Cleaning, Storage and Identification
Analysis
The first stage that any conservationist or archaeologist follows prior to cleaning and conserving ancient coins is one of the most important steps. This is the Analysis Stage. In the Analysis Stage you must be aware of the material composition of the coins you are working with, as different elements can react differently depending on the chosen cleaning method. In Example #1 below, we see a bronze coin. The green layer on this coin is known as a patina, which is formed through the oxidation of the copper in the coin. This layer protects the copper of the coin from further oxidation, which results in corrosion. The cleaning method usually depends on the strength and thickness of this patina layer (in Example #1 this is very thin, so wet or harsh cleaning is not recommended). We must also be aware of any active or passive corrosion pockets on the surface of the coin (see light green area in Example #2) as exposure to acidic solutions, high levels of moisture, or chlorides (found in tap water) can speed up the corrosion process. This green area (in Example #2) is an example of active ‘bronze disease’; a process in which copper from within the coin reacts with hydrogen and chloride ions (from either tap water or moisture in the air) to produce a strong acid (HCl) which eats away at the copper, producing the green powder. This is an ongoing reaction that can spread across the entire coin or even to other coins (treatment will be discussed in the Cleaning section below).
Cleaning
Once the analysis of the coin is complete, the next step is to select the cleaning option, which can change mid-process depending on the success of the method. For very fragile coins (with thin-flaky patinas or deposits of passive ‘bronze disease’), wet methods are not recommended. For sturdier looking coins with thick encrustations, wet methods are more appropriate. I strongly advise against using highly acidic coin cleaners as these often strip the patina off coins, leaving the bare metal open to corrosion.
- Mechanical dry cleaning
The mechanical dry cleaning of coins involves carefully removing dirt from the surface of the coin using a combination of sharp and soft points as well as nylon brushes. It is important to know the design of the coin you are working with so you don’t risk scratching areas of high relief. I tend to find a circular motion using either a wooden or metal point with slight pressure works well to remove dirt. If not too fragile, you can also try soaking the coin in distilled water for short amounts of time and then drying them off with paper towels or a rag. The most important aspect to this step is to know when to stop. I recommend familiarizing yourself with examples of cleaned coins on the internet to understand what the finished product should look like.
- Distilled/de-iodized water wet cleaning
If the coin is slightly more sturdy, prolonged soaks in distilled water is recommended, followed by light brushing, drying and mechanical methods discussed above. It is important not to use normal tap water, as this contains various chemicals - including chloride ions - which may lead to ‘bronze disease’ (Hydrochloric acid eating away at the metal).
- Olive oil wet cleaning
My most preferred method of cleaning involves prolonged soaks in olive oil followed by mechanical cleaning. This method allows fatty acids from the oil (although not too acidic) to soak into the dirt of the coin while, in most cases, not penetrating past the patina layer. Once the oil has soaked in, the bond between dirt and patina becomes weaker and the encrustation is easier to remove. Soaks should begin for an hour at a time, increasing to days and weeks if the initial soak and mechanical cleaning is not sufficient to remove dirt.
- Treating for bronze disease.
As discussed above, ‘bronze disease’ is an ongoing corrosive reaction which is often quite difficult to treat. The most cost-effective method is to isolate the coin affected, remove all surface deposits, bake the coin in an oven (at 250 degrees for 30 minutes) to remove moisture, coat the coin with ‘museum wax’ to ensure the surface is air-tight, and then store the coin in an area of low humidity. You can also try repeated soaks in distilled water (which acts to remove chloride ions and impurities from the metal). For extreme cases, I recommend soaks in 5% Sodium Sesquicarbonate solution for over 2 weeks, or soaks in benzotriazole - ethanole solution for a day or more (benzotriazole is very dangerous and must only be used in a controlled environment).
Storage and Handling
The appropriate humidity for the storage of coins is below 35%. Collections should be stored in enclosed spaces with activated charcoal paper or silica gel to control humidity. Wooden or organic storage products are to be avoided as they release acidic vapors and sulfur. Coins should be stored individually using Mylar or polyethylene terephthalate coin flips to prevent the spread of corrosion. If handled often (especially if the coin is of high value), I recommend using cotton gloves to move the coins. There should be regular checkups to make sure none of the coins show any signs of active corrosion.
Identification
The identification stage of this process is often one of the hardest for new collectors given there are literally tens of thousands of different designs and variations. One of the most important lessons for this stage is to recognize the size and style of the coin, which helps a lot to narrow down the dates. The image on the left shows some of the most common denominations of Roman imperial coins, with sizes and weight (these did change slightly throughout the years). The front side (or head) of the coin is known as the ‘obverse’ whereas the opposite (tail) side is known as the ‘reverse’. The letters surrounding the images is known as the ‘legend’. The legend on the obverse side of the coin usually denotes the rulers name(s) and different imperial titles, whereas the reverse legend could be a variety of propaganda/mythical/battle scenes. One method to identify official imperial Roman coins (as opposed to provincial smaller mints) is to familiarize yourself with the images of different emperors and the names/titles they adopted (see link 1). From here you can select a page of reverse types from that emperor to further identify the coin (see link 2). There is also the option to attempt to find common reverse designs from a particular period to narrow down your search, for example, common 4th-century coins (see links 3 & 4). If you are able to make out any of the lettering around the coins, or recognize any figures on the reverse, these can be searched through various coin search engines to find similar examples (see links 5, 6, & 7). A final piece of advice is to do your own research online to familiarize yourself with the processes discussed above. It is also a great idea to sign up to various Facebook or online groups in order to meet like-minded people and exchange advice for the identification of each other’s coins (see link 8).
http://www.romancoins.info/ImaginesImperatorumStart.html
http://dirtyoldbooks.com/roman/Roman-Emperors.html
http://www.romancoin.info/#Identifying%20Common%20Late%20Roman%20Bronze%20Coins
http://augustuscoins.com/ed/ricix/
https://www.vcoins.com/en/Default.aspx
http://www.forumancientcoins.com/