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Cleaned, Cleaning

Cleaned, Cleaning.  Numismatic items bearing evidence of an attempt to remove surface stains, spots or blemishes; any process of removal of these surface contaminates, but usually an uneven bright untoned surface condition that is unpleasant to the eye because it was done inexpertly. Cleaning is a range of actions from simple washing, even with detergent and water, to as severe a treatment as removal of the top layer of molecules by chemical and physical methods. Cleaning is a typical metalworking process to prepare blanks by removing dirt and grease, DEGREASING, for example, to be struck without the problems these contaminants cause (see metal cleaning). But for coins and medals, cleaning is a method of removing surface toning, tarnish or corrosion . Often the evidence that an object has been cleaned is the existence of an activated surface after which a new toning occurs over a period of time, usually taking on an unnatural appearance.

In numismatics the use of the term "cleaned" is for someone's inexperienced cleaning, for expert cleaning would not leave an activated surface condition on a treated piece. Thus such terms as "once cleaned," "lightly cleaned," or "harshly cleaned" are found in numismatic catalogs. What this indicates is the resultant new toning on a coin or medal's untreated activated surface.

Of all coinage compositions, gold does not tone, bronze tones most of all, but it is silver toning that attracts the most attention – and most often found with the harshest cleaning. Perhaps because of everyone's experience with their own household silverware, and its need of polishing often, plus the availability of silver cleaners for this purpose, that inexperienced persons think silver coins and medals should be cleaned and polished. Nothing is further from the truth! Rare numismatic items have been deteriorated in their state and condition by overactive, inexpert, perhaps well-intentioned but unknowing "cleaners."


     Caution:  No one should ever attempt to clean any    

  coin or medal without knowing what they are doing.      

  Soap and water may be used to remove surface dirt but   

  even this should never be used on certain compositions  

  (paper obviously, wood, and others), and some medals    

  have fugitive patinas which are not colorfast and may   

  even be damaged even by water. If in doubt, don't do it!                                                               


Cleaning blanks.  In most metalworking operations, cleaning the workpieces is a step just prior to some metal forming, as in coining or striking. Removing oil or grease

– and other surface debris – is required of this step of cleaning (see degreasing). Because blanks contain no detail yet, they can be severely cleaned by removing the top layer of metal, as with acid dip. This removes all toning, tarnish and corrosion along with the top layer of molecules. The goal of this cleaning is to provide a surface  that can be formed without any imperfections which the above surface contaminates may cause.

Blanks are cleaned by any of several methods: blanching, abrasive blasting, shot peening, acid dip, heat treating, water hone. But the most popular in coin and medal production is barrel tumbling. Blanks are placed in a large drum along with sawdust, ball bearings, shot or other metal particles and the barrel is set to rotating. The tumbling action causes all these objects to nock against each other. This removes toning, tarnish and corrosion by lightly abrading all surfaces. Afterwards the aggregate is removed and the blanks are separated (by screening). At this stage they will have an activated surface, which, depending upon the environment, will last from two days to two years. Obviously, this method cannot be used on struck pieces and is employed only for blanks.

In this encyclopedia the term “cleaned” is applied to numismatic items that have been treated after they have been issued. The term metal cleaning is the treatment of blanks and partially processed numismatic and medallic items before their issue. All other discussion below is about numismatic items after their issue.

Coin cleaning.  Once they are struck and circulate, coins are subjected to every possible environment; they get dirty, discolored or stained in every possible way. A dirty, discolored or spotted coin is unsightly and it is human nature to want to clean it. But most coin cleaning in the numismatic filed is concerned, not with dirt and grease (both of which can be removed by detergent), but with some form of toning. Toning is the natural coloration of a metal surface exposed to normal environment or the abnormal or concentrated coloration of toning forming spots, blemishes or stains.

Toning results from contamination with sulfur and sulfur permeates our environment. Metal in the coin (particularly copper or silver) combine with pervasive sulfur (no matter how small an amount) to form copper sulfide or silver sulfide. (Bronze also tones from exposure to carbon dioxite – what humans breath out – forming copper carbonate.) The toning effect is somewhat cumulative. The reaction to the constant and varied exposure to environmental sulfur constantly adds toning in time, although existing toning is somewhat self-sealing. Thus no two coins will tone exactly alike (unless they are kept together for their entire existence and exposed to like amount of sulfur).

