Face side

[MusicMan]

Custard, I think that is correct. A similar effect occurs in the tenons of woodwind instruments (the joints in the middle), which are restrained from expansion by the waxed thread lapping that was formerly used for sealing the joints. Of course they are by no means in a controlled atmosphere and suffer over 90% humidity at a somewhat elevated temperature when the player blows them. The wood in the joints gets very wet, but is constrained from expanding against the lapping so expands inwards. Most woodwinds with lapped joints that have been regularly played for 10-20 years or so therefore have slight (maybe 0.02 mm) constrictions at the insides of the tenons, which sometimes affects the playing. Nowadays cork 'lapping' is used which is much more compliant and the problem has gone away.

(Wood wind bores are normally made to a precision of about 0.01-0.05 mm and have been since the 18th century).

Your explanation in furniture is interesting and plausible. I don't think it is elasticity that is lost, but that while wet it is constrained to stay flat by the frame, but when it (the top surface) dries, it will shrink again and go concave.

Keith

 

[Trafalgar]

If the side is glued up of many narrow pieces then it's less an issue, though overall cosmetics may very well be if the article isn't veneered -- a matter of style and taste not the subject of the thread.

Here's an interesting thing, given enough time any horizontal surface almost always tends to cup up at the edges, in other words the top surface tends to become concave. I've seen hundreds of antiques that follow exactly this pattern, which included single wide boards with the heartside both up and down, as well as jointed surfaces that ranged from two boards to many small staves.

I've heard an ingenious explanation which if I can remember correctly goes along the lines that the uppermost surface will initially collect the moisture condensing from the air, so will tend to cup downwards, in other words become convex. But because the surface is generally fixed to the frame below the wood cells in the very top layer eventually become compressed, almost crushed, and over time lose some of their elasticity, so the "resting" position eventually becomes concave.

Whether or not this is correct I don't know, but the statistical prevalence of top surface concavity is too significant to ignore.

Very good stuff.

 

[J_Cramer]

Custard, that is most interesting. I remember finishing expert Bob Flexner mentioning something like this in his writings a few years ago, it was called 'compression set' or 'compression shrinkage' or something along these lines. There are also papers about the subject written by one F.C. Howlett, e.g.

http://cool.conservation-us.org/coolaic ... owlett.pdf


Cheers
Jürgen

 

[Bluekingfisher]

The Mona Lisa was painted on board (poplar) in the very early 1500s, is housed in a rigorously controlled environment, and still the wood moves and gives its curators fits.

And very much an anti climax it is too...............having seen it recently!

David