moisture content

[incola]

How much difference does 2-3% moisture make in wood. Is 8% that much different than 10%?

[Roman]

[MadMark]

You really want to machine the wood at it's driest. This will also be the smallest and when the humidity rises the joints will tighten instead of loosen.

Since this is the Canadian woodworking forum I assume that the pieces will be in a heated house over the winter. Because of this they will become overdry. If you machine at 10% and the pieces dry to 6% you might get some loosening depending on specie. Oak for instance, moves a LOT as the humidity changes.

This is also why lots of furniture shops use kiln dried instead of air dried. By getting the moisture content down to 5% or so the lumber is as small as it will ever get.

M

[Jamie in Brantford]

10% is still acceptable. You will experience a little more shrinkage across the grain when it acclimatizes to the central heated home, but so little, you probably wouldn't notice the difference. If you let your lumber acclimatize to your shop for a couple weeks, there would most likely be no difference between 8 and 10 % after that time. It will end up at what your shop is.

[Hendrik Varju]

I hope you don't mind if I put in my two cents. I have to disagree on a couple of points. Firstly, kiln dried lumber is dried to 6 to 8 percent moisture content and in a centrally heated Canadian home you can see wood reach MC as low as 2 to 4 percent. Also, remember that buying kiln dried lumber doesn't do anything for you if you store your lumber in an inappropriate environment. I think you'll find that a lot of woodworkers purchase lumber at 6 to 8 % MC but then store it in conditions where it might reach 12 % or more.

Secondly, although an increase in MC will tighten joints as the lumber expands in size, it will inevitably lead to loose joints too. For example, as a tenon expands inside of a mortise, it will compress the wood fibres on the walls of the mortise. This will generally happen in the summer when relative humidity is very high (where I am in southern Ontario). Although wood has some elasticity and ability to "spring back" from this compression, often the compression goes beyond its elasticity. What develops is called "permanent set". It means that the mortise walls will permanently become larger. When the tenons shrink in the winter, you will have a joint that is far more loose than it would have been if it hadn't been for the permanent set.

My workshop, which is the only place in which I store my lumber, is humidity controlled. I keep the relative humidity equal to the mid-range of conditions likely to be encountered in southern Ontario. This means that the amount of expansion and contraction that will be experienced by my finished product will be limited to one-half of the total possible movement. If you read a book like "Understanding Wood" by Bruce Hoadley, you will read the author's disappointment that most professional woodworkers do nothing to control relative humidity in their workshops where they store their wood. He notes that woodworkers talk an awful lot about moisture related problems but don't seem to do anything about it.

Just a little something to think about.

Take care.

Hendrik

http://www.passionforwood.com

[jay]

Hi Hendrik what steps have you taken to control the moisture in your shop? Thanks.
Jay

[J.P.Rap (Hamilton)]

This subject is a lot more complicated than I thought and it's got me thinkin.
Someone mentioned there is a differance in the rate that moisture migrates to and from wood , depending on wether its AD or KD.
It was also said that it takes several years to air dry lumber (I wouldn't know ,I've never done it).
I've often dampened wood to raise the grain before final sanding. Today after glueing up a couple of pannels I washed off the squeeze out. lots of water.I usually allow a day or two for drying.
Does this mean the water has soaked into the wood and not evaperated like I thought it was doing?Have I have raised the moisture content of the wood? Are these pannels going to shrink dramaticley and are my tables going to fall apart when I take them out of the dungeon?
Too much information for my tired brain.
J.P.

[Hendrik Varju]

Hi JP.

I highly doubt that you will have a problem. Applying water to raise the grain is a common thing to do. The water will evaporate from the surface fairly quickly. It's not as if you are soaking the entire piece of wood in a bucket. I would allow a minimum of 24 hours drying time before you apply the stain or finish and you will be fine.

