Yesterday John Tuttle asked what to do with a piano, kept in an
unheated and unattended place during the winter.  In my opinion, the
best thing to do would be to leave the piano itself unheated, rather
than to try to keep it warm by some local heating method, such as using
an electric blanket.

The potential problem is not freezing, but keeping the humidity around
the instrument correct, to prevent loosening of the pins and shrinkage
cracking of the joints.

There are two ways to measure the humidity in air.  Absolute Humidity
(the amount of water vapor in air, expressed in grams of water per
cubic meter of air) and Relative Humidity (RH).  That is the percentage
of the actual water vapor present, compared with the maximum amount of
water vapor that could exist before it appears as fog or mist.

Relative humidity, not absolute humidity, is the measure that
determines how much moisture is lost or absorbed by a hygroscopic
material like wood or paper.  Jonathan Holmes, the museum conservator,
recommends 55% relative humidity as the ideal environment for
preserving things (in MMD 010823).  In other words, at 55% RH, the
moisture in the air will be in equilibrium with proper amount of
moisture in the wood at its correct original dimensions, and the wood
will neither gain or loose water.

But the capacity of air to dissolve water vapor varies tremendously,
according to the air temperature -- warmer air can hold far more water
than cooler air.  So the same relative humidity is equivalent to a much
smaller absolute amount of water in the air when it is cold.

Taking an example, where the storage area goes down to 32 F. and the
outdoor relative humidity is 40%.  The maximum amount of moisture
(100% RH) that air at 32 F. can hold is 4.85 gm/m^3, so at 40 % RH
there is 1.94 gm/m^3 of water in that air.

Now suppose that you heat up the air around the piano to 68 F using
an electric blanket, but without adding any more moisture with a
humidifier.  The air inside the blanket still only contains 1.94 gm/m^3
of water, but at 68 F, its maximum water vapor capacity is 17.3 gm/m^3.
Now the relative humidity inside the blanket is only 11.2 %.  That is
far too dry, and moisture will be drawn out of the wood, resulting in
severe shrinkage of the pin block and many other potential problems.

This is the same effect that causes air in a heated house without a
humidifier to become too dry.

In other words, heating up the surroundings of a piano without adding
more moisture to satisfy the increased relative humidity demand can
only make things worse.  Low temperature per-se does little harm.  As
an example, the instruments in the British Piano Museum in Brentford
are housed in an unheated building.  In the winter, the temperature
often approaches freezing, but not much permanent harm is done to them.
They don't dry out too much, because the ambient relative humidity is
usually in the 30 to 60 % range even in cold weather.

Richard Vance