Instruments and Chemistry: Rosin

 In the previous post, we talked about how certain chemicals have properties that allow them to be suitable for maintaining certain instruments. For this last post, we will further explore maintenance, as well as the chemistry behind certain chemicals since they are both complex and vital. Out of the chemicals that are used, rosin is the most complex since it is comprised of various organic and inorganic compounds. The following image shows how complex rosin can be, as well as showing how it is manufactured from the resin found inside trees. The central column only shows two possibilities that the four isomers can undergo since two molecules can be formed by simply arranging one of the bonds next to the additional Hydrogen.


          As one can see, the structure of the original organic compound varies by the location of the two double bonds, causing different concentrations of different acids to be present. These isomeric resin acids belong to the category of tricyclic diterpenes, whose basic structures result from the coupling of four C5-isoprene units (isoprene = 2-methyl-1,3-butadiene). The resin acids are distilled through 20% volatile turpentine and then leave behind some of the needed colophony rosin. However, some non-volatile alcohols and keto compounds are left behind and leave the molecule less pure and stable, so the compounds must undergo steam distillation in order to rearrange the atoms. Mechanistically, the rearrangement entails protonation at a terminal carbon atom of the conjugated double bond system, leading to a resonance-stabilized carbocation. The latter, through proton-transfer to the neighboring (allylic) CH2-group generates a rearranged diene, an unsaturated hydrocarbon with two carbon double bonds. The product ratio in the resulting equilibrium mixture is determined by the relative stabilities of the different conjugated dienes, and the stability gives the rosin the favorable properties that were mentioned in the previous post. While the acids in the final product are more concentrated, they are more stable and pure due to the addition and relocation of the Hydrogen atoms. The original resin is only 72% pure, while the final rosin is 95% pure; the rest of the percentages are non-volatile solids that hinder the rosin’s physical properties. This is why the levopimaric acid is the only substance that becomes less concentrated: it is the least volatile acid out of the four main acids that appear, and it slightly lessens the quality of the rosin as a whole. However, it is needed since the durability of the rosin can be attributed to this acid in particular.

          One of the most important chemicals used in the production of a piano is called lacquer.  Lacquer is more or less a wood polish; it is the chemical added to wood to give it the smooth and shiny properties.  Without it, the wood of a piano would be a lot more difficult to work with, shrinking the lifespan of the common piano by years if not decades.  Without lacquer, the piano and other hammered string instruments may have never become the influential instruments they are today.  But what exactly is lacquer?

          Lacquer is a polymer of many different bases that when coated on a wooden substance, offers a layer of protection and gloss to a piece of wood. The most common lacquer used on piano frames is a urushiol-based lacquer. Urushiol is a common allergen found in many Anacardiaceae plants such as poison ivy.  It has a boiling point of
200˚ C (392˚ F) and can dissolve in substances such as alcohol.  It is composed of a 6 carbon ring with 3 double bonds similar to benzene, 2 hydroxyl groups, and an R group. The R group varies from a large amount of hydrocarbons (such as (CH2)14CH3, (CH2)7CH=CH(CH2)5CH3, (CH2)7CH=CHCH2CH=CH(CH2)2CH3and others).  While harmful to many humans on poisonous plants, urushiol offers a special breed of polymer that can be used as piano frame lacquer.  It is a special type of lacquer in the sense that it settles in a more elaborate and controlled way. While most lacquers dry solely by evaporation, urushiol-based lacquers will oxidize with the air surrounding it and undergo polymerization.  This requires the settling process to be in a humid and warm environment, to avoid any evaporation.  However, if done properly, this lacquer will offer a completely optimized layer of shiny aesthetics and protective lifespan to the frame and other wooden pieces of a piano.

          As mentioned in the string section of our last blog, the best cleaning agents for brass instruments tend to be Trichloroethylene (TRI) and Perchloroethylene (PERC).  Last time, we talked about greases and oils for brass instruments, but sometimes build-ups of these compounds can form in sludge-like globules inside our instruments.  For degreasing and all-purpose cleaning, TRI and PERC are excellent reagents.  These compounds are heavy, maintain constant pH while cleaning, and dissolves quickly due to high solvency. It should also be noted that without adding stabilizing factors, these compounds are rather unstable and can react violently with metals, especially those made from Aluminum and/or Magnesium. These stabilizing factors are also able to protect Aluminum against decomposing.  In addition, more recently ultrasonic cleaning of instruments has been used instead of the degreasers.  This method takes advantage of a phenomenon called ultrasonic cavitation.  When water (or another liquid) is bombarded with the correct ultrasonic frequencies, very small bubbles form causing agitation. This extreme agitation can loosen contaminants sticking to a metal surface.  Adding certain ions to the water can greatly increase the rate of the reaction by reacting spontaneously with the contaminants with the added energy from agitation.  Finally, in the case that you would need to remove the lacquer from your brass instrument, acetone will work well.


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