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Historic Preservation - Technical Procedures

Spectitle:

Marble: Characteristics, Uses And Problems

Procedure code:

0445501S

Source:

Outdoor Sculpture Manual - Center For Public Buildings

Division:

Masonry

Section:

Marble

Last Modified:

02/24/2012

Details:

Marble: Characteristics, Uses And Problems



MARBLE:  CHARACTERISTICS, USES AND PROBLEMS


This standard includes general information on the characteristics
and common uses of marble and identifies typical problems
associated with the material.  See 04400-01-S for guidance on
inspecting stone masonry failures.


INTRODUCTION

Marble is an extremely hard, metamorphic stone composed of calcite
(CaCO3).  It is formed as a result of the recrystallization of
limestone under the intense pressure and heat of geologic
processes.  The effect of this process is the creation of a stone
with a very tight crystalline structure and small but definite
porosity.  Because of its structure, marble can take a very high
polish and is a very popular decorative stone for architectural and
sculptural uses.  The limited porosity of marble, especially
polished marble, makes it less vulnerable to the leaching effects
of water.  Calcium carbonate, however, of which marble is composed,
is highly susceptible to attack by acidic agents.  Marble is
readily dissolved by acids, even very dilute acids, however the
actual results of acidic exposure will vary with the nature of the
acid.  Chlorides, nitrates, sulfates and other chemical compounds
react differently with marble and produce various by-products,
which have a wide range  of solubility and impact on the durability
of marble.  For this reason, it is always important to determine
the exact type of pollutants causing marble deterioration.

Marble itself can be of two types, one composed of calcite and the
other of dolomite.  Dolomitic marble is much more resistant to acid
attack than calcite marble.  The color of marble ranges from the
brilliant white of calcite to black, including blue-gray, red,
yellow and green, depending upon the mineral composition.


TYPICAL USES

Marble has many decorative and structural uses. It is used for
outdoor sculpture as well as for sculpture bases; in architecture
it is used in exterior walls and veneers, flooring, decorative
features, stairways and walkways.  The way in which the stone is
used may be a factor in limiting or controlling the severity of
exposure.  The use or function of the marble may also affect the
feasibility of applying certain treatments, but type of use is not
the primary factor in the major types of deterioration and damage
to which marble is susceptible.


PROBLEMS AND DETERIORATION

The natural forces and agents of weather may have a degrading
effect on the appearance and structural soundness of marble.  These
agents include rain, snow, temperature, wind and atmospheric
pollutants.  Weathering agents almost never work singly or in
isolation, they always act in combination with one another or with
other agents of deterioration.

Rain water, especially in combination with atmospheric gases, can
result in dissolution of the marble, creating higher levels of salt
movement within the micro-structure.  Temperature can effect rates
of deterioration and (in larger stones) movement of the pieces, as
well as patterns of salt migration within the stone.  Higher
temperatures often increase the rate of chemical changes; low
temperatures can create the risk of "freeze-thaw" problems, and
rapid changes of temperature can produce stresses in the material
due to differential expansion.  Most of the natural or inherent
problems which can occur with marble require some degree of
moisture to occur, however other problems such as wind erosion and
vandalism may occur independently.


NATURAL OR INHERENT MARBLE PROBLEMS

WEATHERING:

Marble subjected to exterior exposures deteriorates due to
weathering or the natural effects of wind, rain, and thermal
change.  Marble is extremely durable and because of its limited
porosity does not absorb large amounts of water.  It does, however,
absorb some water and, since it is highly reactive when exposed to
acids or even mildly acidic rain water, it can suffer substantial
deterioration.  In short, while the porosity of marble is low, the
shape of the pores (elliptical) allows greater dissolution than in
typical round pores, and this fact coupled with marble's inherent
solubility in acids can result in two major problems:

1.   Loss of polish

2.   Loss of detailing

The most common symptoms of weathering are a loss of the highly
polished surface (where it exists) and loss of crisp edge details
in decorative carved areas of the stone.

Little can be done to restore edge detailing short of re-carving
the stone which is usually infeasible.  The gloss or polish can be
maintained and, to some degree, restored on the surface of marble
by using marble polishing powder and soft buffing pads.  Obviously
the level of effort or labor required to repolish stone will depend
upon the degree of deterioration of the surface, or loss of polish.
New pieces, restored pieces or well maintained surfaces can be
retained in a highly polished state with much less effort than
surfaces which have been exposed to weathering for long periods of
time with little or no maintenance.  For this reason, polished
marble should receive regular preventive maintenance by polishing.

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 such as dirt, grit, and
other "particles" may selectively wear away detailing.  The effects
will depend largely upon the direction of prevailing winds.  One of
the few effective ways to address this problem is by landscaping
where plantings and/or landscape grade can deflect the wind.  Such
landscaping and/or grading may range from the simple and
inexpensive to a major and expensive intervention.  It would have
to be consistent with the policy for the management of cultural
landscapes.  It may, however, be cost effective when considering
the extended life of the building.

