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 New mortar joints should be of the same size and shape
(profile) as the original. Since historic methods of
pointing were quite different from modern ones,
duplicating them requires special skills and tools. For
large jobs, a skilled mason will be needed, and even an
experienced worker may need practice to match a particular
tooling effect. Be certain not to smear the new mortar
over the brick edges. Mortar should never be smeared
across the face of the stone except in rare cases when
repainting an original rubble finish.
(See Figures 4-6,
4-7, and 4-8.)
o
Terra Cotta. Modern terra cotta is a hard, ceramic
product that was developed in the late 19th century. It
was often used as wall cladding or as a substitute for
carved stone exterior decoration through the 1930s. It
may be found also as tile decorations on buildings
constructed in the 1920s and 1930s. Terra cotta comes in
a wide range of finishes, from matte to gloss; the colors
range from earth or stone hues to brilliant tones. Most
damage to terra cotta is caused by using the wrong
cleaning methods. Do not attempt to clean terra cotta
with hydrofluoric acid or any other strong acid. Cracking
or chipping of pieces is often caused by deteriorating
anchoring systems. New terra cotta is available made to
order to replace failed historic units.
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stucco . stucco , a waterproof type of exterior plaster, is
made of lime, cement, and aggregate. It was originally
intended to protect soft brick or wood construction and to
provide a decorative surface. In historic buildings of
the 18th and 19th centuries, it was often scored to
resemble stone. Like interior plaster, it is usually laid
on in three coats, the first a rough "brown" coat, the
last a relatively smooth finish. In 20th century
buildings, it frequently has a rough, aggregate finish,
used both functionally and decoratively. Stucco made with
a high portland cement content should not be used to
repair most historic buildings.
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Concrete. A mixture of cement and aggregate, concrete
came into general use in the late 19th century. It is a
dense, very strong, relatively impermeable material that
does not absorb water readily. Early concrete building
blocks (similar to today's CMUs, or cement masonry units)
were often molded to resemble stone. Poured concrete has
been around since the 19th century, and became widely used
when steel reinforcement became possible. Although
concrete does not absorb moisture easily, the metal
anchors or pins that hold it can get wet, rust, and expand
enough to damage the concrete, causing it to span,
exposing the reinforcing. Imitation stone made of cement
and crushed stone aggregate has been frequently used in
the 20th century.
4-8
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