Historic: By contrast, conscientious dry or wet abrasive cleaning
of a historic structure would be conducted within the range of
20 to 100 psi at a range of 3 to 12 inches. Cleaning at this low
pressure requires the use of a very fine 00 or 0 mesh grit forced
through a nozzle with a 1/4-inch opening. A similar, even more
delicate method being adopted by architectural conservators uses
a micro-abrasive grit on small, hard-to-clean areas of carved, cut
or molded ornament on a building facade. Originally developed
by museum conservators for cleaning sculpture, this technique
may employ glass beads, micro-balloons, or another type of micro-abrasive
gently powered at approximately 40 psi by a very small, almost
pencil-like pressure instrument. Although a slightly larger pressure
instrument may be used on historic buildings, this technique still
has limited practical applicability on a large scale building
cleaning project because of the cost and the relatively few technicians
competent to handle the task. In general, architectural conservators
have determined that only through very controlled conditions can
most historic building material be abrasively cleaned of soil
or paint without measurable damage to the surface or profile of
the substrate.
Yet some professional cleaning companies which specialize in cleaning
historic masonry buildings use chemicals and water at a pressure
of approximately 1,500 psi, while other cleaning firms recommend
lower pressures ranging from 200 to 800 psi for a similar project.
An architectural conservator might decide, after testing, that
some historic structures could be cleaned properly using a moderate
pressure (200-575 psi), or even a high pressure (575-1800 psi)
water rinse. However, cleaning historic buildings under such high
pressure should be considered an exception rather than the rule,
and would require very careful testing and supervision to assure
that the historic surface materials could withstand the pressure
without gouging, pitting or loosening.
These differences in the amount of pressure used by commercial
or industrial building cleaners and architectural conservators
point to one of the main problems in using abrasive means to clean
historic buildings: misunderstanding of the potentially fragile
nature of historic building materials. There is no one cleaning
formula or pressure suitable for all situations. Decisions regarding
the proper cleaning process for historic structures can be made
only after careful analysis of the building fabric, and testing.
"Dangers of Abrasive Cleaning" an Historic Preservation Brief
