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.
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. A paper discussing cleaning of copper alloy coins is available on digbible.org.
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.
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 but 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. When cleaning with chemicals, take special care if an object is made of differing materials, as a chemical which is good on one material may destroy another.
Some chemicals which can be used for cleaning are listed below. An indication of their safety and toxicity is given, though I would not recommend drinking even those listed as non toxic as any chemical (even water) is toxic if consumed in a great enough quantity. Change the solutions regularly as they can become depleted, or if they become discoloured, as this can cause cross contamination between different metals. Small shallow glass jars are good to use for soaking and will not be affected by the chemicals (though some acids may destroy metal lids over time). You don't want to be putting your fingers in some chemicals so check you can get coins and artefacts out with tweezers before filling the jar with the chemical. Plastic tweezers are best for most chemicals as they will not be damaged by acids and will not scratch objects. All the chemicals listed below are obtainable on the internet from places like Amazon or Ebay.
This can be used to clean soil from coins and one of its commercial applications is breaking down clay. It is useful to remove soil that is stuck in the detail of an object. After cleaning the majority of the soil, leaving the object soaking for about 10 minutes per side then rinsing is normally enough. Is is slightly acidic so long immersion can be detrimental. Use a 10% solution (100 grams dissolved in 1 litre of water). This is a good chemical for cleaning gold coins which can be very pure gold and therefore soft, and even gentle brushing of pure gold can result in micro scratches from the mineral grains in the soil. Leave the coin soaking for as long as required for the soil to dissolve, it will not react with the gold. Sodium Hexametaphosphate is also used as a food additive (E452i), and is also used in toothpaste, so can be considered non toxic.
Citric acid is a pure form of the acid found in lemon juice and should be used in preference to lemon juice as it will not contain all the impurities of lemon juice and the strength can be controlled. Use at 1% concentration (10 grams dissolved in 1 litre of water) to soften encrustation before mechanical cleaning or 5% (50 grams dissolved in 1 litre of water) as a general acidic cleaner. Inspect the object being soaked regularly and rinse when the desired result is obtained. It is non toxic.
EDTA is good at breaking down metallic oxides, returning an object to its metallic state. If the object will not look good as clean bare metal (e.g. most copper alloys) then this is not the chemical to use, though I have used it to some effect to remove patches of corrosion on otherwise clean brass tokens. I have also used it to soften encrustation on stubborn silver coins before electrolysis. Use in a 5% solution (50 grams dissolved in 1 litre of water). It is sold a food supplement though there is some question as to whether it is actually mildly toxic so best not to drink it.
Formic acid is used mostly for cleaning silver, especially more base silver that has copper corrosion. It is very good for removing the copper staining which can occur on base silver coins previously cleaned with other acids. Formic acid is highly corrosive, especially when concentrated and should be handled with extreme care. Wear gloves and glasses, and have a bucket of water on hand so it can be washed off immediately in the event of an accident. I diluted it down 10/1 (100ml of concentrated acid mixed with 1 litre of water) and still handle the diluted mixture with care.
If you want to be a Peroxide Blonde you will be using this stuff. It can be bought in different concentrations but the weakest 3% is sufficient. At higher concentrations it can be quite corrosive. The bottle I have says food grade but also warns it can cause serious eye irritation and to wear eye protection and gloves. Handle it with care. Hydrogen Peroxide has the chemical formula H2O2 and is unstable, wanting to break down into H2O (Water) releasing Oxygen. The fizzing caused by this can help lift dirt and crud from an object. It can also start to oxidise bare metal, so can be used to darken over cleaned objects. It should be kept in the fridge to help extend its shelf life.
Bleach can be used to darken silver. A silver object will turn black immediately if immersed in bleach, though it rubs of again easily if not immersed for a while. The black looks quite flat and unattractive so there is need to polish with a silver cloth afterwards to get a better tone. It is not a process that will benefit a find normally but may help improve the look of an overcleaned object. If you need to use this you have probably devalued your find anyway. I have only used this technique on a coin that needed straightening as the heating destroyed the patina. Handle with care, bleach can cause chemical burns.
Isopropol alcohol is an organic solvent and may affect some plastics so check its effects on anything plastic you intend to use. Its primary use is a degreaser and so will not be effective in cleaning anything that has been dug up, but is useful in removing years of dirt and grease that can build up on old coins that have been handled and left in drawers for years. It is toxic if consumed, and can cause dry skin by breaking down the oils in the skin, so it would be best to wear gloves, especially if you have sensitive skin. It can produce fumes so use in a ventilated area.
Acetone is another organic solvent so everything that applies to Isopropol Alcohol also applies to Acetone. Acetone is however far stronger. I had a coin which was covered in ink from a leaked biro. Soaking in Isopropol alcohol did not remove much, but when dropped in Acetone the coin was clean by the time it hit the bottom of the jar leaving a blue haze in the Acetone.
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 as it will give off toxic chlorine gas, and putting your find in salt water can leave a residue which can cause corrosion). The process removes the non metallic elements of the patina causing the surface of an object to be turned back to metal.
Electrolysis works best to remove encrustation from silver, as with encrustations of horn silver the surface of the original object is often intact. On copper alloy objects the corrosion and patina is removed often leaving the object overcleaned and ugly (Hydrogen Peroxide had help darken it again). 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. It can work well on iron objects to help stabalise them, though it is likely that you will need to scale up the process to handle the larger iron objects that may benefit from this (e.g. horeshoes, etc). Always do electrolysis in a well ventilated area as the process can produce toxic gas.
Many guides sugest converting an old phone charger or similar. The problem with this is that you have no control over the current and it provides no short circuit protection. If you intend on using electrolysis a lot it is worth considering buying a bench power supply which provides over current protection and allows the voltage to be varied to get a desired current. 300-500mA is about right for somethong coin sized. Too high a current can damage an object.
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[pdf].
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 distilled water but they can cost £100+.
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. 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.
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.