Sulfur is used in making paper, thus a coin in an envelope or next to paper (unless it is antitarnish) will, in time, tone.  A coin laid next to a rubber band for a length of time will form a dark streak on the surface from the sulfur used in making the rubber band. It is this constant exposure to sulfur that causes the chemical formation of sulfides on the surface of the coin or medal. Most toning is attractive, forming an even tone in colors from light gold or yellow to dark blues, purples and blacks. However, if this tone, toning is uneven, unsightly or blotchy, the coin is a candidate for its removal.

A number of chemicals can dissolve toning. These include thiourea (commercial

name Jeweluster), or trichlorotrifluoro-ethane (commercial name Dissolve), others. These are acids that dissolve away metal sulfides, as silver sulfide on the surface of a silver coin, and are called surfactanz.

Immersion of the coin in one of these chemicals is called dipping. The immersion time is critical. The longer the coin is left in the solution, the greater amount of surface is removed. Ideally, the goal is to remove only the topmost layer of molecules of the coin's metal (with all the sulfides on top of this) – and no more!  While it is impossible to do this precisely, it can be achieved somewhat by repeated dipping and examination of the surface each time.

The acid must be thoroughly washed off the surface of the coin at this stage (or it

will continue its action). Wash with a detergent and running water several times, then pat dry with a soft cloth (or follow the directions of the commercial cleaner, or both). If this is not done there may be residual action of the cleaner for as long as two years.

At this stage the coin has an activated surface – an unstable chemical status that demands or is susceptible to toning (more so than when the coin was originally struck!).

If left unattended it will combine with the sulfur in the environment and probably tone in a worst state than what was just removed! (This is one reason why inexperienced persons should not do coin cleaning.)

Instead the coin must be retoned under controlled conditions. There are commercial toners that furnish the correct amount of sulfur over the correct period of time. These are preferable to home remedies (like sticking the coin with an activated surface inside a raw potato and leaving it for a month or two). Even so, retoning is tricky and is never guaranteed to produce, an even, attractive toned surface.

(It is unfortunate that retoning a coin cannot be done like recoloring a patinated medal. This can be accomplished within minutes in an adequately equipped medal finishing department. Coin retoning requires time, patience and results in different toning each time.)

There are some conditions that no amount of cleaning can remove: coins or medals with horn silver (silver exposed to hydrogen chloride) and plaster residue (a numismatic item used as a pattern for a plaster cast with plaster remaining in the crevices). No chemicals will dissolve either; horn silver can best be removed by reverse electyroplating plaster residue by chasing. Both need to be done by a professional with the proper equipment.

Medal cleaning and lacquered items.  Better medals are always lacquered. Other

numismatic items, particularly those with a patina finish, may be lacquered as well. The lacquer acts as a protective coating, thus any contaminate would adhere on top of the lacquer. Cleaning lacquered items is easy: wash with a detergent and lukewarm water, then wipe with a soft cloth.

If this does not remove the contaminate then try a mild solvent: toulene, a petroleum derivative, is best for this. Completely immerse the item for a brief time in a shallow pan filled with toulene, them wipe off the residue. Put a lid on the pan containing the toulene and you can use it over and over. Wash off the toulene with running water and pat dry with a soft cloth. Dirt, grease, sticker adhesive and other surface adherents can be removed by this method without damage to the lacquer.


     Caution:  Toulene is highly flammable and can cause 

 skin irritation over prolonged exposure. Do not use any 

 other solvents, particularly acetone, benzene or gasoline  

 for cleaning lacquered items.                      

Treatment with toulene will not adversely affect the lacquer; and obviously, will not affect the metal surface below the lacquer. If the object's metal surface is discolored then it must be refinished; the lacquer must be removed (taking away any contaminate on top of the lacquer), then the piece can be refinished. This removes the unsightly toning, patina, and a new patina is applied anew. It is then relacquered.  See refinish and refinishing.  Cleaning medals by refinishing can only be done by experienced professionals, here again, with proper equipment.

Ultrasonic cleaning.  Modern technology has applied ultrasonics recently to successfully clean numismatic items at any stage, as blanks, during a striking or finishing operation or after the items have been struck and issued. This can be recommended for most all numismatic items except wood and paper items, of course, and medals with fugitive patinas. For all others ultrasonic cleaning appears to be a satisfactory, nondestructive cleaning method.

excerpted with permission from

An Encyclopedia of Coin and Medal Technology

For Artists, Makers, Collectors and Curators


Roger W. Burdette, Editor

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