relative humidity

We call Relative Humidity the ratio between the water quantity actually present in a given volume of air, and the maximum quantity of water that could be present in the same volume, before the water begins to condense in microscopic droplets, forming what it's usually called fog. This limit situation is called saturation point or dew point. A cubic meter of air, at 20°C, and at the average atmospheric pressure at sea level (around 1010 mb) weights about 1.2 kg, and can contain at the vapor status (just at the dew point) about 18 g of water. The Relative Humidity (RH) is always expressed as a percentage: so if in our cubic meter there are 9 g of water vapor we will say that the RH is 50%; if there is no water at all (the "maximum dry" situation) we will speak of a 0% RH, and at the saturation point (the "maximum humid" situation, just at the saturation point or dew point) we will speak of a 100% RH.

The maximum quantity of water that a given volume of air can contain increases when temperature increases: so, for a cubic meter at a pressure of 1010 mb, it is around 4.5 g, at 20°C it goes up to 18 g, reaching 105 g at 50°C. We find an analogous dependence versus the atmospheric pressure: the maximum quantity of water that a given volume of air can contain decreases about 12% for every 1000 m over sea level (and the atmospheric pressure diminishes accordingly). These numbers are to be taken only as rough indications of a much more complex temperature and pressure dependence: however they are more than sufficient for the moment, and they are most important for the correct maintenance of any luthery instrument (thus not only for guitars, but also for violins, cellos, harps and others, made as complex wood, glues and organic varnishes structures). We must never forget that we live in a world in which rapid movements are very frequent, and therefore the instruments can easily undergo to abrupt changes in temperature (car trunks in a sunny parking or in a winter night) and pressure (car trips in the mountains or a travel by plane) in extremely short times. And moving the instrument from outside environment into classrooms or auditoriums has not to be disregarded in this respect.

Some data are most important to be kept in mind:

Inside a car (not to speak of the car trunk) exposed to direct sunshine one can reach easily, in a short time (1 hour or even less) temperatures around 50°C;
In winter one can easily reach temperature under 0°C;
In a plane the passenger cabin air-conditioning system stabilizes pressure and temperature, but produces an extremely dry environment;
In the luggage vane of a plane the air-conditioning system has an even lower performance, and risks are higher.

Especially in a plane, it's very wise to loosen the strings, in order to reduce the overall stress on the instrument.
In winter sudden changes in temperature can have most serious consequences: a guitar at 50% relative humidity and 0°C is in environmental equilibrium without any problem, however it is surrounded by air (e.g.: the air contained in its case) which contains only 2.2 g of water per cubic meter. If now one brings it (and usually in a short time) in a room at 20°C the relative humidity of the case drops to 2.2/18 = 12%: this sudden change causes a rapid evaporation of the water from the wood fibers and structure, called the "dry shock". And the situation can be even more serious the initial situation is already a dry one: for example a dry, cold day with northern winds blowing, when one can reach easily relative humidity around 10%, or lower. We experience dry hand skin, chapped lips, high visibility in a crystal like atmosphere, electric shocks in hand-shaking. If we bring this already dry air inside a 20°C room, we can easily imagine the final value of the relative humidity. A sudden change from a humid to a dry environment implies a heavy and humidity loss from the wood fibers, sometimes so quick that they simply don't have time to settle: some parts dry more rapidly than others, and the inevitable result is wood fissuring and cracking.

The ideal relative humidity for a guitar (and for every advanced luthery instrument, by the way) lies between 50% and 65%, but it can move outside this interval without danger, if one follows some precautions: a continuous and careful monitoring of the environmental conditions is essential, through the use of instruments for humidity measurement and of humidifying devices. However one has to remember that temperature, pressure and humidity variations outside the recommended range (not too much...) can be safely accepted by the instrument provided that they be slow, giving enough time to the instrument to adjust to the new condition.

The apparatuses dedicated to the relative humidity measurement are called hygrometers, and can be either mechanical or electronic. In mechanical hygrometers one exploits the property of several natural fibers to change length with humidity, just as wood which swells in a wet weather and shrinks in a dry day. Human hairs have excellent properties of sensitivity, precision and accuracy even when temperature changes: so in the hair hygrometers, with some lever-spring games, a bundle of human hairs is used to move a pointer. If of good quality (and this speaks: not cheap) this kind of hygrometer is between the reasonably priced instrument, one of the most precise and with a highly constant performance in time. It has not to be confused with the double-layer hygrometers, which as sensitive element use a couple of laminae: they are similar in the outside appearance, but lower in sensitivity and precision, and usually much cheaper.

