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Message:: [QUOTE="Insider, post: 2688446, member: 24314"]All I can say is what was explained to me in very basic steps. When a coin is suitable for conservation, it's a several step process.  Ideally, as much of the contamination/toning/residue is removed before the final step (the dip).  Often, a final dip is not needed.  EDIT:  I forgot to add that in some cases, even after all the steps and a final dip is done, there still my be detracting stuff on the coin.  That was the case with the coin in the OP.  After the final dip, another step was added to remove the spots. As there is a lot of money involved with conservation, the people who offer the service are secretive.  Employees can be sued for releasing proprietary info about the techniques and chemicals. Most dealers/collectors just see something on a coin and stick it into the dip.  They think the concentration of the dip is an important consideration.  In many cases the "stuff" does not come off completely necessitating another dip.  The more you dip a coin the more chance of making it look unnatural.  I've been in several places where coins are cleaned.  I remember one "boiler factory" in the mid-west that treated every silver dollar they sold in an ultrasonic cleaner with hot dip!  That was in the 1980's.  Coin cleaning as done at the various conservation services has gone far beyond that. Nevertheless, ultrasonic cleaners with dip are still used.[/QUOTE]