Author: Terry Stibal
Date: 2007-05-01 14:39
Other than noting (with considerable pleasure, I might add) the use of the proper spelling for the element "sulphur" by Omar above, and that the mixing process for rubber in the past was more of a cooking procedure rather than a precision chemical process, I will only add that the "production" of rubber (particularly prior to World War II) was a very time consuming and manual process.
In order to get the natural, plant-derived latex into what we consider to be "rubber" form, the various additives (primarily including carbon black for the color and sulphur for the cross linking need to be kneaded (as in cooking kneaded) into the resilient mass of the raw stuff. This is the basis of the Goodyear process,
("Synthetic rubber" was only really produced from about 1938 forward, so anything in the musical instrument line prior to that date (and most of it afterwards up to about 1950 or so) was the natural item. And, even today most rubber products contain a significant fraction of natural latex to obtain several desired finished product qualities.)
Rubber is not "mixed" in the same sense as is something like paint pigment; i.e., there is no "uniform" distribution of the components obtained by stirring. Instead, rubber is "milled" in a device called (oddly enough) a "mill", consisting of a large pair of inward rotating roller over a catch pan.
The mill operator takes the raw rubber mass, drops it on top of the mill rollers, and take the resultant "milled" product out of the bottom, As the milling procedure continues, the various additives are worked into the mix, and what was once a sticky, waxy translucent brown mass gradually turns into to the finished appearance.
(Hopefully during this process, he does not get an arm caught in the mill and worked into the mix. Some of the more gruesome fatalities that I have investigated over the years have involved rubber mills.)
Cut through and examined under a scope, you can actually see the layers formed as the material is worked through the mill. It resembles a cross section of a Damascus steel sword blade, with dozens of microscopic layers formed as the material is kneaded back onto itself by the multiple passes through the mill.
Once the "dough" is thoroughly mixed, the mass is then forced into molds and cured there by the application of steam-supplied heat. Whether in the form of a tire (itself "built up" out of multiple layers of rubber and other substances) or a hard rubber rod, the resulting cured item has taken the final form that it can be moved to take. (Mouthpieces and instruments are then created by machining the cured material with machine tools, similar to those used with metal.)
As older rubber "mixtures" age, the various agents outlined in Omar's posting can have an effect on the material. We in the music business have grown accustomed to old mouthpieces and instruments that have gone "green"; this is the result of the sulphur in the mix migrating from the finished product. You may know of someone who covers up the wheels on an infrequently used travel trailer - what that person is doing is cutting down the ultraviolet light exposure that causes the same sort of thing from happening with tires.
(Incidentally, the good folks at Consumer Reports have advised us to always purchase the newest tires available (this by the manufacture date, which can be figured out from coding on the tires), the better to avoid devulcanization problems experienced with older tires.)
(I once bought a house that had, among other charming features, a pile of tires in the crawl space below the building. These dated from the 1930's or so, and they crumbled when we tried to lift them out to clean the place up.)
(The reason that they had probably been dumped under the house in the first place is because you cannot bury a tire with any expectation of it remaining buried. The expansion and contraction of the material with normal temperature changes tends to have them rise to the surface, hence the huge piles of tire dumping sites. I have always wanted to bury an old rubber mouthpiece and see if it too shares this characteristic.)
The rubber milling process works the other way as well. Among the other interesting industrial facilities located in the East Saint Louis IL area, there is a plant (Midwest Rubber Reclaiming) that specializes in recovering the rubber from old tires.
They do this by heating them up (with steam heat again) in large autoclaves, then leaching the sulphur from the rubber mixture with a solvent (carbon disulphide, I think - nasty, wrath of God stuff, but it does the job). Once this is done, they macerate ("chew") the resultant gummy mass up, separate the steel belting and synthetic fibers through a variety of processes, and then end up with an inferior grade of raw rubber (synthetic and natural) stock.
I have one mouthpiece from the 1930's or so that I still use, even though it has turned a yellow green color over the years. Despite this change, I don't notice any difference between it and a copy that I had made in the 1980's to match. So, I'd not worry too much about stuff from the 1900's, at least not that stuff that is likely to still be in use.
leader of Houston's Sounds Of The South Dance Orchestra
info@sotsdo.com
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