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845 McFarland Rd., Alpharetta, GA 30004
phone 770-475-0004
fax 770-817-2898 |
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Stone
Education
GRANITE
Granite offers the designer the opportunity to blend the marvelous
variations found in nature, the basic rhythm of the earth exhibiting
a wide range of amazing colors, with a practical need for high
performance in areas that are subject to high use. Granite is
the only stone that is acceptable for use as a kitchen countertop.
Since granite is formed in nature there are no two applications
that are the same, each installation is unique. Every individual
slab you inspect will exhibit differences even if the slabs were
cut from the same block. Even though the granite is given names,
the names are just an indicator of the color, texture and veining
, never an absolute. It is these natural differences that make
granite unique, valuable and highly desirable. The poet Gerard
Manley Hopkins in his poem Pied Beauty captured the essence of
granite in the line “Glory be to God for dappled things.” Granite
is indeed a rare, unique gift of nature.
Granite is an intrusive
(within the Earth’s crust) igneous
rock; the formation occurs by magma flowing into pockets deep beneath
the Earth’s surface and over millions of years cools and
crystallizes. In some cases the stone could also go through a metamorphic
change (exposure to high pressure and temperatures of a long period
of time) resulting in a transformation of the crystalline structure.
There are basically 25 minerals that can be found in granite each
contributing to the color and physical make up of the stone. The
four mineral families most commonly found are quartz, feldspar,
mica and hornblende.
The various crystalline forms of the minerals
present in the granite are responsible for generating the wide
range of colors. White, grey, green, pink and red are common. Light
colored granite is representative of the average composition of
the Earth’s
crust. Darker granites are likely more influenced by oceanic composition
stone. Dark granular stones, e.g. anthrosite, basalt, diabase,
diorite and grabbo are commercially classified as “black
granite.”
Granite is found all around the world. The more
colorful and therefore more in demand for decorative applications
are found in Africa, Brazil, China, Italy and India. Other exporters
of granite are Argentina, Egypt, Finland, Iran, Norway, Sardinia,
Spain and the Ukraine. Granite is found in abundance in the United
States but the colors are more compatible with monument and exterior
works.
Granite is mined at huge quarries. The basic process is to
use dynamite charges to dislodge large blocks from the mountain.
The blocks are roughly 6’X6’X10’ and weigh up to
nearly 4 tons. The block is moved on to a large gang saw where
it is cut into slices or slabs generally 2 cm or 3 cm in thickness.
Other thicknesses are available but 2
cm and 3 cm represent the “standard” thicknesses
for countertops. The slabs are then sent through a seven stage
polishing procedure to achieve the desired high gloss surface finish.
High pressure is used in each of the seven stages of polishing.
The polishing process will often dislodge softer minerals that
are in the granite surface. The extent to which this pitting occurs
is dependent upon the physical makeup of the granite. Granite minerals
have a Moh’s scale hardness rating of 6 to 7 (Diamond is
the hardest with a Moh’s hardness of 10). Biotite, a member
of the mica family and frequently found in granite, has a Moh’s
hardness of 2.5 to 4.5 and consequently does not polish well and
will appear as small rough spots on the surface. The hardness of
the biotite makes little difference in most applications, as the
overwhelmingly dominant feldspar and quartz minerals are much harder.
The biotite may “pluck” slightly when polishing but
not in general use.
Another naturally occurring phenomenon are
fissures. Fissures are oftenconfused with cracks as they appear
on the surface of the granite. Fissures were created when the stone
was first formed. There is a difference between a crack and a fissure.
Fissures are naturally occurring and represent no issue in the
performance of the stone. The only fissures that should appear
on a grade A stone are found just on the surface. A mechanical
force due to fabrication, installation or other circumstances at
the jobsite causes a crack. The cracks caused by a mechanical force
generally transmit entirely through the slab so it would appear
on the top, edges and bottom of the slab.
The most frequent cause
of cracks in granite countertops is a tradesman who uses the granite
countertop as a step stool. There are definite weak areas for granite
countertops. These areas are around the sink and cook top cutouts
and any overhang beyond the cabinet base. A high quality fabricator
will place steel reinforcing rods around the cutouts to add strength
to these vulnerable areas. Cracks will be visible on the top and
bottom surfaces of the stone. A skilled granite craftsman can repair
some cracks in the field.
Once the slabs have been polished the
surfaces are then sealed. The sealant adds an oliophobic and hydrophobic
barrier on the surface of the granite that causes oils and water
to bead. At this point the slabs are ready for shipment.
A stone distributor
who warehouses the stones and readies them for final shipment to
a fabricator/installer generally handles shipment from the quarry.
