Iron Objects

Introduction

Iron has been used for several thousand years in many different forms. It can be wrought by hand, cast into molds, or formed through a wide range of modern factory processes. It is the most common metal in collections and appears universally in every household.

All common metals are derived from their natural ores by a process called smelting. Iron ores are very stable chemical substances, but when smelted a great deal of energy is put into them to produce the metal. The metal is, therefore, more reactive and less stable and would return to the stable state if it could. We know by observation that metals are relatively unstable - they will rust and tarnish if allowed to do so. This process is called oxidation, although it is not always oxygen that is responsible. As iron oxidizes it moves quite slowly to states of lower energy where it is less reactive and thus more stable.

Iron is converted to steel by adding carbon or other trace elements. In fact, objects of pure iron are very scarce. Working iron during forging introduces carbon as a by-product, resulting in the production of steel. So even wrought iron objects made by blacksmithing are actually made of low carbon steel. Early smiths used this property to advantage, adjusting the amount of forging to make composite sword blades with hard edges and flexible cores. Cast iron objects actually contain quite a high proportion of carbon; as much as 4% in some cases. Thin sheet steel is one of the most common forms of iron, appearing in a wide range of artifacts from lamps and stoves, to cooking ware and containers. Steel is also plated with tin or zinc to give it better protection from the environment.

Causes of Corrosion

When iron is exposed to water and oxygen, it corrodes. The corrosion products are the familiar red-brown rust. Indoors, a certain amount of moisture in the air (above about 65% relative humidity) is usually required before iron will rust. However, corrosion may occur at lower relative humidities if the object's surface is contaminated with salt, dust, or other pollutants. Some iron objects have acquired an adherent rust layer from past exposure to high levels of moisture. This rust layer is often stable and will help slow corrosion of the underlying metal if the object is accidentally exposed to high moisture levels. Salt or other contaminants in the rust layer will stimulate further corrosion, especially at higher relative humidities. A sign of ongoing corrosion is flakes or loose powdery rust surrounding an object.

Handling

Salts and oils on the skin can produce exactly the kind of corrosive environment that causes iron to react. Also, exposure of iron objects to cooking materials such as salts and sauces can accelerate corrosion. If the iron object in question is considered valuable, it should not be used as it was in the past. It should be protected from excessive handling and kept in a dry location. Attics, basements and garages are not good places to store iron objects because of fluctuating environmental conditions.

Cleaning

Objects that were originally finished black, and are in a corroded state, can be darkened again by applying tannic acid. This converts the red rust into a stable iron tannate and gives an even black surface. Such objects include fire irons made of wrought iron, cast stove components, and some tools. The blackening effect of tannic acid is quite noticeable, so it should only be used on objects that were originally black finished. Directions on how to proceed can be found in CCI Notes 9/5.

Lightly corroded objects that were originally brightly finished can be cleaned with fine steel wool and oil. Use the finest grade of steel wool (0000) and a light lubricating oil such as sewing machine oil. Try to rub in the same direction as any tool marks the metal might have. Patches of deeper or tougher corrosion can be rubbed with coarser steel wool, but do not overdo this. At the end of the cleaning, the place where the corrosion was will still be visible as a grey patch. It is not necessary to go any further than this. Full directions can be found in CCI Note 9/6.

Objects that show extensive red or orange corrosion, and have pieces coming loose, should be referred to a metals conservation specialist because incautious cleaning can cause much damage.

Iron objects that might be exposed to high humidity or handled on a regular basis can be coated lightly with microcrystalline wax or a white paste wax for furniture. Rub the wax in well with a soft cloth, and wipe off as much as possible. If the object appears white or smeary, there is still too much wax on the surface. Be cautious when dealing with parts that may have a deep texture, such as cast iron, because the wax can become trapped and appear unsightly. If this happens, the wax can be removed with mineral spirits on a soft cloth. Do this in a well-ventilated area.

Repair

In general, repair of iron objects is best left to a conservator. There is a wide choice of techniques, including soldering, welding, riveting and bolting, but all these potentially cause changes that may not be desirable on a historic object. Some of them can be used, if done well and skillfully, but it is best to ask the advice of a conservator before going ahead.