Wood, Oil and Water
|# Coats||Linseed Oil||Spar Varnish||Polyurethane Varnish||Two Part Epoxy||Paraffin Wax|
*Numbers represent uptake of water vapor by brush coated Ponderosa Pine samples compared to uncoated controls exposed in a high humidity chamber (90% RH @ 80 F) for a period of 14 days. Linseed Oil exposed for 1 day gave values of 88 and 78% for 1 & 2 coats respectively.Oils and Wood | top
VEGETABLE OILS: In fine grained woods, vegetable oils are often used by instrument players to prevent rapid water-induced wood swelling/shrinking. Nature uses Glycerin, an alcohol with three HO- groups, to make these. Each of the HO- groups is condensed with a fatty acid, R-COOH, to form a glyceryl triester (Figure 1). If this is liquid at room temperature it is called an oil. If solid, it would be a fat. A shorthand notation for the fatty acids will help in the following discussions. Their structure can be represented by (1) the number of carbon atoms, (2) how many C=C double bonds exist, and (3) where these bonds are located (indicating the carbon atom # at which they begin, counting from the -COOH); thus 18:2;9,12 represents CH3-(CH2)4-CH=CH-CH2-CH=CH-(CH2)7-COOH. (18 = the number of carbons, the 2 = the number of double bonds, and the 9 and 12 = the # of the carbon atom where double bonds start.)
Some of the acids found in recorder oils are listed in Table II.
UNSATURATION: Compounds without C=C double bonds are saturated. Double bonds create unsaturation, since the carbons COULD bond to other atoms if they were available. For example, Oleic Acid can be converted to the saturated Stearic Acid by the addition of a molecule of H-H across the double bond. The degree of unsaturation in an oil determines its drying characteristics, and can be found by adding Iodine (I-I). The Iodine Number is the # of grams of iodine taken up by 100 grams of oil. Table III lists the approximate Iodine Numbers for four oils made up of the pure glyceryl triesters from Stearic, Oleic, Linoleic, and Linolenic Acids. The name Tristearin comes from TRI STEARic acid glycerIN, etc...
|Trilinolenin||255 (very unsaturated)|
Vegetable oils are mixtures of various saturated and unsaturated acids. With just Stearic Acid and Oleic Acid there are four (4) possible combinations. Using G to represent glycerin, and U and S to represent the unsaturated and saturated fatty acids attached to the glycerin, we have GS3, GUS2 ,GU2S, and GU3. Just imagine the number of combinations possible for 6 or more acids, as is common! Vegetable oils do not contain just one substance, but hundreds. Nature distributes the acids rather randomly among the many glycerin molecules, except that it usually avoids GS3 combinations which are solids. This is called a restricted random distribution. Why is this important? It determines the drying characteristics of the oil which is discussed later.
Table IV shows the fatty acid composition of typical oils that have been used for oiling recorder bores. These are nominal values, since genetics, growing conditions (soil and weather), storage, and processing can dramatically affect the ratios. The Table also gives an average Iodine Number for the oil, and divides the oils into non-drying (Iodine # <100), semi-drying (100-150), and drying (>150). The first remain quite fluid after application to a surface, semi-drying oils become somewhat more viscous over an extended time, and drying oils eventually form a tough film after 3-6 days.
DRYING OILS: For unfinished porous wood, rough surfaces, and occasional oiling the drying oils work best. When applied, the oily touch soon changes to a surface film described as "set-to-touch". The film is sticky, and if broken will transfer an oil residue to your finger. During film formation oxygen is rapidly absorbed by the surface of the oil, up to 10% by weight. After the set-to-touch point is reached, oxygen uptake is reduced by the polymerized surface. Eventually the film becomes hard and smooth. It helps repel bulk water but allows water vapor to slowly pass through. The oxidation process induces a complex polymerization process creating a three dimensional network that entraps smaller molecules, a gel.
What characterizes a drying oil? The ratio of the rate of oxidation of Oleic, Linoleic, and Linolenic Acids is 1:10:25. It is easier to polymerize carbon chains with two nearby C=C bonds (linoleic) than it is with just one double bond (oleic). Three such bonds in close proximity make it even better (linolenic). The presence of the three double bonds also promotes the 3-dimensional gel cross-linking, as does high GU3 content.
