC
0% 0%
25% 5% (This figure is why we shoot for a range of 35-50% (depending where you are) The ideal
50% 9% moisture content is around 6-8%, which according to this chart is higher than 25% (5% MC) 75% 14% and lower than (or maximum of) 50% RH (9% MC))
99% 23-30%

If you constantly control your shops humidity at a dead set 40% all year round, you can expect your wood to be at the 6-8% moisture content. As soon as your piece of wood (or guitar) leaves that shop or environment, and goes to say 75% RH (A regular summer in the midwest, if not even higher) That piece of wood (or guitar) is going to continue to acclimate and expand (take on moisture) until it reaches the matching 14% moisture content to the environments 75% RH. Again when that piece of wood reaches the 14% MC that relates to the 75% RH, that is what is called the equilibrium moisture content.

Cool, a page full of numbers, but what do they mean? How does that moisture affect your guitar or project? Well, the science in those numbers is there to accurately estimate how much your wood is going to move. This is when the slab cut piece of wood is less stable than the quarter sawn. Stable meaning as little movement as possible. By no means does "less" stable mean "not" stable, but less is less, which is the equivalent to "less" desirable. Also one of the reasons why flat sawn necks are frowned upon; there's a better option.

Now, species of wood does affect the rates, as some move more or less than others, there's a general rule on predicting wood movement. Also quarter sawn moves roughly half of the figure flat sawn does.
Most flat grain species of wood - For each 4% change in moisture content (MC) the board will shrink or expand 1%. So in the figure of having an acclimated board to 35% RH (6% MC or so) and then a move to 75% RH (14% MC) That's a spike of 8% moisture content, which according to these figures is movement of 2% of the size of your board across grain.

Now take that figure of 2% movement and apply it towards a 3" thick neck. That's 1/16" movement on a flat sawn board. Because quartered is roughly half the movement of flat sawn, you're looking at 1/32" movement for 3" board.
(THIS INFORMATION IS THE MATH AND SCIENCE USED TO ACCURATELY PREDICT WOOD MOVEMENT IN LARGE QUANTITIES. IT'S EXTREMELY ACCURATE WITH APPLICATIONS OF WOOD FLOORING AND SUCH, AS OVER THE WIDE LAY OF THE FLOOR, IS AVERAGE OF THIS MATH IS SHOWN TRIED AND TRUE. EVERY INDIVIDUAL PIECE OF WOOD IS DIFFERENT SO THIS EXACT WOOD MOVEMENT IS NOT A PROMISE.)

So one other thing to point out with stability of necks, is if you have a fingerboard glued down restricting the movement of the wood, the wood has to move somewhere. That's when you would potentially find more twisting patterns of flat sawn necks versus quartered.

So to some this up, I will stand by the statement that flat sawn IS less stable than quarter sawn, because the science says so. The reason why people can build with flat sawn necks and that they work is because the rules of RH are generally followed. If the piece of wood you are about to build with, despite the orientation of the grain, is completely acclimated and stable with a moisture content of 8%, and this neck never leaves an environment where the EMC is 8%, in theory that piece of wood would never move due to moisture.

This is exactly why we are told to shoot for 40-50% RH year round, and it is so that your wood that is acclimated to 8% moisture content stays that way, and stays stable.

I haven't found a source with the exact information pertaining to old wood, as far as WHY it's more stable, but I have told it has something to do with the constant movements back and fourth as it expands and shrinks which over time gives it stability, and makes it less susceptible to movement. That would explain why instrument builders seek 50+ year old wood. You'll also find if you resaw veneers out of a 2 year old board opposed to a reclaimed board from a very old piece, that even with both acclimated, the new wood will move drastically more than the old. Potato chips anyone?

-Dan

[B. Howard]

