Lesson Learned - 4/17/13

It seems that there is no safe setting to run our ultrasonic cleaner on old or antique instruments. The other day I tried an experiment with different intensities on a trumpet main tuning slide that was rotted, to see at what intensity it would develop holes in the rotted areas. This has been a frequent problem for us in the past, especially on horn leadpipes, which seem prone to collecting gunk and then having it sit there for a long time while the metal of the leadpipe deteriorates. In my little experiment, the mouthpiece survived a 3-minute soak in the chemical bath with no ultrasonic agitation, then survived 3 minutes with the ultrasonic set at 40% intensity. After 3 minutes at 50% intensity, though, pinholes started developing. 
So then my coworker M tried putting an old Conn 22B trumpet in and running it at 40% intensity, figuring that would be enough to clean the instrument without damaging the metal. Of course, this was only based on my pretty poorly executed experiment, so we shouldn't have been surprised when the main tuning crook developed cracks. There was no rot on the instrument. But because it's about 80 years old, it's possible the metal may have just developed some fractures over time.
I've read about "season cracking," which is a problem specific to brass where residual stresses in the metal that are created during the forming process cause cracks when the brass is exposed to temperature changes and, more importantly, ammonia. The problem was initially discovered in British ammunition cartridges that were stored in stables near copious amounts of horse urine. Well, they weren't intentionally stored near horse urine. They were stored near horses, which happen to turn water into that offending substance. I don't imagine this trumpet spent a lot of time around horses, but perhaps there was some other source of ammonia that could have caused the problem, or maybe the temperature changes of some 80-odd years were enough to cause to the failure of this particular instrument without any source of ammonia. Whatever the case, the crook of a main tuning slide would be especially susceptible to season cracking because of the huge amount of stress that is exerted on it during manufacture. It's interesting to note that simply annealing the part after forming will remove these stresses and therefore all but eliminate the possibility of season cracking.
Getting back to the point, though; The lesson is that we haven't yet found a perfectly safe method for cleaning fragile instruments. I don't think anyone has found a perfectly safe method, and that's why there are so many techniques out there. At least for now, I think we'd do well not to run old or fragile instruments through the ultrasonic at all, except as a last option when other methods don't get them clean. Right now it's our first and last resort because it's so convenient and consistently performs well, but that's an approach that needs to be revised when it comes to these types of instruments.