>Ken Follett wrote of travertine:

>Being calcareous it is subject to the same acid rain &
>pollution factors that trouble limestone.

I'm skeptical and politically incorrect when it comes to acid rain. Yes, I
see it having an effect on marble, but I've yet to be convinced it is
damaging to limestone. I've done restoration work on carvings for the
Chicago Tribune Tower, built 1925. That building was subject to a
combination of forces- heavy pollution (Chicago used to mandate that public
buildings burn soft coal, to support the Illinois coal industry), direct
frozen winds off the lake, and all the other abuses of Chicago weather and
politics. Where the carvings were subject to direct rainfall, there was
mild erosion (less than 1/8"); where it was just subject to air pollution
or dripping water it was fine- crisp carvings, chisel marks still sharp.
The dirt had penetrated and stained the stone but hadn't damaged it.

I recall an article in a British stone industry trade journal in the early
80's, showing photos of carving on a London building. The 1925 photo was of
a several hundred year old piece in almost perfect shape. In the 1980 photo
it was in terrible condition. They blamed acid rain. A close inspection
made it evident the damage was due to human action- the rain of bombs which
fell on London during the war. The areas between the broken and fragmented
portions were still crisp, with chisel work evident, almost unchanged from
1925.

Water is powerful stuff- it made the grand canyon. It will wear anything away.
As I recall the Indiana Limestone Institute published a leaflet 12 or 15
years ago refuting the urban legends of the suceptability of Indiana
limestone to acid rain. Basic exposure to the natural elements, and
architectural design that ignores water, is a much factor than acid rain in
the erosion of limestone.

As one Chicago architect put it, (I don't recall if it was Jack Hartray or
Jim Nagel), "all through history architects built buildings to be outdoors,
in the wind, the rain the snow. In the 20th century they started building
buildings to be indoors"... putting up flat roofs in snow country,
eliminating cornices and window sills, etc.

OK, my flame shield is on, I'm ready to get blasted for my blasphemy.

____________________________________
Walter S. Arnold  * [log in to unmask]
Gallery:        http://www.stonecarver.com
Gargoyle postcards: http://www.stonecarver.com/postcard.html

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