Brass, Copper, and Bronze

Introduction

Any number of household objects are made of brass, copper and bronze: decorative objects, housewares, hardware and inlays on furniture and tools. These items are found in an almost endless variety of sizes and shapes. They can be maintained with basic care and attention.

Copper is a soft metal that is pinkish in colour when very clean, but soon tarnishes to an orange/red. Copper can acquire a natural patina that is very attractive and quite stable. Handling and other sources of contamination, however, cause a patchy and unpleasant appearance. Copper objects can also be chemically treated to produce a range of colours and surface textures.

Brasses are alloys of mainly copper and zinc. Standard brass (also known as cartridge brass) contains 70% copper and 30% zinc. Brass objects also acquire a stable patina, often a rich brown colour, although handling and other sources of contamination cause unattractive surfaces to develop. As on copper, artificial patination is sometimes applied chemically.

Bronzes vary greatly in composition, but most are essentially alloys of copper and tin. Other metals are added in small quantities to modify the properties of the alloy. Sometimes brown, brownish-green or green patinas are produced on the surface using chemicals, or a patina can accumulate naturally over time. Patinas on bronzes are highly prized. It is a mistake to over-clean them.

Causes of Damage

Improper handling and storage are the main causes of damage to objects made of brass, copper and bronze. Mishandling can dent, bend and break these objects. The surfaces of these artifacts can also be scratched easily because these metals are not very hard. Over-polishing with substances not normally recommended for heirlooms can remove surface features.

A moist environment can cause corrosion, especially when contaminating materials, such as salts and oils due to handling with bare hands, are present. Also, if left to accumulate, dust and grime will hold moisture and encourage corrosion, even in low humidity.

Some gaseous materials in wood and plastics can also cause corrosion. Paints, wood products, textiles and other household materials that are used to protect metal objects in storage can sometimes cause more trouble than they are intended to solve.

The accumulation of chloride ions on archaeological objects made of copper or its alloys can result in a condition called "bronze disease." Bright powdery green spots appear on the surface. If this corrosion process is not corrected, it can destroy the object. Such corrosion problems should be treated by a conservator.

Varnish or lacquer is sometimes applied to brass, copper and bronze objects to retard corrosion. These surfaces break down eventually, causing uneven tarnishing and spotty corrosion.

Handling and Storage

If possible, avoid handling items made of brass, copper and bronze with your bare hands. Oils and salts on the skin or in lotions and cosmetics can etch into metals and may cause permanent damage. Most people have probably seen how fingerprints develop into dark marks on uncoated copper alloys. It is best to wear cotton or polyethylene (not natural rubber) gloves.

When lifting, hold the metal piece with both hands. Avoid picking up items by their spouts, handles, lips or other extended areas. These parts may be weaker than the main body. As well, it is sometimes difficult to assess how well attached they are.

The best way to avoid problems is to ensure that items are stored in the proper environment. For metals, this means avoiding areas with high humidity. Basements, attics and garages are not appropriate storage areas unless you can control dampness.

Small articles can be stored easily and inexpensively in polyethylene zip-lock bags or polystyrene boxes (available at plastic supply houses) lined with a soft, non-abrasive packing material such as acid-free tissues or Microfoam, an inert foam plastic.

Cleaning

As with handling, it is recommended that cotton or polyethylene (not natural rubber) gloves be worn for cleaning, polishing or waxing objects made of brass, copper or bronze. It is worth remembering that freshly cleaned and polished metal surfaces are especially sensitive to fingerprints.

Before cleaning, examine the item for damage. Surface dirt can often be removed using a very soft toothbrush or artist's brush. There are two ways to remove accumulated surface grime. The first is to use lukewarm distilled water with a mild liquid detergent and a soft cloth. Be careful to avoid scrubbing too hard because this may scratch the surface if there are particles of grit in the cloth. Rinse with distilled water and allow to air dry on paper towels. If this is not sufficient, greasy residues can be removed with a mixture of equal parts distilled water and alcohol. This can be applied with a stencil brush, cotton swabs or a soft cotton cloth. Rinse in distilled water and let dry.

Polishing will remove tarnish. Because all metal polishes contain abrasives, they will also remove some of the metal. Over-polishing can, therefore, result in the loss of surface detail. If excess metal polish has been used in the past, there may be a build-up of residue in detailed areas. This is usually greyish-white or greenish-white in colour. Such residues should be removed. This can be done by using a wooden toothpick moistened in the water and alcohol solution mentioned above. If lacquer or varnish is present, remove it before polishing (but only if adequate facilities are available because the process requires toxic and flammable solvents).

A mild polish, such as jeweller's cloth, is recommended for most mildly tarnished copper, brass and bronze pieces. The cloth contains a fine abrasive, but even so excess metal can be removed by rubbing too hard. Use a more abrasive polish, such as liquid metal polish, only if using the cloth is ineffective. Most polishes leave a blackish residue, which can be removed by softly polishing with a plain piece of flannelette.

Polishing should be done only if the surface can be protected from tarnishing again. There is no point in polishing away material from the surface if tarnish is allowed to reform. Wax coatings offer some protection and can reduce the need for frequent cleaning or polishing. A suitable coating can be made by combining equal parts mineral spirits (for example, Shellsol or Varsol) with a bleached paste wax such as Renaissance wax or a high-quality, white furniture wax. The mixture should be stored in a tightly sealed jar.

To apply, wipe or brush the wax mixture over the object. Then, set it aside so the solvent can evaporate. Be careful to apply it evenly. If the article does not contain wood, ivory or other heat-sensitive material, a hair dryer can be used to melt the wax, ensuring that it gets into recessed areas. Blot away excess wax with tissues while it is still warm. Once the wax sets, buff the object with a clean, lint-free soft cloth.

To remove the wax, use mineral spirits. Be sure to do this only in a well-ventilated room and wear rubber gloves to protect your skin. Small amounts of leftover solvent (or solvent-saturated cleaning cloths) can be disposed of by letting them evaporate until they are dry in a well-ventilated area. Then, put the solid residues in plastic bags and dispose of them.

An application of lacquer is sometimes advisable for objects that will be handled regularly or are exposed to high humidity, pollutants, etc. It is advisable to have this done by a specialist because a poorly applied coating can be disfiguring and does not provide adequate protection.