The stone distributor and fabricator work together to assure that
the end customer has the proper selection of stones available to
meet their needs. Frequently a fabricator/installer will make arrangements
for a customer to view slabs at the distributor’s
warehouse since the selection is usually greater. Premier Surfaces
has a working relationship with all of the major distributors in
the greater Atlanta area.
See Stone Suppliers.
Some technical facts about granite:
- Weight: between 16.7 and 19.8
pounds per square foot at 1.25 inches thick (3cm). It is possible
for some finished pieces to weigh in excess of 800 lbs. When
the weight is too high we often add an additional crew to the
installation to assist with putting the stone in place.
- Slab
dimensions: Generally up to 78 inches in height and 110 inches
in width. It is often necessary to seam pieces since the
dimensions of the slab limit the length of an individual piece.
- Compressive
Strength: 4,700 to 60,000 psi (recommended minimum 19,000)
- Flexural
Strength 700 to 5,500 psi (recommended minimum 1,200)
- Modulus
of Elasticity (in millions) 2.0 to 10.0 psi
- Density lb/ft 3
160 to 190
- Absorption (ASTM C97) 0.02% to 0.40%
Other stones sold as granite
having a similar physical makeup: anorthosite, basalt, diabase,
diorite, gabbro and gneiss.
There are many questions regarding the potential
for bacteria growth on granite since it is a porous material.
Bacteria requires several things in order to thrive and grow:
oxygen, water, sunlight, nutrients, and a substrate to form
on. The minerals in granite are toxic to bacteria. As a result,
there is no habitable environment for the bacteria to live
and grow on a granite surface.
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MARBLE
Marble is geologically classified as a metamorphic rock. Marble can also be formed
through recrystallization. Limestone, which is a sedimentary stone, recrystallizes
under pressure, heat and over long periods of time to form marble. The metamorphic
marbles that are formed under higher pressure are denser and more fracture free
than the recrystallized marbles. Marble consists manly of recrystallized calcite
and/or dolomite. Calcilite has a wide range of colors from black to white and
from transparent to clear. Other colors are caused by minor amounts of substances
during the formation of the stone. Iron oxides make the pinks, yellows, browns,
and reds. Most grays, blue grays, and blacks are of bituminous origin. Mica,
chlorites and silicates cause greens.
Marble comes in four grades A, B, C, and D. The grade A and B are the only ones
Premier Surfaces would consider using. Grade A is a completely metamorphosed
limestone base basically having no geological voids or flaws. Grade B is similar
to A but may contain voids or flaws that are easily filled with a polyester or
acrylic filler. The other two grades have considerable more voids and cannot
be brought up to the quality level Premier Surfaces wants to offer to the customer.
The mineralogical makeup of marble makes it vulnerable to abrasion and chemical
attack. Ordinary kitchen utensils can scratch the surface of marble and acid
containing juices such as vinegar, tomato, lemon or other fruit juices can stain
the surface of marble primarily due to its calcium carbonate base. . Care must
be exercised to protect marble surfaces from damage. Premier Surfaces does not
recommend Marble for use as a kitchen countertop. Marble is best used in bathroom
countertops or for display pieces.
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Limestone
Limestone is a sedimentary stone with at least 50% by weight calcite or calcium
carbonate content. Limestone is
the cemented or recrystalized accumulation of
calcareous organic debris deposited on the sea bottom, then compacted and later
cemented. Substances included in minor amounts during formation cause the color,
veining, clouds, mottlings, and shadings in limestone. These include iron-bearing
minerals, clay and organic material thought to be residual from the soft parts
of tiny marine animals. Iron oxides make the reds, pinks, yellows and browns.
Grays, blacks and gray blues are of bituminous origin.
Limestone contains a number of distinguishable natural characteristics, including
calcite streaks or spots, fossils or shell formations, pit holes, reedy formations,
open texture streaks, honeycomb formations, iron spots and grain-formation changes.
Limestone has half the compressive strength of granite. It is not a recommended
surface for kitchen applications. Surface texture will contain many small gaps
and fissures.
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Travertine
Travertine is a varietal name for a kind of limestone formed under special conditions
of deposition. Travertine is a chemical sedimentary stone and is deposited as
precipitates in land environments. Travertine has striking textural character.
It is very porous, often cavernous with a diverse palette of light hues and soft
earth tomes. Travertine is very soft in comparison to other dimensional stones
The surface of travertines must be filled with an epoxy or polyester resin. Travertine
is not recommended for kitchen countertops. It is best used for bathroom countertops
or display pieces.
Technical information for the above was taken from the Dimension Stone Design
Manual, a publication of the Marble Institute of America.
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