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Subject: Soda blasting

Soda blasting

From: John Horton <jhorton>
Date: Monday, March 26, 2001
Gerron Hite's concerns about soda blasting of gravestones are well
voiced. No matter what the aggregate is, it still is an abrasive
cleaning method that should never be considered for gravestones.
There is simply too much risk of damage even by trained operators.
In A Graveyard Preservation Primer, Lynette Strangstad recommends
for limestone and marble a gentle wet cleaning using water
(preferably by itself), then a non-ionic detergent and soft brushes.
Vulpex soap or household ammonia may also be considered for some
stains. Calcium hypochlorite should only be used for removal of
biological growth. (Reference: Strangstad, Lynette, A Graveyard
Preservation Primer, American Association for State and Local
History, Nashville, 1988, p. 63).

I haven't been able to find much specific information on the use of
baking soda blasting, except for a reference in the revised National
Park Service Preservation Brief #1, which states: "Baking soda
blasting is being used in some communities as a means of quick
graffiti removal. However, it should not be used on historic masonry
which it can easily abrade and can permanently "etch" the graffiti
into the stone; it can also leave potentially damaging salts in the
stone which cannot be removed." The potential dispersion under
pressure of soluble salts into the stone concerns me, as this could
lead to damage later on due to increased absorption of water into
the surface. This is a similar concern often brought up with the use
of sodium based herbicides around gravestones.

Preservation Brief #1 goes on to state: "Some of these processes are
promoted as being more environmentally safe and not damaging to
historic masonry buildings. However, it must be remembered that they
are abrasive and that they "clean" by removing a small portion of
the masonry surface, even though it may be only a minuscule
portion." (Reference: Mack, Robert C. and Grimmer, Anne, NPS
Preservation Brief #1: Assessing Cleaning and Water-Repellent
Treatments for Historic Masonry Buildings, National Park Service,
Washington, DC, November 2000, p.10)

In ASTM's Special Technical Publication 935, David Boyer discusses
the limitations of abrasive cleaning: "Abrasive techniques cannot
discriminate between staining matter and the masonry substrate.
Removal of surface staining requires removal of the masonry surface,
exposing a softer substrate to more rapid deterioration." He goes on
to warn that: "Abrasive cleaning significantly increases the exposed
surface area of the treated substrates to the effects of atmospheric
corrosion, water absorption, and related decay processes."
(Reference: Boyer, David. W., "Masonry Cleaning - The State of the
Art," Cleaning Stone and Masonry, ASTM STP 935, James R. Clifton,
ed., American Society for Testing and Materials, Philadelphia, 1986,
p. 40)

The biggest hurdle with committees entrusted with the care of
historic graveyards is to convince them not to over clean the
stones. Not only does the cleaning process itself often damage the
stone, but the very act of cleaning can radically change the visual
character of the whole graveyard. An archaeologist who had a
business restoring gravestones once told me that over cleaning a
gravestone often did more harm than good because it allowed for
accelerated growth of moss, lichen and other fungi. Especially when
abrasive methods were used, the fungi spores were able to get down
into the stone and adhere more tenaciously, making it more difficult
to clean in the future unless even more abrasive methods were used.

John Horton, RA
Restoration Specialist
NC State Historic Preservation Office

                  Conservation DistList Instance 14:50
                 Distributed: Wednesday, March 28, 2001
                       Message Id: cdl-14-50-002
Received on Monday, 26 March, 2001

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