SEMI- and NON-DRYING OILS: These oils coat the wood surface, also preventing penetration by liquid water and reducing water vapor transport. The oils have a low vapor pressure and do not evaporate easily. However, periodic renewal is necessary. These may be of vegetable origin, a low viscosity mineral oil, or even a citrus oil.What, Which and When | top
Experience suggests it is best not to randomly mix the two oiling approaches. Wax impregnated woods are already protected and occasional application of non-drying oils might help. Unfinished, rough bores will benefit from application of drying oils every 3-4 months for the first year, and then semi- or non-drying oils thereafter. Fine grain bores are possibly serviced best by semi- or non-drying oils from the outset. Always remove excess oil.
WHICH? But which oil to choose? Tung Oil, which has "conjugated" double bonds (C=C-C=C-C=C) drys very quickly, but may leave a frosted surface if you are not careful. Addition of iron ion or lead ion driers can help reduce this. The average player will use linseed oil. This should be the "raw" linseed oil. The nomenclature in the area is arcane and confusing. Raw linseed oil has actually been processed to remove undesirable materials. "Boiled" linseed oil has cobalt and manganese driers added to make the polymerization process faster. Its films can set up in hours, rather than days. Older boiled linseed oils had lead salts as the accelerator, but these have been abandoned because of toxicity. Unfortunately, cobalt driers can lead to frosting, since "top drying" of the film is promoted. This leads to undesirable volume changes in the maturing film that may lead to cracking. Use raw linseed oil.
With semi- and non-drying oils, rancidity must be considered. This implies degradation of the oil to produce offensive or unusual odors. And this often makes the literature confusing. The reports are not necessarily wrong, but oils from different geographic sources, growing conditions, and processing steps respond differently. And rancidity can come from several sources. For example, liquid coconut oil would appear to be a perfect oil, since its low Iodine Number suggests stability. However, under certain conditions, hydrolysis of the esters back to free acids with unusual odors occurs.
Olive oil is a very promising candidate. Pure Triolein is rather resistant to oxidation, but Linoleic Acid appears to catalyze oxidative rancidity. Fortunately, high grade virgin olive oil has natural anti-oxidants present. These prevent the formation of oxidized Oleic Acid products that have unpleasant odors. Unfortunately, this same olive oil may also contain chlorophyll, trace metal ions, or other materials that serve as pro-oxidants, by activating oxygen in the presence of light. Which side wins?
IS AN OIL "THE" OIL?: To make life more complicated, plant growers and nature are altering the genetic make-up of many of the oil producing plants. Peanut Oil is in a transient state of natural genetic modification. Rape Seed Oil is an example where man has intervened. Many of the normal plant species produce an oil with high Erucic Acid content. Animals cannot use glycerides from this acid efficiently. Hybridizing and genetically altering the Rape Seed plant has produced Canbra and Canola Rape Seed Oil with low Erucic Acid and high Oleic Acid content. Is the oil you can get today the same as the oil in an old report or recommendation?
These problems can be avoided by using a commercial mineral oil bore protector that lacks the ester functions and double bonds which give vegetable oils their reactivity, potential rancidity, and tendency to revert back to odors typical of the unrefined oil.An Informative(?) Survey | top
Waldemar Meckes, of W. G. Smith Inc., an oil expert and music lover who provided technical input to this article, asked professional woodwind musicians with the Cleveland Symphony and Institute of Music what bore oil they preferred. Each had a different, strong opinion about what was "the good oil".
At the recent BEMF '95 the author took a survey of the woodwind makers present concerning their preferred bore oiling practice. This informative survey of six artisans revealed the following six different specific suggestions: (1) mineral oil, (2) olive, peanut, or corn oil with Vitamin E added for stability, (3) linseed oil, (4) linseed oil plus almond oil [2:1 or 1:1], (5) almond oil, and (6) boiled linseed oil. A Woodwind Quarterly E-Mail survey revealed that with some instrument makers another alternative found favor: (7) don't oil!
Despite this panorama of suggestions, there did seem to be general agreement that: (a) Drying oils often present considerable problems to instruments with pads and keys and bores when too generously used by owners. Excessive oiling without swabbing out before skin formation has led to instruments being returned for repair. A wooden dowel has often been needed to scrape out the hardened residues! (b) It is better to suggest an oil that owners commonly have around the house, since it encourages regular oiling practices. These suggest that an important variable is how players employ whatever oil they do choose.top
DO IT YOURSELF: Does that mean you have to purchase commercial bore-oils? Moeck Recorder Oil can be analyzed using C13 nuclear magnetic resonance (NMR) which clearly shows each type of carbon atom and allows you to count them. The Moeck Recorder Oil is a semi-drying triglyceride oil (Table 4), perhaps with some antioxidant added.