Calibration is one of the difficulties in hair hygrometers use. A mostly used method is to wrap the instrument in a wet cloth, and after some minutes put the pointer to 100%: I tried this method and found it definitely rough and inaccurate. Much better to calibrate it with a more precise (and more expensive) instrument called ventilated psychrometer, which however is not advisable in guitar maintenance and care, being unpractical both in reading and in use.

Nowadays you find also electronic hygrometers, which are good and precise enough , even if I find them less convenient in reading and precision, if compared to the hair hygrometers. However they are good devices: obviously if they are of good quality.

With a hygrometer you will therefore be able to read the environmental relative humidity, and take the appropriate actions. In a too humid environment (relative humidity over 75%, and constant) you should dehumidify: not an easy task if one takes into account that the necessary device (the dehumidifier) is costly and bulky (similar in dimension to an average bed side cabinet). Today it is a common and very useful appliance, but it's also true that its purchase will be unlikely, in a modern home, for the only sake of a guitar's health. And it's also true that humidity increases (usually connected to meteorological changes), are seldom dramatic, usually slow, and not very harmful for the instruments.

The opposite problem is, on the contrary, much more likely, and in this case a humidifier is needed. In the stores one finds humidifiers of various types, but the best are the vapor based ones, with two electrodes and a small boiler: they use some more electricity, but they are sturdy and above all they are healthy. When using very soft water (a frequent situation in mountain zones) you can use a tiny dash of sodium bicarbonate, to increase the ionic contents in the water, and therefore its electrical conductivity (and consequently the possibility of vapor production).

The ultrasonic humidifiers (the so-called cold vapor ones) don't produce vapor through heat, but use an ultrasound source to produce a very fine fog, which is pushed outside with a small fan. They are very quiet, they use very little energy and they produce a high vapor quantity: however they are more costly and they require a continuous cleaning and water changing, as they do not disinfect the vapor as the previous mentioned appliances do. On the contrary, continuously mixing cold water with air (and dust etc. contained in the air itself) they create a most favorable situation to bacteria proliferation.

Summing up, with these instruments and appliances we will be able to monitor and keep the environment in which the guitar is normally stored in optimal conditions: for example in the room in which one studies. And this, by the way, is very desirable also for our health.

When the instrument is moved (and usually in a very short time) into classrooms or auditoriums, where one can find critical humidity conditions, one must take even other precautions.

If the outside environment is very humid (in foggy days, for instance), we cannot do much: the instrument will swell a bit after a long staying in this situation (even if the case protects it pretty well), its performance will be a bit lowered, but it will not be seriously endangered; provided that one puts much attention when bringing it in very dry, or much heated, environment, like classrooms or auditoriums. In these case the guitar would be exposed to a sudden loss of the high humidity absorbed and stored in the wood fiber structure, with a serious fissuring risk. An excellent precaution to prevent these dangers is to humidify only the guitar, or, more precisely, its interior, with a small, but very efficient inner humidifier. Remember that the inside of the guitar usually is not protected by the varnish, and is more vulnerable because it is directly exposed to the outside environment through the hole. In this way we will create a higher, and protective humidity inside the guitar, avoiding the risk of fissures and cracks. This is exactly the analogous of what we see often in museums and exhibitions, where small bowls filled with distilled water are put inside showcases where delicate and humidity sensitive exhibits are stored.

Beware, however, to be obsessed with these precautions: experience and practice will guide you in an efficient care of your guitar, which (one must always remember this point) is in any case a very sturdy object, even in extreme conditions. A good habit is to always take with you the hygrometer and the internal humidifier: a bit of prevention helps in maintaining the guitar at its optimum performing conditions, also avoiding damages which will call for lengthy, and painful, repairs.

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the varnish

Several types of varnishes are used in guitar making, but the only acceptable ones are the natural varnishes, which can be wax-based, resin-based, shellac-based. As for guitars are concerned the most widespread varnishes, and those that give the best results, are the shellac-based ones.

Shellac is used from ages for wood varnishing, and is secreted, as a protection, by an insect of the cochineal family, living in India. Conveniently treated and purified, it's usually sold in the form of very thin, glossy, amber-colored flakes, and it is the base for many types of guitar making varnishes. Following all kinds of recipes, often many ingredients are added, among which dyes, resins, waxes, and everything is always dissolved in alcohol.

