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What is "Red Rot"?
"Red Rot" is commonly used to refer to any reddish patch of corrosion on a copper-based alloy, such as brass. However, this loose usage covers two distinct but related conditions, surface corrosion and deterioration of the alloy due to de-zincification.
Red rot is found in brasses, which are alloys of copper and other non-ferrous metals, such as zinc. As the alloyed metal, most often zinc, is corroded out of the brass, a lattice-work of copper is left which is reddish in appearance.
If this happens on the surface of the part it is relatively harmless. Only a very thin layer of zinc has corroded away, and the part will likely continue to perform well.
However, sometimes the de-zincification is extensive. True Red Rot is the complete breakdown of the alloy, and it occurs from the inside of a tube and proceeds outward. In this case, the loss of zinc from the brass is substantial and throughout the thickness of the part. It is identifiable as a reddish patch on outside of the tube, roughly circular, with a pin-point dark spot in the very center. At this spot the alloy has failed completely, leaving a weak structure of copper behind. You can put a probe on the spot and push all the way through without effort. The metal has failed.
What causes Red Rot?
Red rot is caused by a reaction with acidic solutions, usually the aspirants from the player's mouth. The acid reacts with the zinc and causes it to come out of the brass. Copper is more resistant to reacting with these mild acids.
There is acid in one's breath, naturally. But acid levels can be increased by consuming certain foods, especially sodas. There are also acids used in instrument cleaning procedures.
What can I do about it?
Once you have Red Rot, there is nothing much you can do about it. If it is localized then you may be able to patch the part and extend its serviceable life. Otherwise, the part must be replaced.
You can help prevent it by reducing the acids you introduce into your instrument. Avoid eating right before playing, and especially drinking acidic drinks such as coffee, tea, sodas, lemonade, and so forth. Brush your teeth before playing if possible, or even simply rinse your mouth well before playing. Empty the water out of your instrument right after playing and oil it before putting it away. Periodically, remove the tuning slide and run a flexible cleaner, or snake, through the mouthpipe or leadpipe of your instrument, rinse the pipe with clean water, and let dry before reassembly.
Your technician can help, as well. After a cleaning with any acidic solution your instrument should be acid-neutralized, rinsed thoroughly, and dried completely. Further protection is offered by oiling critical parts. Check with your technician to be sure that these protective steps are followed.
Some parts are made to be resistant to corrosion by using certain alloys. Yellow brass is most susceptible, so parts are often made of gold or red brass or nickel silver. These features enhance the likelihood of a longer life for your instrument, and should be considered when purchasing an instrument.