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Historic Preservation - Technical Procedures

Spectitle:

Wrought Iron: Characteristics, Uses And Problems

Procedure code:

0501011S

Source:

Developed For Hspg (Nps - Sero)

Division:

Metals

Section:

Metal Materials

Last Modified:

11/06/2014

Details:

Wrought Iron: Characteristics, Uses And Problems



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
deterioration.

REFERENCES:

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.

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 its carbon
content.  Iron/carbon alloys used in construction include the
following:

1.   Wrought iron:  Contains very little carbon (less than
    .035%).

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

3.   Cast Iron:  Contains a high 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.  Note:  Wrought iron becomes stronger the more it is
    worked.

-    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
following:

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
         'boiled'.  

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

    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
         3 feet.  

    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.


TYPICAL USES

Historical uses during the 17th and 18th centuries were typically
decorative and include:

-    Fences, gates and railings

-    Balconies

-    Porches and verandas

-    Canopies

-    Roof cresting

-    Lamps

-    Grilles

-    Hardware

Historical uses during the middle of the nineteenth century
became more structural and include:

-    Nails

-    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-
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

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.

Connection failure:  

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
    susceptible.

                         END OF SECTION
 


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