Veins appear as sharply defined colored
streaks, irregular interlacing markings, to
every gradation possible and sometimes with no sharp line of division.
Veins were formed after the formation of the
host stone as a result of geological
movement which stressed or shattered the surrounding stone causing fissures, cracks
and crevices. Over time super saturated mineral solutions circulated throw these voids
depositing minerals eventually filling them, resulting in veining variations from hard to soft.
Calcite and quartz veins being the most common.
In some marbles, veining is a result of the
stone being subjected to long periods of
continuous stress, causing stress lines. During this
metamorphic process a recrystallization
of various accessory minerals generally occurs along these stress lines producing stress
veining, which are often wavy and harder than the surrounding marble.
Soft-veins are more susceptible to problems than hard-veins. When a soft-veined
stone is used in high traffic areas these veins have a tendency to be "chewed-out".
In wet, moist areas two types of problems can result. In
a area with repeated wet/dry
cycles (showers) or wet/damp humid environments some of the veining minerals may
be dissolved, causing a
lose of veining minerals, leaving pits or recessed veins. If the
veins contain carbonaceous matter (organic
matter) or moisture sensitive minerals
(iron compounds) these may cause blotches, streaks or color variations to occur.
Some marbles that have histories of these kinds of
problems are Creme Marfils, Perlino
Rosata, Breccias and some Perlatos, Rosa Verona, Filetto Rosso, Negro Marquina and
the Serpentines (green marbles).
The more veining and color variation in a ornamental marble
the more brittle and
susceptible to breakage even after installation.
Marble, limestone's
and some varieties of slate have veins. Commercial granite-type stones
generally do not contain veins.
The variety, depth, richness and play of
color is the most obvious feature of stone. This
kaleidoscope of color is the result of several different processes.
With some minerals their color is a
fundamental characteristic directly related to their
chemical composition, they are said to have inherent coloration. The majority of
common rock-forming minerals calcite, calcium and magnesium carbonate (dolomite),
quartz, and feldspars are inherently
colorless, that is,
white or transparent in their pure
state. When these minerals exhibit coloration they are said to posses exotic coloration.
This results when trace amounts of some pigmenting
elements (mineral
solutions and
gases or organic matter) are absorb during different stages of the mineral or the over all
rock formation.
During the metamorphic process when
limestone is transforming into marble, the rock
mass becomes viscous, like soft plastic, hydrothermal super saturated mineral solutions
(water and gases) under great pressure can pass through the mass causing all varieties
of exotic coloration to occur...swirls, clouds, mottling, streaks, and bands.
The reflection of light from the mineral surface also influences
the play of colors in
stone as well. Two unique affects particular to granite-type stones is a Schiller and
Labradorescence affect.
Schiller...is a peculiar play of light that gives the stone a metallic like luster.
Labradorescence...is the play of light resulting in a series of vivid iridescent colors,
usually a brilliant blue or green predominating. Some have referred
to it as the..."Butterfly wing" effect.
Some minerals with exotic coloration's respond to different color wavelengths in
light.
The light the stone receives in its setting can be an important factor in highlighting
or minimizing these colors and contrasts.
Sunlight is a balanced full-spectrum light, however
sometimes it can be so bright that
the colors of the stone can be "washed-out." Incandescent lighting
generally gives off
warm light at the yellow-red end of the light spectrum, halogen lighting gives a much
more controlled, brighter and whiter light producing a more brilliant color rendition at
the
cooler end of the light spectrum green-blue, and fluorescent lighting can provide color
wavelengths in either the full visible light spectrum or in just portions of it.
These minerals have a definite chemical composition and optical qualities, some of
which can be altered, affecting the color of the stone under
certain situations, gradually
or in a relative short period of time. These alterations take
place generally in a few ways...naturally...cleaning
chemicals.
Naturally : As a result of the chemical weathering processes previously
mentioned.
The acids involved in these processes will have a fading affect.The
ultraviolet rays from the sun will fad stones that contain organic
matter as part the coloring agent. Black marble, colored coralstones,
travertine's and limestone are alter this way.
Cleaning Chemicals : Most stones are sensitive to acidic and alkaline
cleaning solutions. Acidic
cleaning solutions
can have a
fading affect. Acids are used to remove rust stains. Those
stones whose colors are based on iron compounds can be
affected.
Optical qualities of the minerals can also be altered
immediately or gradually by etching their exposed
mineral surfaces.
Alkaline cleaning
solutions in the pH
range of 9-12 used
frequently can also have a gradual fading effect If not properly
rinsed the alkaline salts can buildup in the pores and produce a
hazy dulling affect.
Some chemical
alterations take place in a high pH setting and
will produce permanent changes over time.
NOTE : If aggressive cleaning agents need to be used for restoration cleaning,
preliminary testing is mandatory to determine if there will be any
potential
chemical reactions.
The acid by-products of air pollutants will also fad the colors of stone.
