Copper: Characteristics, Uses And Problems

References:

Margot Gayle, David Look, John Waite. Metals in America’s Historic Buildings. Washington,DC: National Park Service, 1995.

L. William Zahner. Architectural Metals. New York: John Wiley & Sons, 1995.

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 an iron 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:

• Natural nor inherent problems based on the characteristics of the material and the conditions of the exposure, and
• 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:

  • Copper is attacked by alkalis, ammonia, and various sulfate compounds that can combine with water to form sulfuric acid.
  • Some bituminous roofing cements will attack copper, as will sulfate-reducing bacteria, which act as a catalyst for corrosion.
  • 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.)

• Galvanic (Electrochemical) Corrosion:

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

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

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

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