[ the craft of the guitar maker ]By TREVOR SEMPLE
5. HumidityHOW important are variations in humidity to the maker and player, and can they be ignored? Changes in humidity cause changes to the moisture content of many different materials. This in turn causes the materials to expand and contract, which is potentially very damaging. Museums go to a great deal of trouble to preserve fragile objects in constant conditions. This is the ideal way to store anything made of wood.
Unfortunately, nature here conspires against us. Island climates are typically very changeable. The lowest humidity levels ever recorded in Britain are around 12 per cent (all percentages are Relative Humidity readings). On a rainy day, levels will rocket to typically around 95 per cent nationwide, while a fog tips the top of the scale, dripping up to 100 per cent. At this stage, the air is totally saturated with water vapour. If you leave an open tray of water standing outside, the water will not evaporate.
This enormous fluctuation depends mainly on the wind direction. When the wind travels across open sea, it picks up moisture as it goes, so for much of the year we experience damp conditions coming from the Atlantic. The driest weather is brought during summer by tropical continental breezes originating in the heart of Europe. Along with many of the season's tourists, they take the shortest route from the continent, straight across the Channel, and consequently have very little opportunity to pick up moisture. Under these conditions, the humidity is typically around 20 - 30 per cent. Polar continental winds (E.or N.E.) similarly cross very little sea to arrive here, and being cold, are also very dry - around 30 - 40 per cent. Prolonged periods at below 40 per cent are quite rare. One such time did occur in the summer of 1976. Due to very dry air settling almost motionless over Britain, there was a period of some 8 weeks of very high temperatures, together with humidity of an amazing 20 per cent during the day! This was severe enough to cause structural damage to some buildings.
Leaving aside the unusual extremes, the general trends are like this: the summer breezes usually carry large amounts of moisture and often give rise to quite humid conditions, while the cold winds of winter can hold far less moisture, regardless of where they come from. Also, towards evening all the year round, as the air temperature falls, the air can support less moisture, and the humidity rises.
So what does this mean in practice? Suppose as an experiment we take a 35Omm square panel of wellseasoned spruce soundboard material in a room where the humidity reads 70 per cent (the national average). We then glue it all round the edges to a square metal frame and then take the lot into a dryer place, say with a humidity of 30 per cent. In these dry conditions, the spruce will contract by about 1.5 per cent across the grain, or a colossal 5mm! The panel would undoubtedly crack wide open, with the crack running parallel to the grain line. Now, 35Omm is about the width of a classical guitar at its widest point.
We are powerless to prevent climatic changes on so vast a scale, but we may seek to limit their effects by regulating the 'local' climate within our own homes or other buildings. The trouble is, things do not always work in quite the way you might expect. In summer, the moist outside air comes into the cooler shaded rooms of the house. As the air cools a little, it will raise the humidity level above that outside! This often gives levels above 80 per cent for days on end. In winter, the situation is reversed. The cold outside air is heated as it comes in, enabling it to carry much more moisture than before. This gives very dry conditions indoors sometimes even below 40per cent.
These effects are well-known to cabinet-makers, who traditionally fix large panels or table tops in some form of sliding fixing to prevent the furniture from tearing itself apart in dry weather. Guitars, on the other hand, have edges fixed all round. So why don't they split during the very dry spells?
The answer is quite simple - it is because of the arching. Both back and soundboard are formed by the maker into a gentle, graceful curve. Prolonged damp conditions cause the soundboard to expand. The wood is forced harder against the sides into a higher arch. It is far less free to vibrate, giving a detectable dullness. The action will also rise. At this stage, it is worth considering some sort of humidity control. However, it is virtually impossible to cause lasting damage through dampness. In very dry weather, the process is reversed, the panels contract, and the arch falls. Remarkably, the transverse bars (on the soundboard you can see them running along the edges of the soundhole) actually exaggerate these movements! Very occasionally I have seen a guitar where the soundboard has contracted to such an extent that the arch has reversed and become a hollow! At this stage the instrument is perilously close to splitting wide open.
The risks of damage to the guitar are greatly reduced if the instrument is built dry. For this you rely on the care and experience of the maker. Many Spanish-made guitars come from a dryer climate than our own, and rarely suffer these problems. Reputable makers in Britain work under humidity-controlled conditions (usually around 50 - 60 % ) and will be happy to advise you of the optimum level to maintain. An instrument built in this way will usually be safe at levels of around 40 percent.
Central heating is the greatest danger for most players. Quite apart from the very dry air, there is also the danger of leaving the guitar near a fire or radiator. This can cause damage very quickly, especially if your case is dark in colour. With a well made guitar, even if you do not use a humidifier, you will only have trouble in fairly rare circumstances, although it does appear that the climate may be changing with dryer summers becoming more common. If you use a humidifier during dry summer periods, and in dry centrally-heated rooms, you will never have any problems at all.
FOOTNOTES: 1. Relative Humidity. This is one of two possible measurements of the water vapour present in air. Absolute humidity refers to the actual quantity of moisture present, and is usually measured in grains per cubic foot, or its metric equivalent. Relative humidity, on the other hand, is the ratio between the moisture in the air at a certain temperature, compared with the maximum that the air could hold at that temperature. Warming the air will lower the relative humidity, although the absolute humidity will remain unchanged. For most purposes, relative humidity is the more useful measurement, as it shows at once a comparison of combined atmospheric changes. 2. Humidity control methods. There are three main types of de-humidifier..- Moisture absorbing crystals. The simplest, but only effective for small air volumes. Sometimes found in small sachets in the packaging of electronic equipment etc. Heat exchangers. They work by cooling the airflow, causing condensation. This is the usual method for most applications. Cycled crystal A complicated and very expensive way to achieve virtually zero humidity. Of use only during some specialist manufacturing processes (eg electronics). Ways of humidifying, on the other hand, are very simple:- Small sponge material to put inside the case or instrument (eg 'Dampit). Water trays to hang on radiators. Electric fan type humidifiers (for large spaces). House plants! 3. Seasoning. This refers to the process of removing some of the water from the newly felled timber, until it reaches reasonable stability. This point is reached when the remaining water corresponds to the humidity of the air. Wood in trees is wet! Extremely wet. Some species may lose 50 per cent of their weight during the process of seasoning.
©1990 Trevor Semple
"Classical Guitar Magazine", December 1990