Wood & moisture content

[cwearing]

How can the turner reach a suggested 10% moisture content for boxes when the relative humidity in the house is at least 25% even during the winter? I assume the wood will be measured with a moisture meter while the house is regulated by something such as an Aprilaire humidifier.

 

[Ken Cawley]

Moisture Content

According to Bruce Hoadley's book Understanding Wood, 25% relative humidity will result in an EMC of approx. 5%. This is well below the 10% you require.

Ken Cawley
Powell River, B.C.

 

[Steve Worcester]

If you rough turn and then allow 1 yr per 1" of thickness, it doesn't matter. You have little control over relative humidity and the climatic factors, so wood will reach it's equilibrium moisture content , with or without you. Anchorseal or no. Finish turning or not. What you ultimately do is release the pent up tension and it will go out of round. Then re-turn.

 

[Bill Boehme]

How can the turner reach a suggested 10% moisture content for boxes when the relative humidity in the house is at least 25% even during the winter?........

The relationship between Moisture Content in wood and Relative Humidity of air is quite complex. Chapter 4 of Hoadley's book "Understanding Wood" give a thorough explanation.

A condensed version of what Hoadley says is that the Moisture Content (MC) of wood is a comparison of the weight of water in a piece of wood to the oven-dry weight of the same piece of wood (in other words, the wood with zero moisture). It is possible for wood to have a moisture content over 100 percent, especially if the dry wood is light weight.

Relative humidity is the amount of water in the air compared to how much water it would take for the air to become saturated at a given temperature and expressed as a percentage. Warm air can hold much more water than cool air can before reaching saturation, for instance, air at 100 degrees F can hold about four times as much moisture as air at 50 degrees F.

If you are air drying wood, the average relative humidity and temperature both play a part in determining when wood will reach its Equilibrium Moisture Content (EMC), which is the MC at which the wood no longer loses any moisture because it is at equilibrium with its surroundings. In the winter when the RH is 30 percent, wood can reach a lower EMC than it could in the summer if the RH is 30 percent. But, a rough rule of thumb is that wood will eventually reach an EMC of 10 to 12 percent for a relative humidity of 50 percent.

Bill

 

[MichaelMouse]

If you are air drying wood, the average relative humidity and temperature both play a part in determining when wood will reach its Equilibrium Moisture Content (EMC), which is the MC at which the wood no longer loses any moisture because it is at equilibrium with its surroundings. In the winter when the RH is 30 percent, wood can reach a lower EMC than it could in the summer if the RH is 30 percent. But, a rough rule of thumb is that wood will eventually reach an EMC of 10 to 12 percent for a relative humidity of 50 percent.

Bill

Bill is mixing relative and absolute humidity. Relative humidity is simply a fraction indicating the percentage of air saturation at any given temperature. Since air with relatively more moisture can accept less from wetter wood or give up more to drier, the only variable in the EMC is the temperature. Molecules get a bit more excited at higher temperatures and slip their hydrogen bonds. As fig 3.4 in the FPL Wood Handbook, which is the same as Hoadley, if you don't have Hoadley indicates, at freezing, 30% RH is an EMC of 6.3, while at temperatures humans are comfortable with it's about 6.1%. Hardly significant, and certainly only marginally affected by the season.

Problem is that wood takes up and gives off moisture at rates affected by the grain orientation and the distance moisture has to travel to escape to the air. The surface is almost always different than the interior. There's a lag. That's one reason why a box lid can bind a bit or flop a bit after time. If you're looking for the "pop" fit, be advised it's temporary.

 

[Bill Boehme]

..........Relative humidity is simply a fraction indicating the percentage of air saturation at any given temperature.........

I think that is what I said. Also, temperature is not the only variable in EMC -- the species of wood is also an important factor, but I was also trying to avoid getting verbose by repeating everything that Hoadley said. My last sentence gives an adequate generalization about EMC and RH.

Bill

 

[MichaelMouse]

Strange FPL doesn't mention species. Probably because they consider that cellulose is pretty much cellulose no matter the configuration. Perhaps the proportion of hemicellulose would make a difference, though it's likely it would be as insignificant as the change in EMC within human-tolerable temperature range.

 

[Bill Boehme]

EMC is affected by the FSP which is different for different species of wood. Because we never remove 100 percent of the bound water, the EMC winds up not being the same for all speciess of wood. At 50 percent RH (a typical value for this area), EMC can vary roughly four percent between wood that has a high extractive content vs. wood that does not.

Bill

 

[Barry Elder]

Wood and Moisture Content

Sorry Guys,

You are sooooooo far over my head it must be nap time!

 

[MichaelMouse]

Sorry Guys,

You are sooooooo far over my head it must be nap time!

OTOH, it's a lot easier to pull yourself up on something that's over your head than something beneath.

Had to be something other than the cellulose involved, though I would not have imagined it would make that much (4%) of a difference.

 

[Andy Hoyt]

My now hurts too much for a nap.

I'm thusly headed straight to the liquor cabinet.