It is possible to add your own antioxidants to a semi- or non-drying high grade food oil you have selected. Find a friend who is taking Vitamin E capsules. Each contains 200 I.U. (200 mg) of alpha-tocopherol, enough to treat a pint of oil. Slit just one capsule open and stir the contents (not the capsule wall) into the oil. Then, keep the material in an amber bottle to prevent air and photo-oxidation. Moeck very carefully uses a brown PVC plastic bottle for their product to avoid light and oxygen diffusion through the container. Polyethylene containers are not safe, since they pass both light and oxygen.
What if you wanted to avoid vegetable oils? Yamaha Bore Oil, analyzed by NMR, is a low viscosity mineral oil. It is a mixture of long carbon chains with hydrogens attached at all points (hydrocarbons). A good grade of Baby Oil clearly labelled as Mineral Oil will suffice. The mineral oil used internally is more viscous.
For the connoisseur, Aerospace Lubricants, Inc. makes a series of lubricants of differing viscosities. NMR and infra-red analysis indicates that these are linear hydrocarbons. They are extremely pure, narrow molecular weight spread materials. This is of importance under high rates of shear, where traditional lubricants may exhibit thixotropic behavior and show a marked decrease in viscosity. They range (low to high viscosity) from Alisyn Valve and Slide Key, to Finger Board, and Bore Oil.
If you prefer pleasant odors, both lemon and (sweet) orange oils have been used. These are both about 90% limonene (Figure 1). This is a simple cyclic unsaturated hydrocarbon biosynthesized from two isoprene units in the plant. Your body uses isoprene units to build cholesterol and the sex hormones. The citrus oils differ in the small amounts of oxygenated materials present. The instability of these can rapidly lead to a terebinthinate odor. Addition of about 5-10% of a non-drying vegetable oil will help prevent this because of their natural stabilizers, like tocopherol; or add your own Vit. E.
IS THERE A MAGIC OIL? Is there one magic formula that is better than anything else? Perhaps, but it is difficult to find controlled experimental evidence to substantiate such a claim. Players and makers have their own preferences based upon experience. All help smooth roughened, checked bores and provide a water barrier.
SWABBING: A convenient way to oil recorders takes advantage of the availability of gun cleaning patches of various sizes. They are tough, lint free, thin enough to discourage "soaking" the surface, and cheap enough to dispose after use. That is advisable for non-drying oils to prevent dirt accumulation, and necessary with drying oils because of the danger of heat build-up in improperly stored rags.
DECONGESTANT SOLUTION: It is also easy to make the decongestant solution sold by many vendors. It is a dilute solution of sodium lauryl sulfate. Reducing Lauric Acid produces Lauryl Alcohol. This forms an "inorganic" ester with sulfuric acid. Its sodium salt is a detergent. It is sold by DuPont under the trade-name Duponol, and you may be able to get small quantities as a sample. Since you only need 1 gm dissolved in 8 ounces of distilled water, a sample will take care of your consort's needs for years. Chemistry can be useful!Wood, Oil and Water Do Mix | top
Wood is magic, and the various species all cast a different spell. With the above background it is now appropriate to examine the physical properties of some typical recorder woods (3,4). Table V shows why the dimensional instabilities of maple have been addressed by many vendors via wax impregnation. Pearwood is borderline, and some pear instruments are impregnated while others are not. The characteristics of Rosewood (Palisander) depend upon the species, so a "rose is not a rose, necessarily". Lignum vitae is included as a final reference point since the best literature source lacks data on the more popular heavily ligninized woods, such as grenadilla.