If the "dungeon" you refer to is your basement, it is certainly possible that you will have some problems. I doubt that your tables will completely fall apart, but moisture content is definitely something to worry about. Put a hygrometer in your basement in the area where you store your lumber. See what it reads. If it is very high (over 55%), then you might want to consider a dehumidifier in that room. A new one is just a couple of hundred dollars and you might even find a used one for $50 or so.

Take care.

Hendrik

http://www.passionforwood.com

[Hendrik Varju]

It is really quite simple, Jay. First of all, my workshop is insulated so that temperature and humidity can be controlled.

I have a hygrometer on the wall, which you can buy for $10 to $15. This tells me what the relative humidity in my workshop is at all times.

Then I have both a humidifier and a dehumidifier in my workshop. I try to keep the RH (relative humidity) around 50% at all times. In the winter, when it is a struggle to keep the RH high enough, I will run the humidifier at all times and keep the RH at 45% as a minimum. In the summer, when it is difficult to keep the RH low enough, I run the dehumidifier and bring RH down to 55% at the top end. I aim for 50%, but 45% to 55% is a more realistic range to aim for. Once in a while, the RH will venture outside of this range even with the humidifier or dehumidifier running 24 hours a day. This situation never last for more than a day or two, which is fine. But sustained periods of high or low RH are what you are trying to avoid.

Keep in mind that the RH values I try to achieve are based on weather conditions in southern Ontario. You would be aiming for a higher RH range if you lived in Vancouver and a lower range if you lived in Nevada. It really isn't all that complicated though. Every day you glance at your hygrometer once or twice while you're working and you adjust your humidifier or dehumidifier accordingly. That's all there is to it.

I hope this helps.

Hendrik

http://www.passionforwood.com

[Steve in Kemptville, On.]

From what I have learned different species of wood do take longer to air dry. Pine form instance can dry in as little a 6 months given the right conditions while hardwoods like Oak can take 1-2 years.

Wetting your lumber on the surface is not a problem. The 60 seconds the surface is actually wet is not long enough for it to absorb moisture deeply into the wood so 24hours dry time is fine. The fact that the wet part is right on the surface also means the evaporation will be fast, not like green wood where the moisture must migrate from the center to the surface.

[Wally]

J.P. -- I find that a paint scraper (which I keep sharp on the disk sander) makes short work of squeeze out wether wet or dry.

Wally/Calgary

[J.P.Rap (Hamilton)]

On the first table top, after the glue was dry and tore out a big sliver of wood near the joint. Fortunatly it was on the bottom side. The second one I washed the glue off.
Thanx for the responce.
J.P.

[J.P.Rap (Hamilton)]

Thanx for the responce.
Yes the dungeon is the basement and its cool and damp. Not wet or real damp (no moisture coming in the walls) but typically damp for a basement.
I only store enough lumber to do one project at a time. If I can't get back to something I have started for a few days, I will usually bring the parts upstairs to keep the RH closer to were it should be.
I do have a dehumidifier but they cost a fair bit to run and we're trying to keep the hydro bills down right now.
Thanx again for the advice.
J.P.

[J.P.Rap (Hamilton)]

I wish.
The dungeon is a bit cool so the wood stays "wet" for an hour or two. If Im in a rush ,sometimes I'll use a hairdryer to speed up drying.
I assumed this practice was ok but if theres one thing I have learned on this forum ,it's that I know a lot less about woodworking than I thought I did.
There are a lot of things I thought I knew the answer to, only to find out I was only part right or compleatly wrong.A lot of the subjects (this one for example) are a lot more complicated than I first thought.
The other thing I have learned is , "the only dumb question is the one that doesn't get asked".
So I'm askin
Peace of mind I guess.
Thanx for the responce.
J.P.

[Wally]

J.P. --If it is soft wood you are gluing up try to scrape most of it when wet. Then you won't get the tearout. When I scrape dry glue off maple I get a nice thin shaving, like a good cabinet scraper.

Wally/Calgary