Temperature and Humidity in Wood

Joseph and I have been working this weekend on paper to illustrate the effects of humidity and temperature on your harp (or any wood) and what you should and should not worry about. Here it is for your edification, as my mother used to say...

Significant temperature and humidity will affect solid, thin, large sound boards on musical instruments like pianos, harps, and string instruments. In this article we are referring to harp soundboards made with solid spruce. 

During the 2021 summer in Vancouver Island, Canada, we experienced a heat dome event with outside temperatures reaching 43°Celsius, with high humidity. That is tropical weather never heard of on our Island and that we only hear about from somewhere else in the world. But it did happen here in Duncan, BC and it will happen again in the future. We need to learn and prepare for those eventualities now. 

The first consideration is humidity in the hot summer. Wood is a hygroscopic material, meaning it absorbs water from the atmosphere; the greater the air humidity, the greater will be the amount of water absorbed by the wood. At 100% relative humidity in the air, wood absorbs the water vapour to about 30% of its weight in water - a 100g piece of wood will have 30 grams of water in it. That amount of moisture will greatly affect its strength, bending and hardness properties, and its volumetric shape (by about 11% for spruce). 

Wood exposed to air at its dew point (100% humidity) will experience the greatest rate of absorption of water, and microscopic condensation on the surface will occur. Wood is like a sponge. Over time, it will absorb moisture even with protective coatings (finishing may slow down the absorption but will not stop it). 

The relationship of various temperatures and humidity to the wood’s moisture content can be illustrated like this: 

At about 20° C and 55% air relative humidity, wood has about 10% moisture content, which is roughly the average temperature and humidity in our shop. In that scenario the dew point will be 10° C, meaning that if the temperature falls to 10° C, the wood will, over time, absorb the water vapour from the air, to a wood moisture content of up to 30%. (Our shop, where wood is stored and the harps are made, is kept at a constant temperature and humidity throughout the year so that the moisture content of the wood stays the same.) 

At about 30° C and 75% air relative humidity (for a humid summer day or evening), wood is at 15% moisture content. However, the dew point is now at about 25° C, which means that over time the wood at that temperature will rise to 30% moisture content. 

It gets worse as the temperature rises. At about 40° C and 75% air relative humidity, wood stays at about 15% moisture content but the dew point is much higher - now at about 35° C. Below that temperature, condensation will occur at a microscopic level on the wood surface. This is dampness that may be felt if the change is rapid, and is not good for your harp. 

A second consideration is the temperature alone. Temperature in the high 20s is not good for your harp because wood and strings soften and loose their springiness. That springiness does not come back when the temperature cools down because the constant load of the strings’ tension alters the internal structures of wood and strings. This effect may be small at 27° C but increases rapidly with temperature rise. Visually you may not notice any differences but the sound board and the harp will have lost some of its acoustic quality, or its brightness. This is a permanent situation and is to be avoided by keeping your harp in a cooler place (though the humidity effect will still be a factor, it is not as critical) 

At high temperature, wood fibres swell and expand, in addition to the swelling and expansion caused by increased moisture content. Expansion, whether from temperature or humidity increase, or both, has a structural consequence, and the faster the rate of change, the more problems result. The sound board wood is locked in position on the harp; the fibres have no place to expand sideways, so they move up or down, detaching themselves from the cells or cell walls they are connected to. This change can often be seen as a surface ridge, an elevation line, or a crease. When the harp cools, these “wrinkles” may stay. 

These elevated wood fibres do not compromise the structural or acoustic quality of the sound board on harps because of the way harps are made. They are, however, visually unattractive, even though they are structurally nothing to worry about. Cosmetically, they can be sanded down and refinished with Tung Oil. At first there will be some wood discoloration but over time it will mostly blend again with the colour tone of the sound board. However, this unnecessary except for cosmetic purposes. 

The opposite effect of high relative humidity – excessive dryness - should also be considered. Shrinkage, the opposite of the swelling from moisture, also occurs. At lower humidity and temperature, the wood will contract, and because the sides and ends are locked with glue to the box, the fibres may separate from each other. You will see gaps in the board that a light can be seen through. This occurs mostly in areas where the outside temperature is below freezing, where the relative humidity in the house is likely to be much less than 50%. These separations are not a problem acoustically, though the cosmetic repairs are more difficult. Many harps are built with veneers that cover these visual alterations, but the veneers may also affect the acoustic response of the soundboard. 

The soundboard is very thin. In comparison to the rest of the harp, it will lose or absorb heat and humidity faster than the other parts, and it will let you know when that happens, especially when those changes are sudden. Alteration to wood fibres is accelerated with rapid changes in temperature due to sun or heat sources, and changes in humidity due to too much or lack of ventilation. 

Some effects will only be cosmetic like the wrinkles or gaps, but others will be structural and much more crucial. The most significant is excess heat. Your harp is under an heavy load from the all the strings. A typical 36 string harp needs to hold about 500 to 750 Kg of tension. Your thin sound board, a few millimetres thick, does most of it, and is the component that is most affected by temperature. 

Lessening the tension in room temperatures over 27° will protect the acoustics and the structure of the harp. Any extreme environmental disturbance will be a challenge for the harp, so the more you minimize the changes, especially rapid ones, the more you protect your instrument. Cases help smooth the transition from one temperature to another, and aiming for a fairly constant room humidity and temperature will help keep your harp in tune, and provide the long years of great sound that you want to hear.