Moisture content: How much is too much?

Joseph Shaul

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Location
Madison, WI
To introduce myself, I'm a college student learning to make things out of wood. I have a few years' experience working with jewelry, and spend enough time mucking about with acoustics and speaker design that I'm trying to learn how to work with it as well.

After looking for a fancy wood for a headphone project, I found a likely candidate: Regnas, otherwise known as borneo rosewood. Aside from a tendency to cause rashes and irritation when green, borneo rosewood seems to be a great wood - it's cheap, and when given a bit of wax or laquer takes on a color from bright red deep red color with black stripes.
Borneo5-3.jpg

http://www.amazonexotichardwoods.co...Product_Code=TB-BOR-553&Category_Code=TB-BORO
The problem with this wood is that it's usually sold as turning stock, which I'm told can have too much moisture to be cut into thin slices or CNC milled without splitting or warping. How much moisture should I be looking for?
 
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Welcome to the Family! :wave:

You really need a moisture meter to find out if it is dry or not, I guess you could weigh it and then see what it is supposed to weigh and compare the two weights :dunno:

Cheers!
 
Joseph, as Don mentioned, it can vary from species to species. That said, I think a lot of woodworkers consider about 12% moisture or less to be "dry". Keep in mind that wood will reach an "equilibrium" moisture content that varies depending on the relative humidity of the air around it. In a humid environment, 15% might be as dry as the wood will ever get. In the desert where Don lives, it might get as low as 4% or 5%.

...You really need a moisture meter to find out if it is dry or not, I guess you could weigh it and then see what it is supposed to weigh and compare the two weights :dunno:

Cheers!

Actually, you can very accurately determine the moisture content with a good scale and an oven. (That's how a good moisture meter is calibrated.) The downside is that you have to cut samples from the block of wood and dry them, and they'd be best from the middle of the blank. Unfortunately, if you try to dry the whole block in the oven, there's a good chance it'll crack.

For those who are curious, here's how to tell moisture content with a scale and an oven. (Leftover knowledge from my days working in a materials testing lab.) While it might not be practical for checking a single turning blank, with just a few representative samples, you can get a good general idea of how wet a whole stack of lumber is by using this method.

  1. Weigh the wet wood.
  2. Dry the wood thoroughly in an oven. It's dry when it's no longer losing weight.
  3. Weigh the dry wood shortly after removing it from the oven. (Otherwise, it can absorb new moisture from the atmosphere.)
  4. Subtract the weight of the dry wood from the weight of the wet wood. The result is the weight of the water.
  5. Calculate the percentage of moisture with this formula:
(Weight of the water / weight of the dry wood) x 100 = % moisture
Example:

Wet wood = 500 grams
Dry wood = 400 grams
Water weight = 100 grams

100 / 400 = 0.25
0.25 x 100 = 25% moisture content
 
Vaughn
When you speak of an oven... are you talking about a conventional oven on low heat or a microwave - or both.. I sometimes dry wood in a microwave if it's really obviously wet... I don't have a scale or a moisture meter so I tend to use a "feels about right" method of determining if it's dry... so far I've been relatively successful with the woods I'm working with.
 
Don't forget the sometimes important fact that some woods need the end grain sealed before drying. Most woods have capillaries (small tubes) that will carry water out of the end grain much faster than out of the face of the board. This uneven drying will cause stresses to build up near the end of the board and then cracking. By sealing the end grain with wax or even a good paint, you force the wood to dry mostly out of face grain generally causing less stress. I would saw a block like that first and after sealing the end grain attempt to dry it. Good luck.
 
Joseph, as Don mentioned, it can vary from species to species. That said, I think a lot of woodworkers consider about 12% moisture or less to be "dry". Keep in mind that wood will reach an "equilibrium" moisture content that varies depending on the relative humidity of the air around it. In a humid environment, 15% might be as dry as the wood will ever get. In the desert where Don lives, it might get as low as 4% or 5%.



Actually, you can very accurately determine the moisture content with a good scale and an oven. (That's how a good moisture meter is calibrated.) The downside is that you have to cut samples from the block of wood and dry them, and they'd be best from the middle of the blank. Unfortunately, if you try to dry the whole block in the oven, there's a good chance it'll crack.

For those who are curious, here's how to tell moisture content with a scale and an oven. (Leftover knowledge from my days working in a materials testing lab.) While it might not be practical for checking a single turning blank, with just a few representative samples, you can get a good general idea of how wet a whole stack of lumber is by using this method.

  1. Weigh the wet wood.
  2. Dry the wood thoroughly in an oven. It's dry when it's no longer losing weight.
  3. Weigh the dry wood shortly after removing it from the oven. (Otherwise, it can absorb new moisture from the atmosphere.)
  4. Subtract the weight of the dry wood from the weight of the wet wood. The result is the weight of the water.
  5. Calculate the percentage of moisture with this formula:
(Weight of the water / weight of the dry wood) x 100 = % moisture
Example:

Wet wood = 500 grams
Dry wood = 400 grams
Water weight = 100 grams

100 / 400 = 0.25
0.25 x 100 = 25% moisture content

Very well explained, Vaughn.

