Lead: 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

Characteristics of lead

• Very soft: without support, it can sag and become distorted.

• Dense.

• Durable.

• Malleable.

• Has a low melting point.

• Long life-span (in milder climates, lead roofs have been known to last 200-300 years).

• Generally corrosion-resistant - has little to no reaction with most compounds and solutions.

• Resistant to corrosion by most acids including chromic, sulfuric, sulfurous and phosphoric acids.

• Corrosive to alkalis (such as lime mortar, portland cement and uncured concrete), tannic acid found in wood, and radiation. Also corrosive to hydrochloric, hydrofluoric, acetic, formic and nitric acids.

Note: Ingestion of lead dust, typically through contact with older walls painted with lead paint, can result in serious long-term health problems, especially in the cases of young children or repeated exposure. See “Reducing Lead-Based Paint Hazards Using a Combination of Abatement and Interim Control Techniques on Windows” for more information and precautions associated with lead-based paint.

Typical uses

Typical historical uses for lead included:

• Lead Pipes: Sheets of lead were formed into tubes by bending and lead burning (welding).

• Flashing, Gutters, Downspouts, and Conductor Heads: In roofing applications, lead was best used for flat or low pitch roofs and built-in gutters due to the heavy weight of the lead sheets.

• Prior to the late 17th century, lead was cast by hand in sand beds. Later, lead sheets were rolled in the mill and were, consequently, much lighter. Lead-coated copper was introduced in the 1930s. This consisted of sheet copper dipped first in a lead-tin alloy, then dipped in pure lead and then rolled.

• Lead-based paint.

• Red lead was typically used as a corrosion inhibitor for use on iron.

• White lead was used more frequently in commercial applications (white lead was not intended for use on iron - its use would increase corrosion, especially on wrought iron).

Typical current uses for lead include:

• Sheet roofing.

• Decorative spandrels.

• Gutters, leader heads and downspouts.

• Cast decorative features and sculpture.

• Cupolas, spires and mullions.

• Sheathing for cables.

• Sheet lead partitions (good for noise reduction).

• Pads for vibrating machinery.

• Shielding for x-ray and nuclear radiation.

• To waterproof ironwork where the iron is fitted into stone.

Natural or inherent problems

Chemical corrosion

• Lead has good corrosion resistance to the following acids: Chromic acid, sulfuric acid, sulfurous acid, and phosphoric acid.

• Lead has poor corrosion resistance to: Alkalis like mortar and cement (evident as a reddish-brown oxide), carbon dioxide and organic acids like those found in wood (evident as a whitish carbonate coating).

• Lead also has poor corrosion resistance to the following acids: Hydrochloric acid, hydrofluoric acid, acetic (i.e. fumes from breweries), formic acid (i.e. from ants and other insects), and nitric acid.

• Lead is also susceptible to corrosion from tannic acid produced by oak, and acids from lichen on a roof that are washed over lead features such as flashing.

Galvanic (electrochemical) corrosion

• Usually not a problem; lead is usually protected by a coating that forms on the surface and insulates the metal.

Mechanical or physical deterioration

• Erosion and Abrasion: From dirt, sleet, hail and rain due to softness of metal.

• Fatigue: Caused by thermal expansion and contraction

• Buckling/Fatigue Cracking: May result from a high coefficient of thermal expansion.

• Creep: Caused by the slow flow of gravity (usually a problem on steeper roofs). Creep and fatigue are often found together. One may accelerate the other.