Basic care – Iron objects

Properties and uses of iron

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 Canadian Conservation Institute (CCI) Note 9/5, Tannic Acid Coating for Rusted Iron Artifacts.

Repairs

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.