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Historic Preservation - Technical Procedures

Spectitle:

Lead: Characteristics, Uses And Problems

Procedure code:

0501014S

Source:

Developed For Hspg (Nps - Sero)

Division:

Metals

Section:

Metal Materials

Last Modified:

02/24/2012

Details:

Lead: Characteristics, Uses And Problems



LEAD:  CHARACTERISTICS, USES AND PROBLEMS


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


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.


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 and downspouts, and conductor heads

-    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.  In roofing applications, lead was
    best used for flat or low pitch roofs and built-in gutters due
    to the heavy weight of the sheets.  Lead-coated copper was
    introduced in the 1930's.  This consisted of sheet copper
    dipped first in a lead-tin alloy, then dipped in pure lead and
    then rolled.

Typical current uses for lead include:

-    Sheet roofing

-    Decorative spandrels

-    Gutters, leader heads and downspouts

-    Cast decorative features and sculpture

-    Cupolas, spires and mullions

-    Lead-based paint:  Red lead is typically used as a corrosion
    inhibitor for use on iron.  White lead is used more frequently
    in commercial applications.  White lead is not intended for
    use on iron - its use will increase corrosion, especially on
    wrought iron.

-    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

1.   Chemical Corrosion:

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

    b.   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).  

    c.   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.  

    d.   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.

2.   Galvanic (Electrochemical) Corrosion:  Usually not a problem;
    lead is usually protected by a coating that forms on the
    surface and insulates the metal.

3.   Lead as component of paint; health hazards: Ingestion of lead
    dust for workers and young children can result in serious
    lint-term health problems.  See 09900-04-S for more
    information and precautions associated with lead-based paint.


VANDALISM OR HUMAN-INDUCED PROBLEMS

Mechanical or physical deterioration:

1.   Erosion and abrasion:  From dirt, sleet, hail and rain due to
    softness of metal.

2.   Fatigue:  Caused by thermal expansion and contraction

3.   Buckling or fatigue cracking:  May result from a high
    coefficient of thermal expansion.

4.   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.

                         END OF SECTION