Dan, Unfortunately things are never quite simple in woodworking. Stability has some different meanings through out the woodworking trades. Instrument making is at the fringe edges of woodworking as a whole and we are concerned with stability on a level most woodworkers never fathom.. For most wood working the term stability means the wood will yield good service in its finished product. This will be a bit different if one is building slab top tables than if making 5 pc. doors and different still for making instruments. You must remember that the bulk of data out there is produced for the benefit of the cabinet/furniture trade and must be taken in that context and transposed to our craft. It is based on species in common use in those endeavors which tend to be entirely domestic and in many cases farm raised. While everything you say is basically true it is an over generalization. Amount of overall wood movement will be quite different by species. For instance Red oak will move about 1/4" per ft width (FS) on a 5% MC swing ( normal service for a 5 pc. door around here) whereas African Mahogany will typically exhibit about half that movement in the same conditions. The relationship of radial movement to tangential is also different from species to species. If we look at these two species as quarter sawn stock we will see that they exhibit the same movement of about 3/32" per ft. So when cut as FS, Mahogany is twice as stable as Oak but on the quarter they are the same.... in this case Mahogany moves roughly 75% as much on the quarter as the flat while the Oak only moves 40% as much. This will vary for every species and how the timber is dried will also effect how the wood behaves in service with a proper kiln dried stock being on average 30% more stable than air dried. The key word there is PROPER, which is not the case in most drying operations these days. There is also the consideration how fast the wood will move when exposed to different environments. This also differs from species to species and is of great interest to us as Luthiers but you will not find much data on it as it does not concern the workingwomen industry.

Then there are the factors like how wood behaves in some of the thin crossections that we use, or the fact we leave our work mostly unbalanced at the end by only finishing one side. This all adds further complication and behavior of the panel in question by virtue of grain orientation can become a concern. Flat sawn wood will have a tendency to cup across the grain because you never really get much truly flat grain in a board, it will start to rift at the edges and may even transform all the way to the quarter depending on the ratio of board width to log size. This tendency will get worse as the board gets wider or thinner. In our case, for a guitar back, we have pushed it in both directions by wanting both wide and thin. Combine that with unbalanced finishing and the flat sawn piece will want to curl up despite stable RH while the quartered piece will stay true throughout a range of RH change. This all has to do with the cellular structure of wood itself, which could be the subject of an entire volume all its own. So, simply stated, RH concerns are not all we need to deal with in our wood selections.

I have spent a lifetime working with wood and while I have come to understand a lot about it, the first great realization everyone needs to understand is that wood was engineered to be the supportive and living tissue of huge plants, not something for the manufacture of goods to suit man. I also know that no matter how well one knows wood, it will always yield surprises, it can never be 100% predicted. No two pieces are the same, even if sawn side by side , book matched from a billet. So again while everything you stated is true on average it is always good to know the specifics about the species you are working with.

[Dan Bombliss]

Hey Brian, thanks for adding information, and pointing out some more details.

In the other post, it said that flat sawn is not less stable than quartered, and based on movement information, I'd have to disagree. I also agree that I barely scratched the surface on wood movement mostly because no one wants to read a forum post that long. This is a very generic post of information intended to get new woodworkers familiar with the idea that wood moves, and help understand how or why the humidity relates.

For anyone interested in learning the most of it, snatch up a copy of "Understanding Wood" by Bruce Hoadly. This is PhD level information that will blow your mind.

-Dan

[johnnparchem]

Thanks guys. These are two really good posts.

[RnB]

Good thing I live In CA. Guess I'm lucky, most of the times I go out to my shop, the humidity is between 40-45 %. Once in a while it will dip one way or the other..

Nonetheless, a very interesting read guys. Thx for posting...!

[David L]

fascinating thread. As Brian stated it isn't quite that simple, lots and lots of variables come into play. A dynamic that I have failed to hear anyone mention is temperature. Temperature causes wood to move as does humidity but not to the same degree (no pun intended). The higher the temperature causes wood to expand, conversely the lower the temperature causes wood to contract. This is not only true with wood but with many other materials in this world including hardened steel (however slight). I used to work in a machine shop and when inserting pins into fixtures with ultra close tolerances the pins were packed in dry ice for 45 or so minutes and with the aid of a 12 lb mall the job was accomplished.
Some Forum members have expressed concern about performing glueing operations in high humidity, and rightly so. I live in Slidell, La (about 25 miles from New Orleans), I have the Gulf of Mexico to the South, A very large lake to the Southwest and I am surrounded by swamps and marshes so I'm no stranger to humidity woes.
I have been known to move my parts to be glued up inside the house several days before the scheduled glue up (for acclimation purposes) and performing to glueing operation inside the house. I understand that this is not an option for some people for various reasons but I have had acceptable results with this remedy, just some food for thought and possible alternatives to the ole humudity bug-a-boos.

