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Historic Preservation - Technical Procedures

Spectitle:

Copper: Characteristics, Uses And Problems

Procedure code:

0501501S

Source:

Developed For Hspg (Nps - Sero)

Division:

Metals

Section:

Copper

Last Modified:

02/24/2012

Details:

Copper: Characteristics, Uses And Problems



COPPER:  CHARACTERISTICS, USES AND PROBLEMS


This standard includes general information on the characteristics
and common uses of copper and identifies typical problems
associated with this material along with common causes of its
deterioration.


INTRODUCTION

Characteristics of Copper:

-    Durable

-    Corrosion-resistant

-    Strong

-    Ductile:  Can be drawn or "stretched"

-    Malleable:  Can be hammered or rolled into sheets without
    breaking

Copper is initially bright reddish-brown in color, but when exposed
to the atmosphere, it acquires a protective patina  that turns from
brown to black to green over an eight to ten year period.  This
patina is a copper carbonate or copper sulfate formed on the
surface of the metal when hydrogen sulfide combines with oxygen or
sulfur dioxide.  This naturally occurring corrosion resists further
corrosion.  Though copper does corrode, this protective patina
makes copper a corrosion-resistant material.


TYPICAL USES

Typical historical uses for copper included:  

-    Sheathing for ships

-    Roofing and flashing:  Sheet copper is light and easily
    formed.

-    Ornamental detailing such as weathervanes and finials

-    Decorative detailing such as running moldings, sheathing on
    oriel and bay windows, rain conductor heads, and other detail
    fabricated to ornament the cornice lines of many buildings.

-    Statues:  Sections of sheet copper were often hammered over
    wooden or other forms to create ornaments or statues.  Once
    the copper sheets had taken the shape of the form, they were
    removed and soldered together over a wooden or metal
    framework.  The most famous example is the Statue of Liberty
    which consists of copper sheeting over a steel framework.

Typical uses for copper in the 20th century include:

-    Decorative detailing:  Limited due to the high cost of copper.

-    Flashing, gutters and downspouts:  Used in small quantities.

-    Piping systems:  Copper's strength and resistance to corrosion
    by most types of soils and water make it appropriate for use
    in plumbing.

-    Electrical wiring, telephone wiring, and heating and air
    conditioning systems:  Copper's high capacity for thermal and
    electrical conductivity makes it suitable for these
    applications.


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

1.   Chemical corrosion:

    a.   Copper is attacked by alkalis, ammonia, and various
         sulfate compounds that can combine with water to form
         sulfuric acid.  

    b.   Some bituminous roofing cements will attack copper, as
         will sulfate-reducing bacteria, which act as a catalyst
         for corrosion.

    c.   Copper is also attacked by rainwater that has become
         acidic through contact with moss, lichen, algae, and
         occasionally wood shingles.  (The use of 16 oz. copper
         sheets will often alleviate the problems associated with
         cedar shingles.)

2.   Galvanic (Electrochemical) Corrosion:

    a.   Galvanic corrosion occurs when two dissimilar metals come
         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.  

    b.   Direct contact of copper with other metals will cause
         those metals to corrode.  Iron, steel, zinc and
         galvanized steel, tin and aluminum are especially
         susceptible to corrosion if put into contact with copper.

         1)   Copper roofing fastened with other than copper or
              brass fasteners will cause the fasteners to
              corrode.

         2)   Mixing metals used for flashing, gutters and
              downspouts, decorative elements, windows or roof
              covering will also cause galvanic corrosion.


VANDALISM OR HUMAN-INDUCED PROBLEMS

Mechanical or physical deterioration:

1.   Erosion:  Erosion of valley flashing and gutters is caused by
    continuous exposure to rain and snow which scours the surfaces
    as it drains.

2.   Fatigue:  Anything which restricts movement due to normal
    expansion and contraction will cause fatigue.  This can
    include:

    a.   The use of asphaltic building papers which will stick to
         the backside of the copper sheets preventing them from
         moving when heated by the sun.

    b.   The lack of an adequate number of transverse joints or
         welts in a length of sheet copper between bays.  Cracking
         of the sheets will often result.

    c.   Improperly sized bays (space between vertical seams) or
         the use of an inadequate number of fasteners.  Using
         copper sheets which are not rigid enough to resist this
         movement will exacerbate the problem.

    d.   Inadequate support from the underside, such as spaced
         rather than tight sheathing boards, will also result in
         buckling and sagging of the sheet metal, ultimately
         causing the metal to crack and tear.

                         END OF SECTION