Trouble with Ultrasound

[styxman]

OK, I am lazy. While I can clean a coin remarkably well, it takes a good long time. So, I went in search of a quicker – easier system. I have seen ultrasonic cleaners supplied by various numismatist related shops and figured this might be a way to go. So, I obtained a 3.5 gallon, 450 watt ultrasonic cleaner that also has a 450 watt tank heater – I thought I could find the correct combination of pre-soak followed by a period of ultrasonic cleaning. What I found was disturbing and want to get your thoughts.

My approach to this topic was to photograph sets of coins (usually Wheat Cents), subject the sets to a variety of cleaning parameters and take another set of photographs. Parameters includes which cleaning solution to use, (only used distilled water), duration of pre-soak at what temperature and then exposure to ultrasound for a period of time. No more that five coins were cleaned at one time to assure that they did not collide against each other in the cleaner.

I found that exposure to ultrasound for as little as 1 minute results in a deformation of the coin. Roughly %90 of all coins showed new defects, some showing large areas while others showing only very small area. The defects had the appearance of being sandblasted with a very fine sand. While I have no idea of what this is called, I made up the name Cavitation Defects. Has anyone else seen this? Is 450 watts too much power?

I have looked for other descriptions of problems with coin cleaning using ultrasonic waves but have found nothing other than damage with coins bouncing into each other. There are many references on the LCF of members using ultrasound – I must be doing something wrong.

Images uploaded show the Cavitation Defects from three coins – some with labels as well as Pre and Post Ultrasound.

Thank you for your time and thoughts!