~50% RH (EMC)
|MAPLE||~32%||0.687||0.721 (12%)||2.6 (3.9%)|
|PEAR||28%||0.665||0.695 ( 9%)||2.5 (2.9%)|
|ROSEWOOD||24%||0.836||0.863 (10%)||2.0 (2.6%)|
|L. VITAE||20%||1.215||1.301 (12%)||1.5 (1.5%)|
|FSP=fiber sat. point, RH=rel. humidity, EMC=equil. moisture content|
BREAKING A RECORDER IN: The hysteresis shown in Figure 5 for sorption/desorption of water indicates that the loss of water is slower than its uptake. As a new, or long unused recorder, is first played the water content in the wood increases. As it stands between sessions it loses water more slowly. If the playing sessions are spaced closely together the water content of the wood, on average, will slowly increase. That is why experts recommend a breaking in protocol of increasingly longer playing sessions spread over time. It allows the wood to adapt to the increasing water content, and avoids stress fractures. Water and water vapor barriers on the wood surface reduce the rates of sorption and desorption; but the build-up pattern remains. Softer woods pick up and lose the water more quickly. Thus, the wood, the oil, and the playing pattern interact in a complex way and each instrument/player combination is unique. Possibly this is why some players might prefer drying oils, others non-drying, and yet others mixtures of non- and semi-drying oils (2).
As the wood surface of the bore interior "weathers" due to repeated moisture sorption/desorption cycles it may grow rougher. Oils can reduce this type of roughness. Drying oils form a hard film, but users should recognize that it will follow the contours of the roughened surface to a great extent. Each treatment also reduces the bore diameter slightly. Semi- and non-drying oils reduce the roughness, but may locally deform due to flow/pressure. A dilemma! (But not a big one, since both will work to improve sound somewhat.)Alternative Impregnation Strategies | top
Just when plastic recorders are making their products look like wood, there is interest in making wood/plastic composites. Sometimes the goal is cost, sometimes fashion, and sometimes it is the elusive immortal woodwind. The technique is not new. Amish farmers have impregnated maple wheel bearings for over a century with lard. The WWII Maytag ringer-washers used mineral wax impregnated maple. Most techniques today are aimed at the floor panel, knife handle, and designer pen body market. But woodwind makers may possibly realize shape stabilization, and water repellency. Only time will tell.
METHYL METHACRYLATE (MMA): Wood Stabilizing Specialists Inc. (Cedar Falls, IA), has produced a billet of cherry impregnated with polymerized methyl methacrylate. Scott Hirsch, a flute maker in Coleville, WA, has made instruments from the material. Wildwoods (Monclova, OH) has produced similar impregnated billets of maple and black-dyed birch for Powell Flutes (Waltham, MA). Tim Bernett of Powell Flute made an impregnated maple alto in the mid '80s. Bamboo has been treated for use in making Japanese flutes; Yamaha has worked with impregnated rosewood for clarinets. Roger Rowell of the USDA Forest Product Laboratory has explored impregnating maple wood with MMA for recorder manufacture.
The pure polymer is the well known Lucite or Plexiglass. Such impregnated woods do pose challenges to the craftsman because of brittleness, the need to use low turning speeds and coolant because of the low melting point of the plastic, and a tendency to clog sanding materials. Finished instruments play well and seem to have good dimensional stability. Some results are reported in Woodwind Quarterly, #2, August 1993. Such impregnations for many commercial purposes began in the 1960's. Ken Caines of WSSI reports load factors of about 50% are normal with common woods. Exotics, such as ebony, accept much less polymer (<10%) and are prone to bleed displaced "wood oils" for many months after treatment. This can be removed by wiping with acetone. Odors from the monomer are often prominent, and are due to excessive water in the impregnated wood (>10%), or improper curing. Drying (40 C) can remove such odors.
OTHER ACRYLATES: Many other efforts in wood impregnation exist. Daniel Deitch, a Baroque woodwind maker in San Francisco, has utilized cyanoacrylate impregnation for wood stabilization. MMA has the structure H2C=C(CH3)COOCH3, while a typical cyanoacrylate has a structure H2C=C(CN)COOCH3. They differ by the substitution of a -CN group for a -CH3. This substitution makes it easier for the material to polymerize into long carbon chains, much like a zipper closes. The cyanoacrylates are used as rapid setting "glues". Deitch swabs out the finished bores with the cyanoacrylates, lets the material polymerize, and then finishes the interior surface. Deitch also reports that he has been pleased with a technique for finishing the exterior learned from Rod Cameron, a flutemaker from Mendocino, CA. A few drops of the cyanoacrylate are added to linseed oil, and the mixture applied as a hardening finish to the exterior. The cyanoacrylate accelerates the hardening of the mixture. Do not be concerned about the presence of the -CN group in the molecule. It is an organic nitrile or cyanide, not an inorganic cyanide. The latter are toxic, but the organic -CN is not. Some artisans find the cyanoacrylic bore finish beads water excessively.