 

[bonsaipeter]

Hi:

You pose an interesting question, the answer to which I don't think will accomplish your true goal. Let me put forth that the 10% EMC (Equilibrium Moiture Content) you seek is not the relavent goal. Let me slightly modify your question to read, "how do I reach an EMC in my finished piece so as to minimize any movement over time when in the environment in which it will live?" This should be your goal. Not withstanding the academic "peeing contest" in some of the responses here (I too have and am well versed in both Hoadley's "Understanding Wood" and the U.S. Forest Products Laboratory's "Wood Handbook" and have a firm grasp of the physics of drying wood), so let's take the pragmatic approch to your question, as modified.

Knowing the absolute EMC in not important here and of no value. Additionally, the use of a pin type moisture meter will prove detrimental since the holes made by the pins from repeated readings may leave damageing effects on the finished piece. Also, a non-pin type moisture meter will not give accurate readings on blocks or turned pieces of wood. These are designed for use on flat surfaces, ideally milled lumber. The best method to determine when you have reached your goal EMC is by the weighing method. The easiest way I know of to impliment this is as follows:

1) Rough turn the piece you are making from whatever wood you wish, no matter how green or dry it is.

2) Weigh the roughed out piece on a balace that has a resolution of 1 gram or less. Record both the date and weight. If you already don't have one yet, you should be able to pick up balance type scale at a yard sale, flee market, ebay, or new. A well worth investment for all sorts of tasks around the shop.

3) Set the piece in a safe place to dry. Ideally where or near the finished piece will be used or displayed. Reweigh and record the results weekly. Any movement in the piece will occur at this stage of the process.

4) If the piece was "wet" it will lose wieght over time. If it was overdried, it will gain weight. When you acheive no change in weight for three consecutive weeks, the piece is at EMC.

5) Once at EMC, finish turn your project and apply any finishes you wish to complete it. Then, sit back and enjoy a stable piece.

This method does not give you an absolute EMC value, it simply insures that you end up with a non-moving, stable piece. That's you goal!

Peter Toch

 

[Bill Boehme]

You pose an interesting question, the answer to which I don't think will accomplish your true goal. Let me put forth that the 10% EMC (Equilibrium Moiture Content) you seek is not the relavent goal............

Although it could be perceived as such, I don't believe that there was a "peeing contest" -- just a lively academic discussion over a nuance that is of limited practical value to a woodturner most of the time, but academics aside, I basically agree with your assessment of the situation. EMC will be whatever it will be and that is the point where the wood has gotten to the point that it is as stable as it will ever get. And, if its environment changes significantly, the EMC will also change by a few percentage points.

I don't own a moisture meter nor a sensitive scale and have found that with a bit of familiarity to the wood that I am turning that it is relatively easy to know when wood is dry enough to finish turning it. I can also tell by such signs as how the shavings look and if the wood changes color on a fresh cut, etc. Since I mostly turn mesquite these days, moisture is even less of a concern as it can often be turned from solid block to finished piece in a single session without concern about moisture.

Bill

 

[MichaelMouse]

Hi:
4) If the piece was "wet" it will lose wieght over time. If it was overdried, it will gain weight. When you acheive no change in weight for three consecutive weeks, the piece is at EMC.

5) Once at EMC, finish turn your project and apply any finishes you wish to complete it. Then, sit back and enjoy a stable piece.

This method does not give you an absolute EMC value, it simply insures that you end up with a non-moving, stable piece. That's you goal!

Peter Toch

Stable? Well, not really. A change in relative humidity will still change the dimension of the piece. Finishes may slow the process of exchange, but cannot stop it. They just make it less vulnerable to short-term changes. A finished piece displayed in the bathroom or placed in a southern exposure will still react as if it were wood, so make that lidded ring dish loose and be prepared for repair on the sun-baked bowl.

Reminds me of my old carving teacher who said sanding a piece of woodenware beyond 100 was misleading the customer, bcause the first time they steamed the surface in the stew the grain was going to raise. Since the bowl will never be perfectly circular except at the MC it was created even if you spent extra effort to keep it circular and uniform, might as well work to the "close enough" principle from the start.

Which means your method is fine, though unnecessary because it cannot guarantee that the surface and interior are at the same MC and says nothing about the ultimate MC where the piece will reside. Moisture meters are even more academic forms of bladder relief.

 

[John Nicholas]

Controlled Drying of a Rough Turned Item

Stable? Well, not really. A change in relative humidity will still change the dimension of the piece.
<SNIP>
Which means your method is fine, though unnecessary because it cannot guarantee that the surface and interior are at the same MC and says nothing about the ultimate MC where the piece will reside. Moisture meters are even more academic forms of bladder relief.

.. though unnecessary ..... Not so. One of my frustrations is attempting to turn something green and have it move as I turn. I would rather spend a few minutes weighing an item every few weeks. Then when I start to return the item it has stabilized to the environment of my shop. Any movement while turning is minimized. No way I know to stop it completely. If you want to turn for a customer's living room, you will have to move your lathe over there. (probably not going to happen)

Each to their own, but I find my turning is better with weighing.

John