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WEATHERING:
Limestone subjected to exterior exposures deteriorates due to weathering
or the natural effects of wind, rain, and thermal change. Limestone
is extremely durable. It does, however, absorb water and, since it is a
carbonate rock, it is highly reactive when
exposed to acids or even mildly acidic rain water, and it can suffer
substantial deterioration. The most common effect of weathering and
erosion is loss of precise detail.
Little can be done to restore edge detailing short of re-carving the stone
which is usually infeasible.
EROSION:
Erosion can be the result of general weathering described above, or it can
be a more localized phenomenon based upon handling or exposure. Wind
driven airborne abrasives may selectively wear away detailing on certain
elevations, based upon the direction of
prevailing winds. One of the few effective ways to address this
problem is by landscaping where plantings and/or grade can deflect the
wind. Such landscaping and/or grading may range from the simple and
inexpensive up to a major and expensive intervention.
It
would have to be consistent with appropriate policy for the management of
cultural landscapes. It may, however, be cost effective when
considering the extended life of the marble.
The symptoms of erosion can be as simple as the loss of edge sharpness as
described above, or it can be very localized, specific wear due to contact
with landscaping and mowing equipment.
Localized
damage due to contact by mowing or other maintenance equipment is
preventable. Where there is evidence of recurrent physical damage,
steps should be taken to protect the resource(s).
STAINING:
Discoloration of the limestone, whether general or localized, is staining.
Staining, may be the result of exposure to a variety of exterior
substances or to internal occlusions in the stone or structural elements.
Some of the most common types of staining and the causative agents are:
1. Oil/grease stains: These stains are usually the result of
vandalism or use. A variety of organic or inorganic oils may be
absorbed into the stone upon contact. The depth of
penetration will depend upon the viscosity of the oil/grease, temperature,
stone porosity, finish and dryness.
The appearance of grease/oil stains will usually consist of a darkening of
the stone at the area of contact. The edges of the staining will
generally be diffused, especially after an
extended period. There are standard techniques for removing oil and
grease stains.
For specific guidance on removing oil/grease stains from limestone, see
04455-10-R and 04455-11-R.
2. Dyes
and inks: The staining could be any color depending on the type and
source of the dye. This type of stain is likely to be extremely
localized around the area of contact. The liquid containing the
coloration may be absorbed into the stone and during the normal process of
evaporation, the coloring pigment is deposited in the stone. For specific
guidance on removing ink and dye stains from limestone, see 04455-18-R.
3. Organic
stains: Organic stains are caused by direct contact with decomposing
organic matter, such as leaves, bird or animal droppings, flowers, tea or
coffee. Regardless of the
source these stains tend to be a slight reddish-brown in color. They
also frequently disappear after the source has been removed. These
stains may be left to weather and bleach or oxidize out after the removal
of the organic source, however a residue may still remain on the
stone. For specific guidance on removing organic stains from limestone,
see 04455-14-R.
4. Metallic stains: Two major categories of metallic staining
occur, they tend to be based on either iron or copper. The source of
the staining may be internal structural components or elements. A
major source is the water wash, or run-off, from adjacent metallic
elements, especially bronze.
a. Rust stains: These stains are reddish-orange and are
caused by the oxidation (rusting) of iron. The source of iron
staining is usually the structural or connecting
components. These components are usually hidden and protected;
however, water penetration from bad joints or cracks can activate or
accelerate rusting. The
discoloration may be within the stone or it may be a deposit of rust on
the surface of the stone. Surface deposits of rust may sometimes be
removed by hand rubbing with a clean cloth. The examination of the
stain should include such rubbing to determine if it is only a surface
deposit. For specific guidance on removing rust stains from limestone, see
04400-06-R.
b. Copper stains: Stains from water run-off from bronze can
range in color from a light green to a dark brown. The staining
results from the dissolved copper salts (from
copper or bronze) which wash onto the stone, then oxidize. The pattern of
the staining is likely to be localized, streaked and in the path of the
run-off from the metallic source. For
specific guidance on removing copper stains from limestone, see
04400-07-R.
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