The symptoms of erosion can be as simple as the general loss of
polish and 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
stone.

STAINING:

Discoloration of the marble, 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 handling.  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 and may often be accomplished by
    maintenance personnel.

    For specific guidance on removing oil/grease stains from
    marble, see 04455-10-R and 04455-11-R.

2.   Dyes and inks:  The staining can 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 within the stone.

    For specific guidance on removing ink and dye stains from
    marble, 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.  Organic 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 marble,
    see 04455-14-R.

4.   Metallic stains:  Two major categories of metallic staining
    are based on either iron or copper. The source of the staining
    may be internal structural components or features. A common
    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
         marble, 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 down 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.  In some cases, especially plaques
         on stone, where biological growth or mildew may form on
         shaded elevations, the copper salts in the water running
         off the bronze acts as a fungicide, killing the growth
         and making areas directly below the metal look cleaner
         than surrounding areas.

         For specific guidance on removing copper stains from
         marble, see 04400-07-R.

5.   General dirt, soot and pollution:  Marble can be discolored
    generally or locally by atmospheric dirt, grime and other
    airborne particulates which adhere to the material.  The
    visual appearance is usually a dulling or graying effect which
    mutes or obscures the original color and gloss. Effects of
    dirt are usually intensified in the protected areas, where the
    rinsing effect of rain water is diminished.

    Dirt can be a complex composition of finely divided solids
    held together by organic material including soot, siliceous
    dust, and other airborne material.  It may include particles
    of metals, glass, ceramics, metal oxides and minerals.

    Dirt is usually held to the surface by a combination of
    absorption and static attraction.  Dirt can become
    incorporated into "crusts" which can develop during the
    deterioration of some stones.  Biological agents can also
    collect on dirty surfaces and the same water used for cleaning
    can stimulate algal growth.  Algae, lichens and moss can
    produce acid by-products which can damage acid sensitive
    stone.  In addition, some water proof and water repellent
    coatings used on stone actually increase the static attraction
    of the surface and result in the stone getting dirty faster.

    For specific guidance on removing dirt/pollution from marble,
    see 04400-01-P, 04400-02-P, 04400-03-P.

CRUMBLING:

This condition is due to a certain brittleness or tendency of the
stone to break up or dissolve.  It may be caused by an inherent
weakness in the stone or gradual breakdown of the binder or
crystalline structure, or it may be the result of external factors
affecting the strength and durability of the marble.

This condition may be caused by the use of de-icing salts, or any
other source of salt migration, such as that which can occur when
rising damp is present.  There is currently little which can be
done to repair the damage once this condition has developed,
however the early detection of potential problems and elimination
of sources of salts is critical to arresting the process.  When
this condition is severe and obviously caused by the heavy or
inappropriate use of de-icing salts, it is sometimes called "Salt
Fretting".  Regular preservation maintenance may eliminate the
causes promoting crumbling, however, once the condition has
occurred, its correction or repair is beyond the level of a
maintenance procedure.  The Regional Historic Preservation Officer
(RHPO) should be contacted for assistance.

CHIPPING:

The separation of small pieces or larger fragments from a masonry
unit, frequently at the corners, edges or mortar joints is known as
chipping.  These fractures are generally caused by deterioration
and repointing, especially due to the use of too hard a pointing
mortar, or by accident or vandalism.

Repairs include detachment repairs, patching and splicing.  Repair
of chipped stone requires a skilled mason and is not a maintenance
procedure.  If chipping is due to occasional impact from mowing or
other landscape maintenance, steps should be taken to prevent
future damage.

For specific guidance on repairing chips in marble, see 04455-03-R.

CRACKING:

This condition is manifested by the appearance of narrow fissures
ranging from less than 1/16 to 1/2 inch wide or more in the stone.
It results from a variety of causes, such as structural overloading
due to settlement, the use of too hard a mortar mix or a flaw in
the material.  Minor cracking may be no problem, in and of itself,
but it can be an indication of structural problems and the cracks
can be a source of entry of water into the interior of the stone,
promoting salt migration.   Marble is a relatively homogenous
material since it is crystallized under intense heat and pressure.
It has, however, evolved from sedimentary stones and may have
structural/physical planes of different density and strength.
Cracking, which allows water or salts to enter the stone, increases
the possibility of failure along the planes with subsequent
spalling.  Repairs include patching and replacement.

For specific guidance on repairing cracks in marble, see 04455-03-
R.

DETACHMENT:

This is not a failure of the marble, but rather a failure of the
construction system, i.e. the connectors and/or joints.  The
definition implies that the failed component survives intact and
may be re-installed using appropriate mechanical techniques.
Detachment cannot occur with a monolithic piece.  Visually,
detached pieces may be separated from surrounding ones.

