Wrought Iron: Characteristics, Uses and Problems

References

Gayle, Margot, Look, David, and Waite, John. Metals in America’s Historic Buildings: Uses and Preservation Treatments. Washington, DC: National Park Service, 1992.

Zahner, L. William. Architectural Metal Surfaces. New York: Wiley 2004.

Introduction

Iron is a dark grey metal and is the major constituent of a range of materials including wrought iron, cast iron, carbonized iron (carbon steel) and steel, each of which has its own unique properties. Iron was first used as a material for tools and weapons. Its uses have since grown to include items for domestic use to architectural building components. The presence of iron in a feature may be detected with a magnet.

Wrought iron differs from cast iron and steel in that it contains less carbon. The three metals are ranked as follows in terms of their carbon content:

1. Wrought iron: Contains the smallest amount of carbon (less than .035%).

2. Steel: Contains a moderate amount of carbon (between .06% and 2%).

3. Cast Iron: Contains the largest amount of carbon (between 2% and 4%).

Characteristics of Wrought Iron

• Soft.

• Ductile.

• Magnetic.

• Strong - high elasticity and tensile strength.

• Malleable - can be heated and reheated and worked into various shapes.

• Becomes stronger the more it is worked.

• Suitable for members in tension or compression (whereas cast iron is suitable for members in compression only).

Stages of the Indirect Reduction Process for Making Wrought Iron

Stage 1: Preparation of puddled iron.

1. Pig iron was first smelted in blast furnaces and subjected to a reducing condition when it apparently ‘boiled’.

2. The boiling iron was worked continuously by the iron puddler and more impurities were removed from the iron, making the iron stiffer.

3. The iron was boiled until virtually no carbon remained, leaving a pasty mass of iron. This was evident when the carbon monoxide would stop bubbling through the iron.

4. The iron was then formed into balls to be molded.

Stage 2: Iron balls were hammered with a shingling hammer, to expel surplus slag or cinder (shingled).

1. Shingling was completed in minutes and the finished product was a bloom of approximately 5 inches x 5 inches x 3 feet.

2. The bloom, still at bright red heat, was then passed through rolling mills, becoming more elongated and thinner in section after each pass, and finished as puddled iron bar.

Stage 3: The bars were reheated and reworked as required to achieve the desired grades.

1. This stage increased the ductility and tensile strength of the puddled iron.

2. The more times the metal was reheated and reworked, the stronger were its mechanical properties.

Typical Uses

Historical uses during the seventeenth and eighteenth centuries were typically decorative and included:

• Fences, gates and railings.

• Balconies.

• Porches and verandas.

• Canopies.

• Roof cresting.

• Lamps.

• Grilles.

• Hardware.

Historical uses during the nineteenth century were more structural and included:

• Nails.

• Iron cramps (i.e. to secure masonry veneer building frames).

• Structural members in tension such as tie rods, bulb-tees and I-beams. The standard sections of wrought iron included bar iron, angle irons, T irons, channel iron (half H iron), rolled girder iron (rolled joist iron, beam iron, I iron, or H iron), various special sections (sash bar, beading iron, cross iron, quadrant iron), iron bars, rivet iron, chain iron, horseshoe iron, nail iron, plate iron, coated iron (tin or lead), and corrugated sheet iron (generally galvanized).

Note: By the end of the nineteenth century, the use of wrought iron for structural purposes had been superseded by steel.

Problems and Deterioration

Problems may be classified into two broad categories: 1) Natural or inherent problems based on the characteristics of the material and the conditions of the exposure, and 2) Vandalism and human- induced problems.

Although there is some overlap between the two categories, the inherent material deterioration problems generally occur gradually over long periods of time, at predictable rates and require appropriate routine or preventive maintenance to control. Conversely, many human induced problems, (especially vandalism), are random in occurrence; can produce catastrophic results; are difficult to prevent, and require emergency action to mitigate. Some human induced problems, however, are predictable and occur routinely.

Natural or Inherent Problems

Chemical corrosion can attack decorative and structural wrought iron features in several ways:

1. Uniform Attack: Corrosion attacks the metal surface evenly.

2. Pitting: Attacks the metal surface in selected areas.

3. Selective Attack: When a metal is not homogenous throughout, certain areas may be attacked in preference to others.

4. Stress corrosion cracking: Attacks areas in a metal which were stressed during metal working and were later exposed to a corrosive environment. Old, hand wrought iron items are more likely to be affected than are machine rolled wrought iron pieces.

5. Rust: Probably the most common form of chemical corrosion of wrought iron. It occurs when unprotected metal is exposed to oxygen in the atmosphere in the presence of moisture. Moisture can be in the form of normal humidity, rain, dew, condensation, etc. Other gases, such as carbon dioxide, sulfur compounds, soot and fly ash will exacerbate the corrosion of the iron, as will airborne salts.

6. Galvanic (or Electro-Chemical) Corrosion: Galvanic corrosion occurs when two dissimilar metals are in contact with one another and an electrolyte, such as rainwater, condensation, dew, fog, etc. is present. Such a reaction will cause one or the other of the metals to corrode. In the case of wrought iron, direct contact with copper or zinc, and to a lesser extent galvanized iron or steel, will cause galvanic corrosion.

Vandalism or Human-Induced Problems

1. Mechanical or physical deterioration:

   1. Fatigue: Failure of metal that has been repeatedly stressed beyond its elastic limit.

      1. Wrought iron is generally fatigue resistant because it is so tough. It will deform considerably, within its elastic limit, without failure.

      2. Even if past overloading has caused deformation, wrought iron fixings will usually continue to function.

      3. Defects in the wrought iron itself, or stress points can cause a feature to fracture.

   2. Heat: Usually in the form of fire, will cause wrought iron features to become plastic, distort, and fail.

   3. Distortion: Permanent deformation or failure may occur when a metal is overloaded beyond its yield point because of increased live or dead loads, thermal stresses, or structural modifications altering a stress regime.

2. Connection failure:

   1. Chemical and mechanical processes can breakdown or reduce the effectiveness of structural metal fixings such as bolts, rivets, and pins.

   2. Stress failure is often a contributor to breakdown situations. Iron water traps are particularly susceptible.