Cleaning and Treatment of Ancient Metal Artifacts and Coins

Part - 3 - Three

If you want to try manually cleaning coins or artifacts, and I recommend it whole heartedly, start with cheap ones. Get yourself a good microscope or table magnifier, and a few dental picks. Sharpen the ends of them into small blades and probes and such of different shapes. Add a bamboo chop stick with a nice sharpened end, a few tooth picks, and whatever else you can think of. Work on small areas of the coin or artifact, keeping the edge of your tool at a close angle to the surface you are working on. If you are lucky or skilled, and you have patience, you can clean down to a fine hard patina with remarkably beautiful results. The most important elements in this technique are patience, and knowing when to stop. As with all cleaning methods, when you first start, you have a high probability of doing damage.

Embrittlement, as stated previously, is the condition commonly know to collectors as crystallization. Embrittlement is primarily the result of inter-crystalline corrosion at the microscopic level due to the selective precipitation of copper from the silver alloy at the crystal boundaries. Discontinuous precipitation of copper may soon provide a new method for the dating and authentication of silver artifacts. Discontinuous precipitation occurs primarily in silver alloys with a 1.5% - 10% copper content, and shows up as a crystalline or jig-saw grain pattern on the surface of the artifact. Higher copper content silver alloys are resistant to this phenomenon.

 Electrolytic Cleaning: Electrolytic Cleaning is the technique of reducing the patina and encrustations on the surface of an object back to metal, by suspending the object as a pole in a electrolyte solution and running a charge through it. It is a remarkable technique which I have used a number of times for badly ncrusted coins.. The same process can be accomplished on a table top by various chemical methods involving metals and chemistry that together generate an electrical flow. I will cover this process in detail in a future eBay guide.

Chemistry: If you choose to use chemicals to clean your coins or artifacts, read the following section on restabilization to fully halt the chemical processes that you begin.
Always use the weakest chemistry appropriate to the job. Read all the instructions, wear appropriate safety equipment and clothing, and work in a well ventilated area or outside. Practice on cheap coins or artifacts first, and don't use chemicals on important or expensive coins.
I am only going to share a few items which give reasonable results. There are a large number of kitchen, household, and bathroom cleaning agents which people have used for cleaning antiquities.
 
Harsh Acids: Acids are the fastest removers of artifact encrustations. They are also the most potentially destructive to the artifacts. I would never use them on good coins. Vinegar and Lemon juice are fairly weak starting points. Muriatic swimming pool acid will remove dirt and encrustation. The result is frequently an ugly red artifact that must be brushed and artificially retoned. Another readily available chemical group which strip coins and artifacts quickly are commercial bathroom cleaners. Chemicals should be considered only as a last resort. They should be used safely in controlled surroundings, and the article to be cleaned should be emersed for no more than the minimum amount of time necessary to do the job. With experience and knowledge, acceptable results can be achieved in some cases. This is a matter of experience, trial, and error only, however, as the initial condition of each coin or artifact defines the amount of exposure necessary to achieve a desirable result with that item. Despite all of these warnings, acid treatment is, for some artifacts (seldom if ever coins), and for some end results, the appropriate and desirable alternative, second only to electrolysis. All acid treated items must be restabilized.
 
Darkening Harshly Cleaned Metals
The ethics of repatination is a fiercely debated topic. Few will argue, however that harshly cleaned or stripped coins benefit greatly from some darkening or toning. As with cleaning, however, take into account that changes you create are permanent. Always ask yourself if this object might really be better off just the way it is. To accomplish toning or darkening of a coin or artifact, you have to initiate one of several different chemical reactions.

By far the easiest and safest way to tone a bronze or silver object is to bake it in an oven at 350 or so Fahrenheit. This will stimulate an oxidation reaction in either metal and darken it somewhat. Getting the item to darken evenly may be a challenge.
 
The second technique for darkening these metals, and by far the most common, is application of sulfur compounds. This causes copper and/or silver sulfides and sulfates to form. The result in either metal is a darkening. The products used for this purpose are generally called flour of sulfur, liver of sulfur or something of the sort. Many of the commercial preparations prepared for darkening or toning metals are also sulfur compounds. Any time you tone or darken an item with a sulfur compound or any of the many commercial compounds on the market, you need to fully restabilize the item or the reaction may continue slowly on, possibly leading to severe damage to the artifact or coin.

Restabilization and Storage of Artifacts
This section is more important than its length would seem to indicate. Restabilization is the all important process of removing chemicals and halting the various chemical reactions that you might choose to start in the process of cleaning or repatinating (darkening) an artifact. Any time you treat an artifact with any chemical, it is advisable to go through a process of restabilizing it. Restabilization is accomplished by simply placing your coins or artifacts in distilled water and allowing them to soak for a week or so. Two weeks is preferable. Change the water each day. Boiling will accelerate the process somewhat and make it more thorough..
Following soaking, dry your item thoroughly in a heated place. You need to get all of the water out of it. Alcohol can be used to help remove water. Just wet the object in alcohol before allowing it to dry, and it will dry quicker and more thoroughly. You can stop there, or you might choose to use archival, inert micro-crystalline Wax to coat your artifact to protect it from further degradation from environmental exposure. Don't coat an uncleaned or inadequately dried item. You can as easily trap harmful compounds in, as keep them out.

Bronze Disease: It's Cause and It's Cures
Bronze disease is a destructive reaction which occurs on the surface of coins and artifacts fashioned from copper alloys. Bronze disease will appear a green or blue-green color and will be soft and powdery. It can be rubbed off with a finger, but it will re-appear as more powdery growth. Anything too hard to gently rub off is probably not bronze disease. It is relatively rare but very destructive. Those living in dry climates will probably never have a problem with it, because the reaction requires a relative humidity of 39% or higher to become active.

The reactive chemicals involved are cuprous chloride which combines with oxygen and water in the air to form destructive hydrochloric acid which eats the metal, forming cuprous chloride again and so on... Of all of the various reactions occurring on the surface of an artifact at any given time, cuprous and cupric chloride are the most rapidly destructive.

The most effective cure seems to be a three step process:
1) Light brushing to remove as much of the powdery growth as possible while not harming the surface of the coin or artifact.
2) Then let the coins soak in distilled water, changing at least every day, for two weeks to a month. This serves to remove the chlorides. They are carried away in the water slowly. The length of the soaking process and the frequent changes of water are both critical.
3) Then bake the coins in a normal kitchen oven at 300 degrees F (500 C) for two to three hours to drive out all of the moisture present.

At the end of this process, if all has gone well, the green powdery bronze disease will have vanished. The coins may be slightly darkened due to oxidation while baking, and there will be slight scarring where the bronze disease was present. The artifact may be lightly polished with a soft cloth if necessary.
If you are confident that you have removed all of the offending agents, and that the article is totally dry, you may wish to coat the item with wax to protect it from repeat exposure.

End Part - 3 - End of Guide

Archeology Pty Ltd, does not accept any liability or responsibility for any outcome or end result achieved or attained by using or following any or all directions contained in this eBay guide. This information is provided on the basis of good faith and use of any or all information is completely at the readers/users risk. Remember, this is a general guideline and should not be applied without testing and adequate safety precautions.