The failure of anchors or metal connectors which lead to detachment
may be caused and/or accelerated by the penetration of water into
the structure behind the stone, causing rust and corrosion.
Adequate pointing and caulking will prevent leakage and penetration
of water into the system.

For specific guidance on resecurring detached marble, see 04455-21-
R.

EFFLORESCENCE:

The appearance of a whitish deposit locally or uniformly over the
surface may be efflorescence, the surface deposition of soluble
salts.  There are numerous sources for the soluble salts which
create the hazy appearance; salts can come from mortar, improper
cleaning agents, rising damp, de-icing salts, chemical landscaping
treatments and air pollution.  Whitish deposits of salts may be
much less obvious on white marble than on darker stones and brick.
On polished surfaces, deposits will reduce the gloss and present a
diffuse or hazy area.

Efflorescence can be a salt residue resulting from improper
chemical cleaning, i.e. too strong a chemical cleaner or inadequate
rinsing.  It can also be an indication of water problems.  Salt
migration and/or sub-florescence and efflorescence should be
considered a symptom which should be investigated to identify the
source of the soluble salts and/or the source of moisture.
Corrective action should then be taken to eliminate the source of
the problem once it is identified.

Some efflorescence may occur naturally with new stones, mortar and
installation materials.  Normally, this efflorescence will be
removed by natural rain and weathering processes and/or by regular
washing.  The new or continued appearance of efflorescence is a
stronger indicator of problems like rising damp, inappropriate
cleaning methods, all of which should be referred to the RHPO.

For specific guidance on removing efflorescence from marble, see
04455-25-R.

EROSION:

Erosion is the wearing away of the material surface by the natural
action of wind, windblown particles and water.  It can occur with
marble as well as any exposed materials. Inspections should include
examination  for any apparent loss of detail and edge sharpness
which could be due to erosion.

Erosion may be less of a problem on rock-faced or quarry-faced
marble, but may be a more serious problem on stone with more
precise detail.  Little can be done to correct this problem once it
occurs, other than to protect the surface from further exposure to
stop or retard the process.

FLAKING:

This is an early stage of peeling, exfoliation, delamination or
spalling evidenced by the detachment of small flat thin pieces of
the outer layers of stone from a larger piece of stone.  Flaking is
usually caused by capillary moisture or freeze-thaw cycles which
occur within the masonry.  

The problem can also occur due to sub-florescence, so that if
flaking occurs, the area should be examined to determine if salt
crystallization is occurring in the flaked areas.

PEELING:

Peeling is flaking away of the surface from the substrate in strips
or layers.  It may result from the improper application of masonry
coatings which result in failure of the coating and/or stone
surface.  It may also result from a defect in the stone, or from
weathering.

Encrustation of the surface caused by chemical reactions with
environmental elements may also peel or flake along the bedding
plane.

RISING DAMP:

Rising damp is the suction of ground water into the base of masonry
through capillary action.  Moisture is drawn up into the marble and
may rise and fall due to conditions of temperature; humidity; site
grading; absence or failure of damp courses, and/or treatments to
the masonry surfaces which affect evaporation.

During active wet periods, rising damp may be visible as a
darkening of the marble along the base at ground level.  Due to the
continuous changing of the moisture level due to varying exposure
conditions, staining or efflorescence may be visible at a range of
several feet up from the ground.  Continuation of the problem can
lead to more severe problems of flaking, peeling and/or spalling,
but the correction of the problem requires the elimination of the
source of water or the interruption of its path into the stone by
physical or chemical damp-proofing.

SPALLING:

Spalling is the separation and breaking away of pieces of stone due
to sub-florescence, freeze-thaw, improper repointing with too hard
a mortar mix or portland cement, or structural overloading of the
stone.

Spalling is less frequent with marble than with sedimentary stones
which are also less dense.  Marble is dense enough to resist
internal forces which would cause spalling in other natural stones
or fabricated masonry.

For specific guidance on repairing spalling marble, see 04400-03-R
and 04455-03-R.

SUB-FLORESCENCE:

This is a potentially harmful internal accumulation of soluble
salts deposited under or just beneath the masonry surface as
moisture in the wall evaporates.

The build-up of salts and their crystallization can create
substantial pressures within the marble, causing pieces to break
off along the planes of deposition.  Efflorescence at the surface
is an indication that sub-florescence is possible.  Techniques for
mitigating the problem include poulticing, removal of identified
salt sources, elimination of moisture in the stone and damp-
proofing.

SUGARING:

This is a gradual disintegration of the surface of the marble,
possibly caused by salt migration and exposure to moisture.
Excessive moisture may have the effect of dissolving the binder.

Carbonate stones, especially fine grained marbles, are particularly
susceptible to this form of deterioration.  The surface takes on a
rough granular, crystalline or sometimes powdery appearance.

                         END OF SECTION