Dave,
The maximum recommended overhang by the
Marble Institute of America is 10" without a support bracket. On 18" no matter what stone, I would use corner posts and skirting boards much like a wood kitchen table between the legs and back to the cabinets. Tennon the skirting into the legs and opoxy to the stone at the top. I have even pegged the legs at the floor so noone can kick it out by accident.
You would be surprised how much weight people try to put on an overhang when sitting at the counter - the last thing you need is someone breaking off your overhang and it landing on their toes. Plus it dresses it up.
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 | | Marble | MARBLE: CHARACTERISTICS
This standard includes general information on the characteristics
and common uses of marble and identifies typical problems
associated with the material.
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. | |  |
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