For the varnishing procedure a pad (a cotton or wood wad wrapped in a cotton or linen piece of fabric) is used, following a very ancient technique, hard to learn and lengthy in execution, which today is used only for handcrafted musical instruments and for handcrafted highly valued furniture.

The French polishing plays an important role in sound quality, gives to the wood a wonderful appearance, and is unsurpassed in exalting the beauty of wood grain. Thickness is very tiny, of the order of 0.1 mm (the thickness of a photocopier paper) or less, and the less varnish one is capable to use, the better is final result. However the varnish coating is delicate is not so much resistant to scratches, heat, sweat, and needs periodical and regular maintenance, in order to keep its lustre and to constantly protect the wood. In this respect it's very different from synthetic varnishes, which are often used in guitars produced nowadays, and which are often applied in coatings that can be thick as much as 1 mm; these thicknesses are therefore even ten times (or often more) the shellac ones, and are actually comparable with the wood thickness of the soundboard itself. These varnishes are definitely highly resistant to scratches and wear, but they have the disadvantages to cause a worsening of sound quality, and they do not exalt the beauty of underlying wood grain.

A so special and delicate varnish requires some minimal knowledge about its characteristics, if one wants to maintain its properties at their best, owing to the fact that the varnish is basically different from the varnish concept we are used to. The shellac coating is, in fact, totally compatible with the wood inner structure, and, once applied, penetrates deeply inside it, where it is slowly absorbed: the final result is more a polished wood, than a varnished wood.

So, first of all, one has to pay attention to the guitar temperature: shellac and resins which form the varnish are very heat sensitive, they soften easily at high temperatures, and can be marked by anything that comes in contact with them. So it can well happen that when opening the case, warmed up after staying in a car parked in the sunshine, one finds the texture of the case padding carefully reproduced on the beautiful glossy back of the guitar. Analogous problems one can have just playing the instrument in hot summer day: it's very easy to find marks of our clothing reproduced on the varnish just in regions in which there is direct contact with our body (breast, legs, right forearm). It's always advisable, in these cases, to protect the instrument with a soft cloth or with a buckskin. One must also remember that the varnish is very much sensitive to sweat and to the salts dissolved in it: they can cause unpredictable deteriorations in the varnish coating, like loss of luster, shrinking, changing in toughness and texture. It is truly indispensable to protect the instrument in hot weather.

Would the varnish be damaged, one should not panic, however: a not minor advantage of a shellac based varnish is its reversibility, and this is an indispensable characteristics in case of repair or restoration. As one can repeat indefinitely the process of dissolving of resins and of shellac in alcohol, the varnish can be renewed, retouched, polished, and the final appearance is absolutely equal to that of a brand new guitar (and sometimes even better). For minor inconveniences a complete revarnishing is usually unnecessary, and local retouches are usually quite sufficient. Synthetic varnishes, on the other hand, are very highly resistant to scratches, temperature and sweat, but they give serious problems in repairing: being irreversible one must take off completely the varnish coating from the whole instrument, and re-varnish it completely (i.e.: soundboard, sides, back, etc.).

Please remember also that the wearing of the most used parts of the instrument (neck, back in the region which is in close contact with the breast, sides in the regions in contact with legs and forearm) is quite normal, and requires a regular and periodical maintenance, needed by the instrument in its life, in order to keep it at its best aesthetical appearance. So at regular time intervals the instrument must be taken to the luthier both for a general checking and for varnish retouches, should them be necessary: this will avoid exposing the wood to filth and to the aggressive chemicals produced by the skin, which could stain it permanently. The only part which, normally, is not re-varnished is the soundboard (mostly for acoustic reasons), and this is one of the less sturdy parts of the guitar, being fir a very soft wood. So please pay much attention to scratches and: once made, they will stay there forever. If it's true that small and tiny marks can also be a pleasant testimony of use and of history; it's also true that, if deep and conspicuous, they are definitely unappealing. To clean your guitar use a soft cotton cloth, better if plushy, and only recommended products. Failing all else, you can use a soft paper towel or a soft rag, wetted just a little bit, lightly rubbing the instrument and immediately wiping it dry with a soft cloth.

And, please, remember: in cleaning your guitar absolutely never use furniture products, be they spray, liquid, oily or creamy, and especially if alcohol or silicone based. Absolutely never use cleaning products of any kind in presence of damages and fissures: one could run up against major problems should repairs be necessary.

Always remember that the varnish is a resin, and that, penetrating deeply, beautifies the wood and enhances the sound richness of the guitar, but it's delicate and needs care and attention in order to save its precious lustre for a long time to come.

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