Ok purely hypothetical question here. I know that both the options below are less than desirable but what one do you think will affect wood movement more? Temperature difference or moisture difference from place of build to final home of the piece.
Example 1. You build your project at 70 degrees and its final resting home will be at 70 degrees, but you build with wood that is 12% moisture content and the final home for the peice is going to be at 7%.
Example 2. You build the same thing in 30 degree and its going to be inside at a cozy 70 degrees but the moister content of the wood is 7% at build and the final resting place is also at 7%.
I know there are many issues with both, glue temp in the cold and what not but what one would you think will give you the most trouble with wood movement and growth?
Replies
Wood moisture content has almost nothing to do with temperature. It has everything to do with relative humidity.
So, look to changes in relative humidity as the thing that will lead to changes in Equilibrium Moisture Content which will lead to wood expansion/contraction.
Howi
Yeah i realize that moisture content and temp are not necessarily related. Thats the whole question.
What causes more wood movement, moisture % change or Temp change?
"What causes more wood movement, moisture % change or Temp change?"
Moisture. The thermal expansion coefficient of wood is quite small. Take a 6"-wide piece of oak from northern New Hampshire at -40° and move it to Phoenix at 120° and it will have expanded by less than 1/32".
-Steve
>> What causes more wood movement, moisture % change or Temp change?I though I was clear on that point. Temperature change has little or no affect on wood movement. Equilibrium Moisture Content change is the driver of expansion and contraction in wood.Howie.........
WT, The wood movement is all about gaining or loosing MC. THis changes relative to RH relative humidity. The way this relates to temperature, is little hard to explain simply, but warm air has the potential to carry more moisture than cool air. When cool air is near 100%, the same actual volume of moisture may only be 50% in warmer air. When warm moist air cools and the RH gets to 100% it rains as long as the temperature is dropping. Here is a good link that you should studyhttp://www.woodweb.com/knowledge_base/MeasuringMonitoring_Moisture_Levels.htmlIf you are think that heating wood will cause it to expand like it does metal, that isn't the way it works. .
"If you are think that heating wood will cause it to expand like it does metal, that isn't the way it works."
It does work that way. It's just that the thermal effect is completely swamped by the moisture effect. The thermal expansion coefficients of woods are in the same ballpark as those for metals, averaging a little lower.
-Steve
Well thanks Steve for correcting me on that. I guess I had forgotten about that. I had to get my Hoadley book down to look for tables that I could not remember. As it turns out, he only gave it one paragraph. Seems a 100º change in a 8' length ? 1/64" change. That is not a lot, but it is enough to measure, so I stand corrected. Ha, you win. :-)
"Seems a 100º change in a 8' length ? 1/64" change."
The expansion coefficient across the grain is about ten times as much as it is along the grain, enough that you might need to take it into account in certain circumstances.
-Steve
Almost certainly the moisture changes, though the part that makes it complicated is that temperature changes in the air are associated with changes in relative humidity. But given the wood is at specified moisture contents in each case, the temperature would effect would be negligible. Hoadley Understanding Wood, 1980 discusses this.
If you want to see this in numbers, the Forest Products Laboratory has a manual with all those formulae.
Neither of your examples is very realistic since 7% MC (RH?) is way below the "norm" in most homes, and finished pieces react differently than raw wood to changes in RH.
Your second example is probably a bit more realistic although I really doubt if anyone works in a shop that stays at 30*. How would your glue, stain, and finish ever dry? - lol
The effect of a "sudden" change in the ambient conditions (between the shop and the home) might cause some problems if the piece wasn't built to allow some amount of expansion/contraction, but the movement of a finished piece will be mostly due to temperature differences.
One exception to all of this would be kitchen cabinets above a coffee maker, electric skillet, or near a stove. Each of these creates a lot of heat and steam which can (and does) play havoc with the cabinets above them. At first, it's all about the temperature swings, but the steam really degrades the finish and eventually both temperature and moisture will cause problems.
I'm looking at the maps in Hoadley's Understanding Wood, and see large swaths of the country that have winter time average moisture content (MC)of interior woodwork at 7% or less, and another very large band with averages below 8%. These are for finished wood, it would be quite common in northern areas to see unfinished wood dip to 4% moisture content at their winter low points.
But the thing that confounds the intuition is that heat reduces relative humidity. The air in a space at 30° with a given relative humidity and associated equilibrium MC takes on a lower relative humidity when it is heated, without introduction of more water to the system, to 70° and consequently the equilibrium MC decreases as the wood surrenders more of its moisture to the air. Though the temperature increase is causing the wood to swell through thermal expansion the effect is so little that is quite thoroughly overwhelmed by the shrinkage effects of lower MC content, at least in the scale relevant to furniture making.
Edited 11/18/2007 9:52 pm ET by SteveSchoene
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