Cleaning metals: basic guidelines
Once you have thoroughly examined your object and identified any potential problem areas, prepare your piece for basic cleaning using the following guidelines:
- Metals and their surface finishes are softer, more porous and more easily damaged than is often imagined. Inappropriate or excessive cleaning is a common mistake. When it comes to cleaning metals, less is more.
- Work in an area with good light, ventilation, a padded surface (for example thin foam or cardboard covered with blotting paper).
- Stop often to inspect your swab and the surface as you work - check what's on the swab and check the surface. If in doubt, stop and consult a conservator.
- Test clean an unobtrusive area of the object first.
- If in any doubt, do not attempt any cleaning and seek advice from a conservator.
There is a difference between removing dirt and dust (cleaning) and removing tarnish (polishing). Polishing uses abrasives, so every time tarnish forms and is polished away, part of the original surface is lost. Repeated polishing may eventually lead to the loss of decorative details, plating, chasing, filigree work or even hallmarks.
The ideal treatment is to remove unwanted tarnish or corrosion and then do your best to maintain your object in that state. This may mean using an anti-tarnish products and/or applying a protective coating.
Polishing typically produces shiny and different coloured areas. It is almost impossible to remove dark patches or polish one area to match an adjacent lighter area. If you wish to try to leave some tarnish behind, work over the surface as a whole, stop regularly to rinse off polishing residues and check that the surface has an even appearance.
Solvents - take care!
Apart from distilled water, the solvents mentioned below are all listed on the COSHH (Control of Substances Hazardous to Health) Register. It is very important to read the instructions and hazard labels before you use these .
- Distilled water - available from DIY stores and chemists, used to clean and as a neutral rinse.
- White spirit - good for removing waxes and oily dirt, available from DIY and hardware stores.
- Methylated spirit - good for removing degraded lacquer and degreasing, available from DIY and hardware stores.
- Acetone - a useful solvent which will remove most coatings, available in small amounts from chemists. This is the most aggressive solvent in the list, so it may be prudent to start with others.
Gloves (plastic or Nitrile, not fabric) should be worn to protect the hands, work should be carried out in a very well ventilated area, and the minimum amount of solvent should be used.
To ensure safe handling of solvents, decant small quantities for use into glass or ceramic containers - this prevents contaminating your bulk supply. Label small quantities of solvent (use the bulk container as a guide) and store all solvents appropriately (e.g. out of reach of children and away from sources of ignition etc.).
The following tools are essential for safe cleaning:
- Artist's sable brushes for dusting - tape over the ferrule (metal band between hairs and handle) with masking tape to prevent scratching.
- Cotton buds or home-made swabs (make using a kebab stick and cotton wool - pull off a small quantity of cotton wool, push the stick into the cotton wool, then twist the stick so that the cotton wool rolls onto the end.
- A swab pot or small waste container (that can contain solvent fumes from used swabs). Take an empty soft drink can and label it on both sides 'used swabs'. To use, remove used swabs from the stick by pushing the swab into the hole and pulling the stick back against the edge of the opening. This will loosen the swab and it will then drop into the can. When the can is full leave it somewhere well ventilated to allow any solvent residue to evaporate, and then dispose of it in your household rubbish.
Taking things apart for cleaning
Metal objects are often constructed in several parts that are screwed or bolted together. Occasionally it is better to remove parts, for example wooden knobs from a metal object, before cleaning. However, dismantling can be problematic. Some metal items are assembled under tension, so when a screw is removed the pieces spring apart and are difficult to reassemble. Sometimes so much tension was used that it causes stress in the metal, which can fracture as you try to reassemble it. It is often a safer option to protect non-metal parts by covering them with a plastic food wrap such as Clingfilm.
If you decide to dismantle, make a careful note of where each part comes from - screws should be returned to their original holes, for example. You may need to label each component, and use a drawing or photo of the object to identify each part. Use good quality tools that match the job, for example screwdrivers should fit the slot snugly, otherwise the screw heads and the object itself can be damaged. Use masking tape on all parts of the tool that either come into contact with the object, or that might inadvertently scratch the surface of your object, for example, the metal ferrules on brushes.
