Historic Preservation - Technical Procedures
- Wrought Iron: Characteristics, Uses And Problems
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- Metal Materials
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- Wrought Iron: Characteristics, Uses And Problems
- Last Modified:
WROUGHT IRON: CHARACTERISTICS, USES AND PROBLEMS
This standard includes general information on the characteristics
and common uses of wrought iron and identifies typical problems
associated with the material along with common causes of its
Margot Gayle, David W. Look, John Waite. Metals in America's Historic Buildings. Washington, DC: National Park Service, 1992.
L. William Zahner. Architectural Metals. New York: John Wiley & Sons, Inc., 1995.
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 its carbon
content. Iron/carbon alloys used in construction include the
1. Wrought iron: Contains very little carbon (less than
2. Steel: Contains a moderate amount of carbon (between .06% and
3. Cast Iron: Contains a high amount of carbon (between 2% and
Characteristics of Wrought Iron:
- Strong - high elasticity and tensile strength
- Malleable - can be heated and reheated and worked into various
shapes. Note: Wrought iron becomes stronger the more it is
- Suitable for members in tension OR compression; whereas, cast
iron is suitable for members in compression ONLY.
Wrought iron was typically produced by the indirect reduction
process. Stages in the production of wrought iron include the
1. Stage 1: Preparation of puddled iron.
a. Pig iron was first smelted in blast furnaces and
subjected to a reducing condition when it apparently
b. The boiling iron was worked continuously by the puddler
and more impurities were removed from the iron, making the
c. 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.
d. The iron was then formed into balls to be molded.
2. Stage 2: Iron balls were hammered with a shingling hammer, to
expel surplus slag or cinder (shingled).
a. Shingling was completed in minutes and the finished
product was a bloom approximately 5 inches x 5 inches x
b. 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.
3. Stage 3: The bars were reheated and reworked as required to
achieve the desired grades.
a. This stage increased the ductility and tensile strength
of the puddled iron.
b. The more times the metal was reheated and reworked, the
stronger were its mechanical properties.
Historical uses during the 17th and 18th centuries were typically
decorative and include:
- Fences, gates and railings
- Porches and verandas
- Roof cresting
Historical uses during the middle of the nineteenth century
became more structural and include:
- Iron cramps (i.e. to secure masonry veneer building frames)
- Structural members in tension such as tie rods (or strapwork),
bulb-tees and I-beams. The standard sections of wrought iron
included bar iron, angle and 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-
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
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
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
VANDALISM OR HUMAN-INDUCED PROBLEMS
Mechanical or physical deterioration:
1. Fatigue: Failure of metal that has been repeatedly stressed
beyond its elastic limit.
a. Wrought iron is generally fatigue resistant because it is
so tough. It will deform considerably, within its
elastic limit, without failure.
b. Even if past overloading has caused deformation, wrought
iron fixings will usually continue to function.
c. 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.
1. Chemical and mechanical processes can cause the breakdown or
reduced effectiveness of structural metal fixings such as
bolts, rivets, and pins.
2. Stress failure is often a contributor to breakdown situations.
Iron connections which are water traps are particularly
END OF SECTION