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THE ULTIMATE DISPLAY, THE
ULTIMATE PROTECTION
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To understand why humidity is so important to your guitars, we must first familiarize ourselves with the types of humidity and how they are constantly at work in the world around us. If we say humidity, we are talking about the actual amount of water, in the form of water vapor, that exists in a given mass of air. This is commonly represented as grains per cubic foot of air. You would think this would be the most accurate way of representing humidity, but it has one major problem. You cannot predict the way the water vapor in the air is going to act without knowing the temperature of the air as well. This complex and dynamic process has its' own field of study, known as psychrometrics. From this study, we are able to determine relative humidity. This is a representation of the amount of water vapor in the air relative to how much it could possibly suspend. That is why it is given as a percentage. 45% RH means the air being measured is supporting 45% of the total amount it could suspend at that temperature. Like the name says, this is the study of moisture in the air relative to the temperature of the air. We're going to look at the factors that impact our guitars. The first is that water vapor is suspended in air, similar to how a teaspoon of sugar will dissolve in a cup of water. Second, the amount of water vapor a given quantity of air can hold (grains per cubic foot) depends on the temperature of the air. Warm air holds more water than cold air. Third, the process is very dynamic; moisture is always moving from areas of high humidity to areas of low humidity until they balance. When a meteorologist sees a mass of cold air moving south from the Midwest towards the warm moist air of the Gulf of Mexico, he knows rain will move along with that cold front. The reason is psychrometrics. The warm air is holding a lot of moisture because its' high temperature allows it to absorb a lot of water vapor from the ocean. As it meets the colder air from the north, the two air masses mix and the temperature of the Gulf air drops. Colder air cannot hold as much moisture, and that moisture has to go somewhere. It condenses out of the air as water and falls as rain. The same thing happens to the outside of a glass of iced tea on a summer day. The air temperature around the glass drops and moisture condenses out as water.
Now that we understand what relative humidity is, let's look at how it affects your guitars. Any luthier or woodworker can tell you that the moisture content of wood is critical. It is also constantly changing. The moisture content of wood is directly related to the humidity and temperature of the surrounding air (relative humidity). The equilibrium moisture content (EMC) occurs when the wood has reached an equilibrium with its environment and is no longer gaining or losing moisture. We said earlier that moisture is always moving from areas of high humidity to areas of low humidity. We now see that includes objects in the air. If you take your guitar, with all that fine spruce, rosewood, and ebony and leave it in a room that has an RH of 20%, the dry air will pull the moisture out of the wood until the EMC is reached. The result is very dry wood, with all the resulting problems. There is good news, however. One factor of relative humidity, air temperature, stays pretty constant in your home. The second is that wood is very reluctant to give up (or absorb) moisture. It is a gradual process. That is why you can play your guitar for hours in a dry, heated room without the top cracking. Your instrument spends most of the day in storage, and this is where it must be protected. This is the reason for the Acoustic Life Saver Cabinet. Finally, about our picture at the top of the page. The piece is called "Hilo Raindrop" by Perry Policicchio. He says, "Hilo's raindrops are the largest in the world. This bowl is a tribute to our rain." A perfect representation of the balance between wood and water. You can see his work at The Grove Gallery.
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