Coating metal after cleaning or polishing
Once tarnish has been removed, a coating is sometimes applied to slow down re-tarnishing. If a coating is applied, metal must be coated completely and evenly. If it isn't, preferential corrosion will occur and parts that were missed will corrode faster than if the object had been left completely uncoated. Old lacquer should be removed and replaced at the first sign of breakdown (i.e. tarnish forming) unless the lacquer is original and forms part of the historical value of the object. Lacquer coatings that are handled as little as possible and protected from sunlight can last for many years.
Beeswax and other natural waxes have been used for hundreds of years to coat a wide variety of objects. However, they can become acidic if not regularly removed, and may cause corrosion to start. There are brands of synthetic, microcrystalline wax on the market which are acceptable for use, as they remain neutral. Apply a thin, even coat using a hogs hair brush or soft cloth, and then buff the surface gently with a soft lint-free cloth. Repeat two or three times to minimise the possibility of gaps in the coating.
If there are accumulations of wax in corners of decorative details, gently remove excess with a brush or wooden swab stick. Don't use wax on surfaces that are very ornate or detailed as it can be difficult to remove from around raised details or crevices. Wax needs more maintenance than lacquer; it may need removing and renewing every two years.
Commercial metal coatings may be best left to a conservator as many of them incorporate harmful solvents that require extraction equipment for safe use. If you decide to do it yourself, follow the manufacturer's instructions and apply a thin coat using a flat squirrel hair brush.
Shellac (a lacquer derived from beetles) has been applied to many metal fittings and mounts associated with furniture since at least the mid 18th century. It has the advantage of good adhesive properties and is usually easily reversible in alcohol (methylated spirits) or acetone. Shellac is difficult to apply, and the object to be lacquered must be heated, which may not be appropriate. Seek advice from a conservator as to the best method of use.
Traditional or home remedies - to be avoided!
Traditional or home remedies usually rely on harsh abrasives, acids or alkalis to attack tarnish and should be avoided. An overnight soaking in cola, baking soda, false teeth cleaner or lemon juice is aggressive and uncontrolled. The effect - a bright and shiny 'as new' appearance - is not usually appropriate or desirable on an antique. Traditional remedies are used for a 'quick fix' - the opposite of conservation, which requires a slower, more cautious approach and regular progress checks.
- Ammonia - also known as ammonium hydroxide, ammonia will darken many woods and attack transparent and decorative surfaces. Cast brass or bronze is porous and the longer the contact with an ammonia solution, the greater the danger of deep penetration of both water and ammonia into the body of the metal. In some cases ammonia may leave the surface with an undesirable pink colouration. Surfaces newly cleaned with ammonia tend to tarnish rapidly and thus soon lose their brightness. Ammonia should not be used on copper or its alloys (brass and bronze). The risks associated with ammonium hydroxide far outweigh potential benefits for cleaning.
- Commercial polishing products - avoid any such products which have a chemical action. Any chemicals which can strip metal corrosion products from a surface are also capable of attacking the metal itself to cause pits. When using commercial brands, less is more. It is important to remove all residues of cleaning product from the object's surface. Manufacturers can also change the formulations of their products without warning. Always check first that the product does not contain ammonia and if in doubt contact a conservator.
- Water - avoid immersing metal pieces in water. Cast metal tends to be porous - there are often micro-voids in the surface that can trap water or other liquids. Ornate or three-dimensional objects, or pieces that have components soldered on, may have gaps in the solder which will also trap liquids. Water/other liquids may cause corrosion in the long term.
General care guidelines for objects in regular use
Metal objects in regular use, such as copper pans, silver flatware or teapots, pewter mugs and plates etc will benefit from these basic care procedures:
- Make sure the objects are in good condition, without cracks or soldering at the joints.
- Clean as soon as possible after use - particularly after contact with garlic, fruit juices, eggs, salt or onions.
- Wash with warm, soapy water, and rinse thoroughly.
- Dry thoroughly with a soft cotton cloth or kitchen towel.
- Wipe over with methylated spirit to remove any fingerprints.
- Store in tarnish inhibiting bags and in a dry, warm place.
- Avoid dishwashers.
- Avoid soaking.
- Avoid storing in sealed plastic bags, wrapping in newspaper or tying with rubber bands.