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be removed at a later date without leaving an impaired historic
4.2 DETERIORATION OF MATERIALS.  Even well-maintained buildings
are constantly in the process of changing as the materials they
are made of deteriorate.  To understand why building materials
fail and how they can be made to last longer, it is necessary to
know something about their makeup.  The following is a brief
summary of the major building materials and some of their most
frequent problems.
4.2.1 Masonry.  Masonry is a term that applies to stone,
brick, terra cotta, adobe, concrete, stucco, and mortar.  It is
one of the most durable of all building materials.  A well-built
and well-cared for masonry building can last for centuries, but
it can be quickly ruined by the wrong maintenance or repair
techniques or harsh cleaning methods, such as sandblasting.
Brick.  Brick is a mixture of clay and sand that has been
shaped in molds, partially dried, and then baked at high
temperatures to produce a hard surface.  Generally, the
older the brick, the softer and less dense it is and the
more irregular the color and surface texture are.
Hand-molded brick, used in buildings constructed before
about 1830, is very soft.  Pressed brick, used later in
the 19th century, was mechanically formed under high
pressure to make a hard, dense product with a smooth,
uniform surface.  Around the turn of the 20th century,
pattern bricks in a variety of surface textures and in
colors other than red or brown (usually yellow and cream)
came into use.  Especially in the earliest periods, bricks
were often rubbed or shaped to achieve a decorative
effect.  Variations in color and texture produced by
different natural materials in the clay and varied firing
temperatures also produced decorative effects.  Very soft,
low-grade brick (salmon brick) may be found as filler
behind face brick in buildings of all periods.  This
filler brick is especially subject to crumbling when
exposed to water.  The quality of brick, its size, color,
and texture, and the various brick bonding patterns found
in historic buildings often suggest when they were built
and how they were used.
(See Figure 4-l.) Brick surfaces
may span or crumble, or the brick may crack.  Harsh
cleaning may remove the "crust" that is essential to the
weathering ability of the brick.  Repairing damaged brick
is almost never feasible.  A major advantage of masonry
construction in general, and of brick buildings in
particular, is that when building units decay or are
damaged, they can be easily cut out and replaced.
Sometimes bricks from a less visible part of the building
can be swapped for damaged ones, or matching salvaged
bricks can be found.  Since bricks of almost every type
are still being made, there is no reason to settle for
less than an exact replica when replacements are needed.


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