Harp Myth 6

This article is the sixth in an extended series of articles addressing some of the issues and myths surrounding traditional harps and their construction. This iteration is the second of five which deals with materials. Cracks are the nightmares of all stringed instruments. Preventing them is the goal of every luthier and player.

Why Instruments Crack

Cracks in stringed instruments can have a variety of different causes. Foremost is poor construction and/or mechanical design. Perfect wood cannot save a badly built or conceived instrument. Conversely, a beautifully crafted and designed piece can easily be ruined by improperly or incompletely dried woods. Yet, even perfectly crafted instruments of optimally dried woods can be cracked due to environmental forces beyond the control of the lutherie. The normative moisture content (MC) range for lutherie woods averages between 6 and 11% MC at between 65° and 75°. Chart 1-1 shows a recommended guide for MC depending upon the area into which an instrument is being shipped. Beyond ensuring proper construction techniques, instrument design and wood preparation, the hands and abilities of luthiers are limited by the environment in which the instrument resides.

Relative humidity (RH) is a slippery subject. The warmer air is, the more moisture it can hold. Here, in the Western Sierra at 3500' elevation, we are in a forested portion of an arid environment. The humidity in our shop hovers around 50% year round. When left to equalize in the open air of our shop with 50%RH at 70°, the equilibrium moisture content (EMC) for our woods is 9.2%. By warming the wood storage area beyond the ambient in the shop, the temperature of the air rises but there is no additional moisture actually available in the air. The EMC of the wood then falls because more moisture evaporates out of the wood into this artificially drier air.

To illustrate the effects relative humidity (RH) can have on an instrument consider a 16" wide spruce top. The top is at equilibrium at 70°F, 35%RH, and has a moisture content of 6.9%. A drop in temperature elevates the RH to 85%, and correspondingly the MC of the top to 17.9%. The top swells to 16.2609" - over 1/4"!

- Luthier's Mercantile Catalog-Handbook for Stringed Instrument Builders, ©1990

Now imagine this. Let's say you live in Utah and it is 100° outside. The RH is 15%. If you check Chart 1-2 again, you see that the EMC of your instrument has just fallen to a perilous 2.27%. Air conditioners further complicate matters by driving out even the small amount of moisture in the air. You know all that body lotion, hand cream, hair moisturizer and lip balm you need? Your skin can not take the prolonged losses of moisture and neither can the fine cellular structure of the instrument woods. We generally are proponents of the belief that if you are uncomfortable in a given environment, your harp will be as well. The problem with desert climates is that people can cheat. Your harp has no access to lotion and would not know what to do with it if it did. [The same is true for any solid wood furniture in your home.]

	5-15%	20-30%	35-45%	50-60%	65-75%	80-95%
30-50º	2.57	5.47	7.90	10.40	13.57	20.08
60-80º	2.43	5.37	7.93	10.07	13.13	19.48
90-110º	2.27	4.97	7.20	9.43	12.27	18.90

Chart 1-2 Avg. Moisture Content of Wood at EMC,

given temperature and Relative Humidity.

Especially at risk is the thin harp soundboard which is under hundreds of pounds of string pressure. The optimal answer is to humidify your home and solve the problem for your own skin and household goods along with your harp. The less expensive approach is to increase the local humidity (see Harp Care Hint "A Sponge In Time";). A quantitative rule of thumb is that if the RH falls below 35%, take precaution.

Cracking is a danger to instruments but fear not. Instruments are more resilient than they appear and most cracks can be repaired and the instrument returned to its original vigor. We do have one warning however. Do not allow concerns about relative humidity turn you into a Weather Channel addict. William is, well... hopeless. I have actually caught him watching "The Best of Weather", which recaps the highlights of the past week in weather. Do they have "Weath-Anon" groups?

[General Articles] [Harp Care Hints] [911 Harp]

[Main Menu] [Organization] [E-Mail Us]
[Download Resource Articles] [Search the Rees Web Site] [Web Site Map]

© Wm. Rees Instruments, 1997, 1998, 1999
222 Main Street, Rising Sun, Indiana 47040 • voice: (812) 438-3032 • web: http://traditionalharps.com
The address of this page is: http://traditionalharps.com/HarpGeneralCracking.html