When cleaning finds you are unlikely to be increasing the value of an object but it is very easy to decrease its value. If there is any possibility that it is valuable it is better to leave it alone. Use distilled or deionised water for cleaning. Tap water can have dissolved impurities. The gentlest cleaning method is to leave an object soaking in distilled water to soften the dirt. Change the water when required. Gentle brushing can be used to remove the loosened soil but be careful if the object has an unstable surface. Removing the corrosion can result in removing all detail at the same time. Be especially careful with gold. Gold is a very soft metal. If you rub or brush a dirty gold object you will easily cover it with tiny scratches. Always practice on an object of no value first.
A paper discussing cleaning of copper alloy coins is available on digbible.org.
Gold is a very stable metal. If it is discoloured or corroded it is not good gold. Using water to wash off any soil is all that will be required. Be gentle as it scratches so easily.
Silver is a stable metal and normally washing the soil off is all that is required. Silver can oxidise to a black or brown patina. Collectors prefer to see the patina of age. A shiny polished coin can be worth a fraction of what it was worth before it was cleaned.
Copper alloy objects form a brown or green patina. Unfortunately they are more prone to corrosion and are often found with a corroded surface and not with a nice patina. If they have a stable patina then washing the soil off with distilled water is all that should be required though after 2000 years it can become well attached and soaking techniques are required. If the object has a corroded surface then more care is required. Any surviving detail may be formed by corrosion. Removing the corrosion will also remove any detail. The best way to clean corrosion is mechanically by hand. This is how a museum conservator will do it. Use a small tool to pick away the corrosion gently preserving the detail. The tool should be a softer material than the object being cleaned. Professional conservators use a thorn held in a pin vice. See the description of how the Staffordshire hoard was cleaned. A binocular microscope is very useful for this but they are not very cheap. Basic models can be found on Ebay. Mechanical cleaning is very time consuming.
Lead And pewter
Lead will oxidise to a grey / white colour. This can be powdery and is toxic. Lead should be washed with water. Preferably use gloves when handling lead. Pewter is an alloy of lead. It is harder but very susceptible to corrosion. Storing lead and especially pewter in a dry acid free environment is the key to preventing further corrosion. Pewter can literally crumble to dust if stored incorrectly.
Using chemicals to clean an object is just a method of automating a mechanical cleaning. The end result is the same. The end result is far less controllable and many chemicals used will be dangerous or give off dangerous fumes. It is best to avoid chemical cleaning unless experienced. Do not try cleaning with vinegar, coca cola or other household chemicals. They clean because they are acidic and remove the corrosion or patina by dissolving the surface of the object. They can be very harsh and can take an object back to bare metal normally removing most of the detail with the corrosion. They also contain many other chemicals which can react in unpredictable ways and leave a residue which can continue reacting with the object.
These sound good but in practice a cleaner intended for jewellery is not powerful enough to remove encrusted dirt. I have found they can help remove soil far faster than soaking methods if the underlying coin surface is stable but can damage coins that have a corroded surface. Buy one with a 1/2 hour timer. Adding a little washing up liquid can help.
A patina is formed when the surface of an object reacts with its environment. Corrosion is a similar process but where the reaction eats into the surface or forms a material which is not stable. Electrolysis is a process where an electrical current is passed through water with a dissolved electrolyte such as baking soda.(Do not use salt indoors as it will give off toxic chlorine gas). The process removes the non metallic elements of the patina causing the surface of an object to be turned back to metal. The corrosion and patina is removed. There is still a risk of detail being lost in the process and the end result looking too clean or new. If there was no detail remaining under the corrosion electrolysis will not recover anything. Less is always better. It is always possible to clean a little more later. It is not possible to unclean an object. I have found electrolysis to be a good way of removing verdigris but the resulting surface may be pitted. Always do electrolysis in a well ventilated area as the process can produce toxic gas.
metaldetectingintheusa.com has an article with more information.
One of our members, Andrew, describes how he used electrolysis to clean a small hoard of roman silver coins, available as a pdf here.
Soaking in distilled water should be a first step for most cleaning. Use distilled water and not tap water which contains impurities. As there is nothing dissolved in distilled water so it has a higher capacity to absorb dirt. There is a benefit in replacing the water regularly. Distilled water can be bought in many places like B&Q or Halfords. If you have a condensing tumble drier or dehumidifier the water it collects is suitable. You can also buy machines specifically for producing water but they can cost £100+.
Olive oil and petroleum jelly
Ensure your coins are completely dry before soaking in olive oil by baking in an oven at about 100 degrees as water and oil does not mix. Soaking in olive oil may loosen soil a little quicker than water but soil plus water is easy to rinse off. Soil plus oil will not rinse off and should be removed with a degreaser. I use No nonsense heavy duty degreaser from Screwfix. You will need to soak the coins in neat degreaser for a while to remove the majority of the oil. Do not leave them longer than necessary as it can affect the patina. Do not mix coins of differing metals either. I have seen patinas on bronze coins be affected by aluminium and cupro-nickel coins. Washing up liquid in water will not work well enough. When the coins have been washed and dried soak them in isopropol alcohol to remove any remaining oil. Isopropol alcohol is highly flammable and evaporates at room temperature so you need to use an airtight and preferably unbreakable container. Using olive oil can darken the patina. Olive oil should not be used to preserve an object. Olive oil is slightly acidic and being a natural product it contains water. Preserving an object with olive oil means it is in a damp acidic environment. The oil may also go rancid leaving the object smelling bad. Do not use vegetable oils for the same reason. Petroleum jelly such as Vaseline does not dry so any object coated will attract dust over time.
One process I have used is to rinse all loose soil from the coins then soak in distilled water for several weeks. I then spread the coins on a baking tray and freeze them overnight before putting them in the oven at 100 degrees to dry thoroughly. When water freezes it expands so this can help loosen encrusted soil. The coins are then soaked in olive oil for several weeks. I remove as much oil as possible with kitchen towel before soaking the coins in degreaser. They are then washed and dried before soaking in isopropol alcohol. Sort out the coins that require further cleaning and repeat as required. Some coins will not clean successfully by soaking as corrosion products can be mixed with soil forming a crust. These will need chemical or electrolytic cleaning. Of course there is no guarantee of any detail surviving under the dirt.
Storage of finds
Finds should be stored in a dry environment. Desiccant sachets are the normal method of removing excess moisture from a container. Finds should be stored in an acid free environment. Envelopes made of acid free paper can be used. Plastic containers should be made of polypropylene or polyethylene (polythene) which are chemically inert. Jiffy bags (grip seal bags) and jiffy foam are made with polyethylene.
Cleaning and storage products are available from many sources on the internet. Here are a few:
Prinz sells a range of coin holders and acid free paper envelopes. CPC sell Grip seal bags. Weston boxes sell a range of polypropylene boxes. Amazon has merchants selling grip seal bags, jiffy foam and acid free card.