David L

[B. Howard]

Wood movement due strictly to temperature alone is miniscule. Wood does not behave at all like metal in regards to temperature change. The fact that relative humidity is tied to temperature and will change with temperature is what is actually causing the movement in the wood. This correlation is easiest to see when one tracks the dew point of the air which is a direct measurement of how much moisture is actually in the air. If the amount of water available remains constant and only the temperature changes the relative humidity changes as well. That is why it's called relative humidity. Warmer air is less dense and therefore has greater capacity to hold water molecules. For example if we have a dew point of 62F and a temp of 62F we have 100% RH. Now if the temp rises but the dew point (amount of water actually in the air) stays at 62F but our temp rises to72F we now have 57% RH. This is why I have always stressed that when considering environmental control of a wood working space one should concentrate on RH rather than temp. The ability to add or remove moisture is key because as long as the RH stays within parameters the temp can wander quite a bit with no ill effects on the wood.

[David L]

Brian, while I'm not disagreeing with you at all, as I am quite clear in the fact that you've probably forgot more about wood than I'll ever know I am puzzled at some of your rationale and perhaps you can enlighten me (I'm always eager to learn, Knowledge is power). I install flooring for a living (unfortunate for me). When we install wood flooring in a house the material is brought to the jobsite and stacked inside the house (climate controlled at this point of the build) so that it may acclimate for obvious reasons. My pea brain tells me that the acclimation is more for temp rather that humidity. Also houses with wood flooring need to be kept relatively stable in regards to temperature (within certain pararmeters) or problems could arise, buckling or separation issues. I understand that temp and humidity are closely related and they influence each other. In my own house I conducted an experiment with temp and humidity. Let me say that I used my homemade Ben Franklin hygrometer so lack of accuracy was not an issue. What I did was raise and lower the temperature inside the house drastically (don't remember exactly how many degrees) and the flucuation in the humidity was negligible. I did this several times and allowed the temp to stablize for more than an hour each time. What is your take on that? Keep in mind where I reside geographically.

David L

[B. Howard]

David, my thoughts. First I assume that by a Ben Franklin Hygrometer you mean a wet/dry bulb thermometer? That is the same way I monitor my humidity in the shop these days, calculated based on dew-point. Based on your location I also assume this test was done in an indoor space regulated by a modern air conditioning unit. That second fact is kind of crucial because air conditioners not only regulate temp but also desiccate the air so it would be absolutely no surprise to me that one could alter the thermostat a wide range and have the air remain at around 40-45% RH. I can tell you that up here in a much less tropical climate where we use both AC in summer and heat in winter that is not always the case. In my home which is heated via hot water radiators when the heat kicks on and those radiators fill with hot water the air immediately looses RH, within 3 minutes my auto humidifier in the music room will start putting out water vapor to make up the difference. In my shop I have always let temp wander all over ( within reason) as i do not spend money to heat or cool space I am not currently working in. In the winter I keep it at 58F overnight and turn it up to 66F when I get in. If I monitor the hygrometer by my bench I can see an initial drop of about 4-5% RH as the temp rises faster than the dew point. It can take 3 hours or so for my humidity control equipment to get my RH back to the middle of my desired range, unless it is below about 20F outside in which case the best I can do is keep the RH at 40% as the heat runs so often it outpaces my RH regulating equipment. Some days I quit early because I can not maintain enough RH with the heat running. In the summer It is a different situation , but kind of the same problem. My ac unit is sized very large to accommodate a work environment and be able to handle extra heat generated by equipment and sweat of the brow. While it lowers the air temp which should cause dew-point (and RH) to rise the machine actually dries the air way to much and the RH drops, fast! I wind up actually needing to run a humidifier at that point. So I guess the point I am trying to make is that due to modern mechanical intervention in the way of climate control the natural correlation between temp and RH can get skewed quite a bit. As further proof that RH and not temp is the main concern as to woodworking, while you can find lots of charts regarding the expansion of different woods at various RH and MC you will not find one that correlates wood movement by temperature. At least as far as I know of.

[tippie53]

remember that humidity is relative to temp warm air will hold more water than cold thus the drop in dew points. Also
what and how was the wood dried ? Old air dried plus 25 yrs is usually pretty stable. Kiln drying will usually be more stable than air dried.
Then you get into the type of kilns so there is many variables as was stated. The one point is that , is you understand what happens and why , you can control it better. As for my shop , I have a lot of money invested in woods and I keep the shop heated and RH controlled .
sticker and pressing wood helps. Let the air get around it.