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 o Stone. Although we think of stone as a hard material,
many building stones are quite soft. For instance,
limestone, sandstone (especially the type known as
brownstone), and marble can be dissolved by plain water.
They are all subject to pitting, staining, and erosion.
The sulfuric acid in acid rain can dissolve the surface of
limestone buildings. Structural stress can cause
cracking, or moisture from the freeze-thaw cycle may cause
spalling. The hard, protective crust on stone formed when
it was new can be destroyed by harsh cleaning methods.
Stone has a natural bedding plane, or direction in
which its layers were formed in the earth, somewhat like
the grain in wood. Often its structural strength as well
as its beauty depends partly on its being laid with this
plane perpendicular, rather than parallel, to the face of
the building. Otherwise, it can delaminate, or separate
along the bedding planes, and exfoliate (the face of the
stone peels off).
(See Figure 4-2.) Deicing chemicals
can destroy masonry steps and walks and can leach into
nearby masonry walls, causing chemical damage. Unlike
brick, damaged stone often can be repaired using modern
epoxies and consolidants. However, any repair more
extensive than reattaching a small section must be done by
masonry experts.
(See Figures 4-3 and 4-4.)
o
Mortar. Mortar is the bedding material that separates the
building units in a masonry structure. The mortar used in
most 18th and 19th century buildings was made of lime and
sand, or of lime, sand, and a little cement, mixed with
water.
(See Figure 4-5.) Because it is elastic and soft
in comparison with the masonry, mortar allows the
structure to shift and "give" slightly under stress; it
also cushions the individual masonry units from too much
pressure. Early mortar, made mostly of lime, is very weak
and porous compared to modern cement. However, its
weakness is an advantage from the standpoint of old
masonry, which would be overstressed if a hard mortar like
portland cement was used. The absorbent quality of lime
mortar can also be helpful, because it allows excess
moisture to transpire, or "breathe" out of the building.
On the other hand, its absorbency becomes troublesome when
ground water is pulled up into the walls through capillary
action, creating a condition known as "rising damp."
Mortar mixes containing a high ratio of portland
cement, an extremely hard mortar developed in the late
19th century, should be used only with compatible hard
modern masonry. Do not use mortars high in portland
cement on historic buildings. When soft brick is
repointed with a hard cement, building stresses are
transferred to the brick rather than to the mortar.
Furthermore, excess moisture attempting to leave the
building gets trapped within the brick rather than
escaping through the mortar joints. Eventually, pressure
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