Soaking bronzes in Olive Oil is the oldest and most reliable and least destructive cleaning technique that I know of. You cannot leave an object in olive oil too long. This technique has been used for hundreds of years, and it is the only cleaning technique of which I have never heard a condemnation.
It is, however, also the slowest technique I know of. You may not see any results for a month or more and complete cleaning of an article can take many months. Some encrustations will not respond at all to this technique. It is, however, the only technique other than microscopic cleaning with hand tools that I personally will use on expensive or fine coins.
To speed up the cleaning process the temperature can be raised, but you need a thermostatically controlled cabinet to regulate it. The oil should not be heated to above handleable temperature. Don't use gas burners or ordinary electric hotplates to heat the oil! Olive oil is very flammable.
To use olive oil, place the object to be cleaned in olive oil in a closed container. To hasten the process; use a toothbrush to occasionally gently scrub the coin. Change the oil monthly or bi-monthly.
High Pressure Water and Mechanical Washing:
Techniques such as water picks, dishwashers, and placing items in a sock in the washing machine are potentially effective in removing dirt type encrustations only. Beyond this you will need to use your judgement as to the potential effects on fragile artifacts and expensive appliances.
Ultrasonic cleaning devices are useful for removing loose layers of dirt, and they are to some extent effective on more permanent encrustations. With a good ultrasonic device, the crud will just fall off, but over cleaning is possible. This process is very gentle, but many ancient items are structurally fragile or brittle. They can shatter, break, chip, or develop shiny spots if left in too long or if they were weak to start with. Note, the $10 ultrasonic jewelry cleaners you can get at the jewelry store or department store will have no effect. You will need a professional model for it to be of any use.
The brass brush is a fast and usually safe approach for preliminary removal of dirt and clay deposits on bronze items. Some people hate this technique, some love it. I have tried it with great results in some cases, and damage to the artifact in others. The real trick lies in knowing when to stop. Some brushes are too hard and will scratch your coins; some are to soft and will rub off a layer of brass on your items. I like the brass tire and suede brushes manufactured by Kiwi (as in the shoe polish). You can find them at Wal-Mart in the states. Never use this technique on a high quality coin, or on any item made of gold or silver.
Using a rotary rock tumbler to clean coins is generally a bad idea.
Vibrating tumblers of sort used in jewelry trade are somewhat better. There are many different kinds of media for use with this type of tumbler, from walnut husks, to wire clippings, to steel shot, and fast cutting ceramic and plastic abrasives. The results, therefore, can vary from the equivalent of a gentle scrubbing with a toothbrush in soapy water, to the equivalent of sand blasting away half of the mass of the article. A remarkable degree of control is possible with a lot of information and experience, and efficient destruction is possible without. The challenge here is that a tumbler of this type is a fairly high tech piece of equipment that must be used correctly with the correct media, time, soap, and judgement about the condition of the article. Experience and knowledge is the key.
Dremels, Foredoms, and related tools:
While they are wonderful tools, there is absolutely nothing that I have discovered to date or have ever heard of that you can do with a Foredom or Dremmel that is in any way beneficial to any ancient coin ever. High RPM's is the opposite of control. So, put down the power tool, step away from the defenseless coin and take ten deep slow breaths.
Hand Cleaning or tool cleaning is one of the best ways to clean ancient artifacts, particularly coins. Hand cleaning is the process of cleaning a coin or object by sharpened instruments with the aid of a magnifying tool. The ideal setup is a binocular microscope and light source and an assortment of steel, plastic, glass, and wooden tools fashioned for cutting picking, and probing. The reason this technique is desirable, is that it gives the maximum control in applying appropriate techniques to appropriate areas on the surface of the artifact, according to the conditions of the surface. This is always desirable over global techniques, which apply uniform indiscriminate processes to the entire surface of the object.
The reason I have placed this technique near the bottom of the list with the potentially destructive ideas is that without lots of practice, you can do a whole lot of damage to a valuable coin with a sharpened blade with just a slip. Geta just doesn't look nearly as handsome without his nose.
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 chopstick with a nice sharpened end, a few tooth picks, and whatever else you can think of. Your tool kit will rapidly refine itself according to your tastes, preferences, and dexterity.
You can try adding a lubricant to the equation to protect the object's surface and loosen the encrustations if you feel comfortable with the idea.
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.
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 never attempted. The same process can be accomplished on a tabletop by various chemical methods involving metals and chemistry that together generate an electrical flow. I understand the process to be particularly effective in the treatment of horn silver. Following is one technique:
For table electrolytic reduction of horn silver, wrap the artifact in aluminum foil so that the foil is fully in contact with the surfaces, but slightly open at the edges. Put a small amount of MALT vinegar in a shallow dish and emerse the coin in it for a short time (30 to 60 seconds). You will probably see small bubbles coming out as the silver chloride is reduced and chlorine is released. This must sometimes be repeated a number of times. The silver will form a pure silver deposit on the surface that must then be removed. If it is a very light coating it will probably rub off easily with your fingers, but often it is heavier (forming a bumpy surface) and more difficult to remove. A short soak in diluted ammonia will help loosen it, but in the end it often must be removed with pins and needles under a microscope.
If you want to get more involved, you can build an electrolysis machine with a battery or plug and rectifier, some wire, a tank and some chemicals. I am sure instructions can be found on the net. Note this involves liquids, electricity, and some harsh chemicals. Don't screw with it unless you are serious about cleaning artifacts, and do your research first.
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.
Soaking in Calgon Water Softener works rather nicely for cleaning many bronze and silver objects. The article is soaked in dilute or full strength solution for a time ranging from 1 day to 2 weeks, with daily scrubbing with a toothbrush. The results are adequate, but overcleaning is possible. Left long enough, Calgon will strip all patina so check the progress frequently.
Many collectors praise the days when you could get these now prohibited trichlorotrifluoroethane based products. They apparently worked wonders in removing encrustations and left a nice patina. The chemicals were commonly used (by the thousands of tons) as refrigerants, and were outlawed because they destroyed the ozone. If you have some lying around, the few grams necessary for cleaning coins probably aren't going to do much environmental damage. The chances of finding one of these solvents are remote.
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. So will jewelers pickle, dilute hydrochloric, and any number of others. 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 such as Lime-A-Way. I have even heard of Naval Jelly being used also. 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 stabilized.
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. As with cleaning techniques, I will tell you that you will screw some of them up so don't experiment with anything that is important to you.
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. You may be able to get similar results through boiling in distilled water for several hours, but I have never tried it.
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. Unless I am mistaken, the result of mixing flour or liver of Sulfur with water is a Sulfuric Acid solution, and caution is called for. The best results I have achieved in experimenting with these materials came from mixing a very weak solution (a couple of grams or less to a cup or two of water), and allowing the solution to sit for several hours until it is mostly inactive. Then, in this super-weakened solution, I soak the item for 30 minutes or so until toned to the desired extent, remove it, and rinse it. 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 microcrystalline Renaissance 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. Renaissance wax can be got from Art Rubino & Co. Numismatic & Philatelic Arts of Santa Fe., Phone (505) 982-8792.
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 Renaissance wax to protect it from repeat exposure. The source for renaissance wax is listed at the end of the preceding section on Restabilization.
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