I have used an ultrasonic cleaned with moderate results-- you have to be careful and just give the coins short bursts or the patina can be removed pretty quickly.
The stats for mine, it also has a basket which keeps the coins of the bottom, if not they would bounce around too much
Commercial Grade Kendal 6 Liters 380 Watts HEATED ULTRASONIC CLEANER
3 x strengthened transducers make the ultrasonic more powerful (42 KHz and 180 Watts ultrasonic cleaning power) and provides remarkable cleaning results.
Large tank -- Suitable for long dental equipment and tube. Tank Size: 11.8" (L) x 6.1" (W) x 6.0" (H). Tank working Capacity: 6 Liter (1.59 gallon)
Heater -- Powerful heater (200 watts) makes the temperature reach the presetting one quickly -- significantly enhance cleaning.
Digital program cleaning cycles ranging from 1 to 99 minutes--easy to operate. Digital temperature control with temperature ranging from 20 to 80 degree in Celsius (68 to 176 degree Fahrenheit).
Drains with flow control valve for easy cleaning and rinsing. Power Supply: AC 100-120V 60 HZ.