Chuck - a conventional oven is preferred because a microwave is too fast; it's easy (DAMHIKT) to scorch wood in a microwave, even if it's run on a 1:10 duty cycle. A conventional oven set to 150-200 (fahrenheit) works well, and best at about 150 (the minimum lowest setting for most conventional ovens). There's no need to bring the temperature all the way up to the boiling point, risking scorching - 150 will drive the moisture out very effectively & quickly.
 
...Actually, you can very accurately determine the moisture content with a good scale and an oven. (That's how a good moisture meter is calibrated.) The downside is that you have to cut samples from the block of wood and dry them, and they'd be best from the middle of the blank. Unfortunately, if you try to dry the whole block in the oven, there's a good chance it'll crack.

For those who are curious, here's how to tell moisture content with a scale and an oven. (Leftover knowledge from my days working in a materials testing lab.) While it might not be practical for checking a single turning blank, with just a few representative samples, you can get a good general idea of how wet a whole stack of lumber is by using this method.

  1. Weigh the wet wood.
  2. Dry the wood thoroughly in an oven. It's dry when it's no longer losing weight.
  3. Weigh the dry wood shortly after removing it from the oven. (Otherwise, it can absorb new moisture from the atmosphere.)
  4. Subtract the weight of the dry wood from the weight of the wet wood. The result is the weight of the water.
  5. Calculate the percentage of moisture with this formula:
(Weight of the water / weight of the dry wood) x 100 = % moisture
Example:

Wet wood = 500 grams
Dry wood = 400 grams
Water weight = 100 grams

100 / 400 = 0.25
0.25 x 100 = 25% moisture content

Maybe...

The formula will only work if you consider the 'dry' wood to have achieved 0% moisture - which is a virtual impossibility.

You will have removed 20% of the initial weight of the wood, and most of it will have been moisture. A few grams of other substances will also have been removed, but so little as to be of no consequence.

The actual moisture content, though will have been something greater than the 25% in the example, because of the amount that's still in there.

Once dried in this manner, it's still not at it's 'natural' moisture content. That will be achieved when the wood reaches an equilibrium within its final environment. As previously mentioned, that level could be anywhere from a couple percent up to maybe fifteen percent or more.

Don's Arizona summer level might be only a couple percent, but Royal's Hawaiian tropical climate may result in something more like fifteen percent.

For most furniture, an average moisture level in the 8~10% range will be quite normal and acceptable. The climatic extremes cited above will result in much more wood movement - and will also require planning for it during the construction.
 
Very well explained, Vaughn.

Chuck - a conventional oven is preferred because a microwave is too fast; it's easy (DAMHIKT) to scorch wood in a microwave, even if it's run on a 1:10 duty cycle. A conventional oven set to 150-200 (fahrenheit) works well, and best at about 150 (the minimum lowest setting for most conventional ovens). There's no need to bring the temperature all the way up to the boiling point, risking scorching - 150 will drive the moisture out very effectively & quickly.

I know about scortched wood... stinks worse than microwave popcorn...DAMHIKT :D:D

I use the defrost cycle and never over 2 or 3 minutes then let cool for 20 or 30 before next cycle.... only had problem once and because I rushed some pen blanks. :eek:
 
Maybe...

The formula will only work if you consider the 'dry' wood to have achieved 0% moisture - which is a virtual impossibility.

You will have removed 20% of the initial weight of the wood, and most of it will have been moisture. A few grams of other substances will also have been removed, but so little as to be of no consequence.

The actual moisture content, though will have been something greater than the 25% in the example, because of the amount that's still in there.

Once dried in this manner, it's still not at it's 'natural' moisture content. That will be achieved when the wood reaches an equilibrium within its final environment. As previously mentioned, that level could be anywhere from a couple percent up to maybe fifteen percent or more.

Don's Arizona summer level might be only a couple percent, but Royal's Hawaiian tropical climate may result in something more like fifteen percent.

For most furniture, an average moisture level in the 8~10% range will be quite normal and acceptable. The climatic extremes cited above will result in much more wood movement - and will also require planning for it during the construction.

I should have been more clear...the intent with the oven is to dry a sample, not the whole blank. And yes, it must be dried to 0% moisture. That's why I noted that it should be weighed shortly after coming out of the oven, before it has time to pick up atmospheric moisture*. After it's been dried and weighed, the sample can be tossed. Reaching 0% is not impossible at all with enough heat, time, and willingness to ruin the sample.

In the case of soil samples in the testing lab, I don't recall the temp of the ovens we used, but I think it was in the 350º to 450º range. We would usually leave a pan of soil in the oven for several hours (or better yet, overnight) to dry it. I would not recommend trying to dry a turning blank like that. ;)





* If you weigh it immediately out of the oven, the heat waves emanating from the sample are enough to throw off the reading on a sensitive scale. We used to give soil samples a minute or two to cool before weighing them.
 
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