Rate of response of wood to RH changes

I have a question:

Given that pianos are affected by relative humidity changes in their immediate environment, and given that the response of the wood to its environmental conditions appears to be one where it seeks an equilibrium with that environment, (cf. Wood Equilibrium Moisture Content Table) does anybody know what the rate of response of wood is to changes in its environment? That is, how quickly (or slowly) does wood respond?

For example, let's say we have a piano 'at equilibrium' in a room with stable RH of 60%. We change the RH to 40% and keep it there. How long before the wood in the piano reaches its new equilibrium? Hours? Days? Weeks? And if the answer is on the longer (slow) side, does that mean that in a typical environment with fluctuating RH, the piano is permanently playing 'catch up' as it first chases one equilibrium, then another, etc?

Also, is the rate of response for acquiring moisture the same as that of releasing it? In other words, if we took the same piano now at equilibrium at 40% RH and switched back to 60% RH, would it reach the new (original) equilibrium in the same time, or more quickly, or slowly?

I'm curious about this because the issue of how quickly or slowly wood responds to RH changes appears missing in most discussion of RH change and its effects on pianos and it seems to me important for anybody monitoring changes in RH as part of maintaining their piano.

With thanks in anticipation,

P.

All else being equal, my guess is that wood takes up and gives up moisture at the same rate.
If it has a finish on it, depending on the finish, it slows the rate.
For wood to respond to RH changes I don't know exactly but I would guess hours - on the slow side.
When tuning is altered because of environment, it is more likely the temperature that is affecting the steel strings immediately and then the iron plate a bit later followed by the wood responding to RH/temp.

Wood absorbs and gives off moisture far more from the end grain than its surface. This is why curing wood cracks out near the end of the timber first. Expensive woods are often treated in this area to retard the drying there.

I typically see a week or two pass on a newer piano before a full continuos humidity swing takes its full toll on movement. Some very old pianos (+75 years) can take far longer to fully shift. A colleague of mine seen a 100+ year old grand piano take well over a year to adjust when its Dampp Chaser system was turned off. The denser more tightly structured grain of the old growth wood is likely responsible for this. There is no material/finish that works as a perfect vapour barrier for soundboards.

This study found a time lag of 41 hours in historic furniture http://www.english-heritage.org.uk/content/imported-docs/k-o/musmicchistables.pdf

A raw unfinished board measurably changes dimension across the grain relatively quickly when exposed to rh/temp changes. Within hours.

When considering the change in a simple block of wood, the factors are at least these:

The type of wood
Some woods have a more porous structure.

The relative amount of end grain to radial or tangential grain

The type finish on the wood

The thickness of the finish and its condition

The gradient of the change in relative humidity - how big a change there is
(In my house, it can go from 70%+ to under 30% in two days, if I don't do something about it.)

The buffer capacity of the room
(A library full of books or a room full of plants both act as buffers in very different ways.)

The rate of air circulation around the wood

In a piano you have rapid response areas, such as the bridge caps and the sound board, depending upon the type, condition, and thickness of the finish. You have intermediate changes in case parts with old, crazed finishes. Then you have components which do change, but change very slowly, such as the top beam in an upright, or the deep potions of a well-finished pin block. (Few pin blocks have a finish, but they should have it.)

A harsh change (big gradient) can knock a sound board out of kilter in three days or less, with the tenor section or mid section often going out terribly as a section. A pin block could take a week or a few weeks, if it has a good finish and tight pins and bushings. The symptoms of that change usually show up as individual strings within unisons going out with each other. How this detuning presents itself depends upon whether the tuning pin is unbushed, or is bushed. A bushed tuning pin often acts as a lever against that bushing when the wood of the pin block shifts, detuning in the opposite direction than what you would expect because of that.

You have asked a complex question. It's a mess, huh? How does any piano stay in tune?

Control of humidity is a good goal.

Years ago I compiled some data from the Forest Service wood technology publications and the average rate for soft woods for absorbing increasing humidity was about five times as fast as the rate it would give out moisture.

That is why when you belly a soundboard it takes several days to drop the moisture into the 4% EMC range and only a few hours for it to crown up significantly when out of the hot box.

This is why you only have a time window of about 20 minutes to fit the dry board, (panel) to the rim before bellying, and only then a couple of days after that-about 20 minutes to glue all the ribs to the panel. If you can't fit the panel to the rim that quickly-you have to wait a day or two with the board back in the hot box to try again. Once you start gluing ribs you gotta get to the finish line in about 20-30 minutes or you are screwed.

That is why protecting a piano from humidity spikes is more important than exposing it to a few days of lower humidity. Go damp chaser with humidistat!
I do not get paid for saying this.

One of the most basic variables has been left out - thickness of the wood. Obviously, the thicker the wood, the longer it will take to reach to reach equilibrium. Perhaps there is a formula for surface area vs. volume of the wood, with the different surfaces (end grain, radial grain, flat grain) having different rates of course.

There are obviously far too many variables at play to give a simple answer the the original question:Yes, the piano is always lagging behind the environment, but it may catch up if the environment's conditions are stable for a time.

Very interesting. Thanks for this, Ed. (It's summer here, and we do get humidity spikes - had one just last week, where it shot up to 85% for two days.)

Thanks for all the responses. Yes, there are multiple factors at work, but I wanted a rough idea of the differential propensities and speeds to absorb/release humidity--all other things being equal--and I think I have enough information for my purposes from the various responses.

Many thanks,

P.

EUh, OK, but what parts are in wood in a piano ?

In central Georgia our periods of very low humidity are normally brief, although our humidity can often be high for much longer and quite often. I have gradually come to the conclusion that, for our climate, a dehumidifier rod with a humidistat is the practical route for most pianos - useful in most situations. The rest of the system (for adding moisture) is not so critical unless the heating system tends to dry the air excessively (as do many modern commercial systems which have a high fresh air intake ratio.)

What Ed said about wood gaining moisture faster than it looses it reinforces my assumptions for our area. This counter intuitive, so I am glad to know that careful research bears this out.

In colder areas in which the indoor relative humidity can stay under thirty percent (even under 20%) for weeks at a time, the use of an all-weather system, such as the Dampp Chaser Piano Life Saver is almost a necessity. When cycled through the torments of northern climate, it is hard to see how a sound board can perform well for more than a couple of decades. On the other hand, good control of humidity swings could result in very long life for a sound board, regardless of how the crowning of the board was initially achieved, but especially helping the compression-crowned boards have extended vitality.