Cleaning and Preservation of Coins - A Complete Guide, page 14
HOW TO CLEAN COINS WITH ULTRASONIC CLEANER
One of the safest ways to clean ordinary collectible coins at home is to use a commercially manufactured ultrasonic wave cleaner (or ultrasonic sound cleaner, also PUC - Pulsed Ultrasonic Cleaner, which uses low freaquency for turbulence and high frequency for cleaning). A generator within the cleaner produces high-frequency sound vibrations, not perceptible to the human ear, between about 20,000 and 10,000,000 cycles per second, which are transferred to the containers via a transducer.
These vibrations exert a very strong mechanical action: hundreds of imploding vacuum bubbles dislodge dirt without abrasion by this action; the cleaning fluid is forced into all the irregularities and the impurities are drawn out of them. At the same time, the metal surface is set into intense vibration so that corrosion and other foreign deposits are broken loose and separated.
Small-sized ultrasonic cleaners are used by many jewelry and watch firms, and larger machines are used for cleaning needles at tatoo shops. The ultrasonic cleaning method allows to accomplish a simultaneous chemical and mechanical treatment of maximum efficacy: cleaning occurs even deep in the metal and, therefore, practically in the smallest pores and indentations.
The small ultrasonic cleaners are able to remove dirt, gunk, grime and other moderate deposits which have built up on the coins over long time. But small ultrasonic cleaners are useless on most hard deposits. Heavy, hard and thick encrustation would require usage of larger ultrasound cleaners; however, using the big soaking vat type cleaners involves a risk of taking off the coin's true patina, hence caution must be observed. It is best to use the small cleaners for gentle coin cleaning that ensures loosening the crust and crud (after performing many cleaning cycles in the cleaner) which can be removed using a different cleaning technique. If the cleaner is too powerful, it may damage the coin.
Professionals ultrasonically clean the coins long enough only to remove heavy dirt from the hard-to-get places like around small inscriptions and fine details, then finish the cleaning process with a lighter technique such as brushing. This saves a lot of time in scrubbing and picking. It is always better to underclean a coin than overclean it. And the ultrasonic cleaners CAN overclean! This is why while cleaning copper, bronze and brass coins by ultrasonics, you have to watch out and stop the process as soon as you observe satisfactory results.
To ensure the trouble-free ultrasonic cleaning of your coins and see amazing results, follow these tips:
1) Use only distlled water for cleaning solution as ultrasonic vibrations produce heat and, therefore, the solution being used will evaporate. If there are salts and other chemical compounds in the water, they will not only affect the coin being cleaned but also may cause health hazards if their vapors are inhaled.
2) The cleaning solution should be made up of distilled water, a dab of dish soap (use a toothpick to add a dab into the solution to provide a better action of ultrasound) and a dab of defoamer. Using a mild detergent solution, ultrasonic cleaners work very well on silver coins. Copper and nickel coins may require stronger detergents or oxide removers to dislodge grime and corrosion. You may add commercial cleaners designed for specific metals.
3) Prior to the ultrasonic cleaning, soak your coins in distilled water untill some encrustation and/or dirt deposits are about to come off, use a hard nylon brush to scrub away most of them.
4) Suspend a plastic cup or a small container or a basket (the smaller untrasonic machines come with plastic baskets) into the cleaner's pan. Rig the cup with a common wire hanger in such a way that the cup is in the water but not touching the stainless steel pan which is filled with just distilled water. Pour the solution into the cup and up to the level of water in the cleaner's pan. The cup does not reduce the cleaning effect at all. During the process, the cup will catch the dirt falling off the coin and stop it from falling to the bottom of the pan and affecting oscillation. Any dirt or crud being present in the main pan will affect the cleaner's preformance and can greatly shorten the machine's lifetime. The cleaning solution in the cup can be changed without dirtying the water in the main pan.
5) DON NOT put the coin on the bottom of the cleaner's pan at all! The vibrating bottom of the ultrasonic cleaner and the dirt coming off the coin will cause numerous micro scratches and polishing marks on the coin's surface; thus, devaluing the coin. And the banging of coins against the steel bottom will cause flat spots on the coins.
6) Suspend the coin attached to a paperclip on a string off a stand (made of an old coat hanger wire) into the cup and within an inch of the solution level. The coin should be hung in a perpendicular position to the bottom. Such coin placement will provide maximum "cavitation" - sudden formation and collapse of low-pressure bubbles in liquids by means of mechanical force such as ultrasound waves. The more cavitation you can create, the faster the ultrasonic cleaning process. Addition of the little dab of dishsoap helps in increasing the cavitation.
If you do not have the old wire hanger at hand, you can use a nylon pyramid tea bags (without tea of course) with the string for placing the coin into it and suspending it into the cup. The only drawback of using a tea bag is that it is difficult to position the coin properly. Other means can be a piece of a plastic window screen from which you can make a "coin bag" for the coin to remain in a perpendicular position to the bottom.
7) Turn the cleaner on, and allow it to run for a few minutes. Switch it off, take out the coin, and inspect it to see how much dirt has been removed. As you see results after the first run, you will be able to estimate the extension of cleaning required. If the coin only had a light coating of dirt or gunk on it originally, it should have been cleaned by now.
8) During next run, do NOT let your machine run for more than 50% of its duty cycle. Stop it on time and allow the cleaner to cool off. Change the solution or just add distilled water to the cup. Brush all the loosend dirt off the coin.
9) To get the crusty spots, run a few cycles of cleaning followed by brushing after each run.
11) Keep checking frequently as to the progress and stop when your feeling happy about the job. Do NOT ultrasound the coin for any longer than necessary as the coin's patina would also be attacked and may be destroyed.