Problems with wood... not that!

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Donna at MetroAF

Donna at MetroAF

CGF II, Certified Grumble Framer Level 2
Joined
May 6, 2005
Posts
355
Loc
Roseville, MN
I was just reading the Framer-to-Framer forum on PFM's website, and I saw something wierd.

There was a post started about assembling a piece, and why there is a need for the allowance. A few people said it was needed for the expansion and contraction of the mats/foam etc. But, one said it was for the expansions & contraction of the frame (which, correct me if I am wrong, sounded right to me).
Anyway... another response rips into the guy for even suggesting that the wood would "shrink". It's just not possible, etc., etc., and so forth.

So, doesn't the wood expand and contract? Or am I crazy. I'm pretty sure that is why we use the allowance system, right?
 
Some info here

http://www.wisegeek.com/what-is-wood-movement.htm

Just twigged the significance of the title of your topic - LOL!!!!

We have to leave play in the rebate, we can't make the glass/mat/mount/backing an "engineering fit" and squeeze it all in, so I don't think we need worry too much about that, but I suppose a lot of damage on old ornate frames is down to 'movement'
 
Interesting article. Thats what I would have guessed. More movement across the grain than with it. That would explain why we see bowing & twisting in moulding lengths . Also evident in mitred corners, especially wide ones, where the fresh cut exposed wood is very prone to drying & shrinking eventually leaving a once perfect corner with a gap. Or a closed corner with a crack. I think the mat boards would probably be worse than the wood as far as changing dimentions but it doesn't really matter. If it is or becomes too tight it will bind & buckle.
 
Of course wood shrinks and expands.

Even a non-framer would know that, just by observing that doors in a house in certain seasons close / don't close as easily!

Of course, certain woods do more so than others.
 
Matboards also expand and contract - just try to "squeeze" a mat into a frame and revisit it in a couple of months when it is nice and wavy...

Has to do with relative humidity (RH) in the air - for the sake of Pete!
 
Thanks!

That's what I thought. But, you never know. You've never been in a business long enough to know everything. ;)

Thanks everyone!
 
The reply on the PFM forum pointing to the URL was mine.
When humidity is high all the components expand, the art sandwich gets larger and the frame rails also get larger, thus making the internal size of the frame smaller.
Whilst all the amounts of movement are very small in themselves, they become significant when added together if the internals are tight in the frame.
 
That makes total sense. I saw you reply, it was good. It was this one that got me thinking:


"posted 9/7/2006 2:25:00 AM by steve
-------
How the heck would the outer frame shrink? No such thing. Wood frame does not shrink. Are you using a rubber frame or something? maybe rubber would shrink based on temp. "


Thanks again!
 
Hi John,

Unfortunately the PFM forum is being badly hit by spammers at the moment, but there are often some interesting questions and answers on there

I picked that link because it wasn't to technical.:)

I spent many years as a craftsman woodturner, and the moisture content was very important, as artistic work could take on a totally different shape as the wood dried out.
 
If a 3" wide moulding that has a sectional mass of 5.7 (which a Louie 15th does) is made when the moisture content is between 6% and 8% [an acceptable range for furniture mfg.]; Over time in a warm room, and especially near the heating source, the moisture content can drop to less than 4%.

If the frame is 16"x20" or larger, the width of the moulding across the presenting face, can be expected to shrink from 1/32" to 1/16". This may not sound like much... but that is ALL four rails.

The result is that the 45 degree mitres become approximately 47 degree mitres. Resulting in the inside of the corners opening up. Most people confuse this phenomenon as the "sight" shrinking the length of the rail or leg... when in reality that is the optical illusion caused by the shrinking across the width of the moulding.

The finer mouldings carved in the 16th -18th centuries don't usually exibit these characteristics because the better the masters the more care was taken in the prep work of the wood. The french used to air dry the lumber, then case dry to a suspected 3-4%(by weight) by layering the boards with quick lime. This also had a whitening effect... which has nothing to do with French liming, or whiting.

They also copied the Dutch practice of quartersawing the lumber which is much more stable as the grain runs the short length from the back of the frame to the front, instead of side to side. Therefore any swelling or shrinkage would occur in the depth of the frame instead of the width.

Many of the "cracked at the corner" closed corner frames were made after the French revolution and Napoleon had disbanded the guilds and much of the talent was lost to the use of machines and compo.
 
The allowance on "normal" sized work is primarily for ease of fitting.

On larger works, especially those where glazing that has a high expansion coefficient are used, the allowance also needs to accommodate the fluctuation in the size of the glazing. Note that the measurable movement of glazing is relative to temperature alone.

Wood does expand and contract, primarily tangentially and second radially with the grain. There is some longitudinal movement, but for most applications it is so small that it can be disregarded. Note that wood only moves in measurable amounts relative to moisture content.

As far as the shrinkage/swelling of the wood changing the frame dimension, it will never get smaller, always larger, because of the nature of a miter joint. And in fact it will be the shrinkage of the wood that has the potential to make the rabbet size larger by a greater degree. Because the points of the miters will keep the outside size of the frame the same, the gaps opening up on the inside as a result of drying/shrinkning mean that the inside dimension of the frame is getting larger. In an extreme example where the wood wasn't properly dried, say to 12% (equivilent to air-dried in most locales), and stabilizing at 3% near a woodstove in winter, a 4-1/2" wide basswood moulding would grow 1/4" (each leg shrinking 1/8" in width) in total frame dimension. Swelling will also make the inside larger, as the inside "points" would stay in the same place and push the rest of the frame including the rabbet further away. However, because the width of the rabbet is generally so small, the amount will be negligible - maybe as much as 1/64" total. In both cases these are assuming that the joint failed completely and there was no compression of the wood as a result of an initial bond.

Also, see http://artfacts.org/standards/frm_396.html for FACTS information regarding Determining Frame Allowance.
 
David N Waldmann said:
As far as the shrinkage/swelling of the wood changing the frame dimension, it will never get smaller, always larger, because of the nature of a miter joint. And in fact it will be the shrinkage of the wood that has the potential to make the rabbet size larger by a greater degree. Because the points of the miters will keep the outside size of the frame the same, the gaps opening up on the inside as a result of drying/shrinkning mean that the inside dimension of the frame is getting larger..[/B][/B]
Click to expand...

Thanks David.

I was getting a bit lost in the thread but you put it back on track for me. Makes sense now.
 
I don't go to PFM anymore but your point shows that not only TFG is plagued by KIAs( Know it all) who spot off there opinions and with out rebuttals from sensible people who use documented Internet information to refute those errors many would grow to have the complete wrong information as Gospel according to an Industry source.

Not only does wood swell and contract with changes in humidity but many other materials do also like mats and a lot of other paper products .So in answer to your original question which swelling is the allowance for ;it is for both or any swelling .

In a related but very old similar topic when old computer systems utilized key punch cards ,The Computer rooms had to be kept at a constant temperature or the cards would swell and jam the machines. There was much less tolerance in those machines than in a picture frame . However paper is a wood pulp product and as such i suspect the cell structure is the culprit.

But as Paul N pointed out almost ever wooden door experiences jamming or tightening in increased and decreased humidity conditions . But it should also be noted that with out any adjustment the door will normally return to normal when the original humidity is meet. that is unless you have a Katrina and they are submerged under water in which the cells may expand beyond their ability to contract. But getting back to the original concept many materials respond to changes in humidity and allowances just make good sens just as ignoring the comments that speak about things they don't really check out.
BUDDY
 
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