From: Gerry Z Hill <gerryinga(a)juno.com>
From Malinda Jones on the Parham list mthiessE(a)swbell.net
CondorPnsa(a)aol.com wrote:
Good evening all; I am Pete Dohms, husband of Benigne who is your
regular correspondent at this e-mail address. I am a professional
geologist and
this topic has landed squarely in an area of which I have some degree of
knowledge.
From direct observation and over 30 years of professional experience,
I
can assure you all that there is some degree of truth in what Bob has
stated, but
there are limitations. Allow me to explain.
There are several types of "stone" that are typically used for creating
tombstones. In the early days of America (particularly in New England),
slate was used. Slate is a dark colored rock that breaks naturally along
a very strong preferential plane (cleavage). It is also fairly soft and
fairly easy to carve. It is also highly resistant to chemical
weathering (which is the erosional phenomena Bob was alluding to).
Slate tombstones that are hundreds of years old can usually be easily
read.
In the past 100 years, though, most American tombstones have been
created from two other types of rock. Those are marble and "granite."
In a
moment I'll explain why I put "granite" in quotation marks. First to
marble.
Marble is a crystallized form of what began as limestone, a chemical
precipitate formed as layers in the bottom of a shallow sea. After deep
burial, and the application of heat and pressure, the limestone grains
(calcium carbonate) flowed together into the larger and more
crystallized
forms seen in marble. Marble is, like slate, relatively soft and quite
easy
to carve. It is fairly easy to saw and grind, which is good since it
doesn't
cleave like slate. If you've ever been to a U.S. Military cemetery,
virtually all tombstones you see in the long precise rows are marble.
The effect is quite striking.
Marble, unfortunately, does not resist chemical weathering at all well.
Many tombstones from 100 years ago have become illegible, hence the
"tips"
that pass around on "how to raise the inscription." In the case of
shaving
cream, in the absence of affirmative knowledge to the contrary, I
imagine the
surfactants (wetting agents) in the shaving cream penetrate the
microscopic
discontinuities along crystal grain boundaries, making those areas
easier for > rainwater (which in the NE USA is quite acidic) to
penetrate. Once
penetrated, the acids go to work on the crystal grains, enlarging the
discontinuities and exposing new marble (calcium carbonate) to chemical
attack. As the process continues, tiny flecks of marble spall off the
stone,
leaving a white, sugary, irregular surface behind. In cold climates, ice
also penetrates into the stone and the freeze-thaw cycle adds physical
weathering to the chemical attack (recall that water expands upon
freezing, which pushes the cracks open even further).
What I called "granite" is, in reality, a trade term for a wide variety
of igneous and crystalline metamorphic rocks that contain high
concentrations of silica (silicon dioxide, or quartz). The other major
components include various feldspars, dark minerals (pyroxenes,
hornblendes, micas), and other rock-forming minerals. As a class,
"granite" is difficult to work into tombstones. It is both hard and
tough. This different mineral composition and more challenging geologic
history, however, provides it with an excellent ability to resist
chemical weathering. A year ago, Benigne & I found the grave of her
grandmother's uncle, who was buried in Northeast
Pennsylvania in 1914. Rain in this area is as acidic as any location in
the country and
more acidic than most. The fine granite tombstone that was described in
his
Will looked as though it had been erected the week prior to our visit;
it was
untouched by the 85 years of weather it had experienced.
Having said all that, please understand that the forgoing are general
statements. There will be specific locations where 200 year old marble
tombstones are in remarkable states of preservation, and there will be
cases where 200 year old slate tombstones have dissolved to low piles of
shaley rubble, but those are exceptions to the general conditions
described above.
This has gone on long enough, most of you are probably bored by now, and
i f anybody has a comment or question, I will be happy to respond "off
web"
(or on-web if preferred). Thanks for bearing with this.
Pete Dohms, Geologist
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