OTHER FORMS OF POLYMERIZATION: The polymerization of both the cyanoacrylates and methacrylates depend upon chemical induced free radical formation to begin the polymerization. A free radical is a carbon with only three groups around it, and one lone electron. This poses some interesting problems in getting complete penetration into all the void space of the porous wood. The cyanoacrylates react with air and polymerize quite rapidly, providing good near surface penetration. Free radical initiators added to the methacrylates allow somewhat deeper penetration before blockage occurs. Firms such as Applied Radiant Energy (Lynchburg, VA) have developed an alternate strategy for achieving an even more complete penetration. The methyl methacrylate is forced into the wood by first applying a vacuum, then flooding the chamber with the liquid acrylate, and finally bringing the head space up to atmospheric pressure. They can achieve loadings of 10-100% in this way, filling a great deal of the void space. The polymerization of the material is then induced by exposure to gamma radiation. This ionizing radiation penetrates the wood completely and evenly, allowing uniform polymer hardening throughout. The degree of polymerization can be carefully controlled by the amount of irradiation. This permits careful adjustment of the working properties of the resulting composite. Don't be concerned about the radiation initiator. Gamma radiation is just like an X-Ray beam. Once the beam is turned off, its gone. The route has been used to sterilize food for long term preservation. Gamma radiation DOES NOT consist of radioactive particles which have long half-lives, and which slowly emit hazardous particles and radiation.
Billets up to 8"x8" have been treated. Ash, oak, pine and poplar are common. More exotic woods include cherry, ebony, and jatoba (Brazilian cherry). Best results come from using the heart wood. Robert Turner, a Charlottesville, VA recorder maker, is experimenting with some of the materials.
PHYSICAL AND PSYCHEDELIC PROPERTIES: It should be noted that these treatments do not prevent the penetration of water vapor into the wood. It is merely slowed down, as described earlier. The composites have increased tensile strength and lower impact resistance due to their brittleness. They are harder. Little objective acoustic data on composite woodwinds are available, although strong opinions exist.
Most composite manufacturers are focusing on dyed impregnated woods, often with multicolored, swirling patterns for designer products. Imagine a recorder playing cool jazz or hot Blues with a color scheme to match! Avant-garde players could have complete sets. On the other hand, although the Pacific Rim produces the greater part of ABS plastic instruments, most players there seem to prefer buying perfect, natural material woodwinds. But good grenadilla and rosewood are becoming scarce. Interesting marketing problems.top
Buffet has announced its Green-Line series of commercial clarinets. These are made from grenadilla wood powder, polycarbonate fibers, and a bonding polymer. But, will recorders made from such synthetic and impregnated woods sound the same? Wait and hear.
The recorder player sees and feels wonderful instruments made from different bulk woods and hears differences. The artisan sees and feels different bore surfaces and hole edges and hears differences. The acoustician sees and feels (in the mind) the boundary air layers of the played instrument and conceives possible differences. But they use different languages in expressing cause/effect. Leibniz, in discussing a metaphysical approach to language, suggested the need for a Universal Language. We must try top hear these different views. Most players feel they detect tonal differences in what appear to be identical instruments made from different materials; but, are they really identical. Experimental and theoretical evaluations (10) suggest that the bore wall construction material does not noticeably influence the timbre of sound produced from a thick-wall cylindrical "wood"-wind.
A WOOD IS A WOOD IS A WOOD? Considering mechanical distortion, differing construction materials can affect sound quality in thin square or rectangular tubes (organ pipes), or where the material is essential to sound production (like string instruments), or in very thin wall wind instruments with very large side holes. Thick walled circular bore instruments are quite different. Compare the relative compliance associated with expansion of the bore under pressure, and the compression of the air in the tube. The ratio is about .001 for even thin walled cylinders. Direct sound radiation from wall vibration is insignificant. This is not surprising, since it would require local changes in the radius of the bore, coupled to air column vibrations. The walls are just too stiff. Such vibration levels are estimated to be some 60 dB below that of the air column. It is like hearing a cat purr next to a subway train.
Only a few percent of the energy of the air stream makes it way out of the tone holes as perceived sound (0.5-1%). Where does the rest go? Tube walls can influence vibrations of the contained air column because of viscous air and thermal losses across the air/wood boundary layer. This is where free movement of the air is affected by the proximity of the wall and viscous and thermal losses occur. Wall materials all have thermal conductivities much greater than air so this cannot be a differentiating factor. Surface roughness only becomes a factor when it develops to a scale relevant to the thickness of the boundary layer. The viscous and thermal boundary layers are frequency dependent, but range from about 0.1-0.05 mm (100-50 micron). Machining practices yield a surface roughness average of 10-1 micron. Wood grain and smoothness can affect the damping coefficient or drag, but in a filled, fine-finished and oiled bore such differences are small compared to those caused by the edges of chamfers, the blade, finger holes, key pads or even finger tips, which are sources of turbulence. These variables are determined by the working nature of the material, the skill of the artisan, and how much time can be afforded by player and maker.
CHEMICAL CHANGES: Sorbed water, or chemical modification of the wood material, can change the visco-elastic properties of the wood composite. Here, the word viscous refers to a material that absorbs sound energy and converts it to heat. Elastic refers to a material that can give back such energy, like stretching and releasing a rubber band. Wood is inbetween, and is called visco-elastic. Sorbed water makes it a little more viscous. Benders of wood "knew" this, since steaming wood (heat + water) allows you to form the complex shaped ribs of a violin. The water acts as a plasticizer, lowering the glass-transition temperature of the composite. This is the temperature where longer segments of associated molecules in the composite can begin to slip past one another, but basic form is retained (rubbery). The heat raised the rib material above this temperature. In the finished violin there is a major air resonance from the box, as well as a top-plate wood resonance. The latter can certainly be affected by wood composition and treatment. In recorders the cylindrical bore is used below its glass-transition temperature and any affects are less than most musicians can detect.Acknowledgements | top
The authors would like to thank Scott Hirsch for his direction and encouragement, and particularly for his reference to the source of the data shown in Table I; Ben Dunham for his continued enthusiasm for the "Wood-Works" project; Roger Rowell for technical editing assistance; and Ching-Wan Yip and Yue-Ling Wong for the computer graphics. Wood impregnation vendors were most helpful, including Rodney Bell and James Myron of Applied Radiant Energy, Jim Fray and Nicholas Forosisky of Wildwoods, Ken Caines of WSSI, and Tim Bernett of Powell Flutes. John Martin (University of Queensland) and Susan Thompson (Yale University) were most collegial in sharing their opinions, as was John Gates of ALiSYN and Wally Meckes of W.G. Smith. Finally, the following artisans freely shared their experiences: Lee Collins, Tom Prescott, Jonathan Bosworth, Daniel Deitch, Robert Turner, and Daniel Noonan. The support of the Gomer van Awsterwyke Institute has been invaluable.Prior Cellulose Publications | top
This article originally appeared in the Winter 1996 issue (#11) of Woodwind Quarterly, 1513 Old CC Road, Colville, WA. A shorter form appeared in the November 1995 issue of American Recorder. Response to readers questions, and detailed analyses of the woodwind/composition controversy appeared in the Spring 1996 WQ, with an abbreviated version in the January 1996 American Recorder The authors appreciate the permission of WQ and its editor, Scott Hirsch, for permission to mount this material on the Web.Authors | top
Raymond Dessy is Emeritus Professor of Chemistry at Virginia Polytechnic Institute and State University, Blacksburg, VA. His research areas include microsensors and computer applications in science. He and his wife are interested in playing the recorder, from Kunstlieder to the Blues; and in discovering how recorders work. They raise and train dressage horses.References, Suggested Reading and Source Books | top
The figures are omitted from this version of the article.
b. Lignin Precursors
c. A Carboxylic, or Fatty Acid
d. An Alcohol
e. An Ester
f. A Glyceryl Triester
V=Vessels, F=Fibers, R=radial parenchyma
Micrometer bar in lower right is 10 micrometer (micron)
(from Ref. 6 with permission)
(from Ref. 6 with permission)
A. Triolein (Vegetable Oil)
B. Mineral Oil (typical chain)
a. A Plot of Equilibrium Moisture Content (EMC) versus Relative Humidity (RH) for Brazilian Rosewood (Dalbergia nigra). The separation of sorption and desorption curves is common and is called hysteresis.
b. A Plot of Radial and Tangential Shrinkage and Swellage of D. nigra at various EMCs. (r=radial, tg=tangential) (from Ref. 7 with permission)
We give detail advice about oiling on our Recorder Care page entitled Complete Recorder Care.