The greatest problem in developing practical guidelines for cleaning
any historic building is the large number of variable and unpredictable
factors involved. Because these variables make each cleaning project
unique, it is difficult to establish specific standards at this
time. This is particularly true of abrasive cleaning methods because
their inherent potential for causing damage is multiplied by the
following factors:
- the type and condition of the material being cleaned
- the size and sharpness of the grit particles or the mechanical
equipment
- the pressure with which the abrasive grit or equipment is applied
to the building surface
- the skill and care of the operator, and
- the constancy of the pressure on all surfaces during the cleaning
process.
Pressure: The damaging effects of most of the variable factors
involved in abrasive cleaning are self evident. However, the matter
of pressure requires further explanation. In cleaning specifications,
pressure is generally abbreviated as "psi" (pounds per
square inch), which technically refers to the "tip"
pressure, or the amount of pressure at the nozzle of the blasting
apparatus. Sometimes "psig," or pressure at the gauge
(which may be many feet away, at the other end of the hose), is
used in place of "psi." These terms are often incorrectly
used interchangeably.
Despite the apparent care taken by most architects and building
cleaning contractors to prepare specifications for pressure cleaning
which will not cause harm to the delicate fabric of a historic
building, it is very difficult to ensure that the same amount
of pressure is applied to all parts of the building. For example,
if the operator of the pressure equipment stands on the ground
while cleaning a two-story structure, the amount of force reaching
the first story will be greater than that hitting the second story,
even if the operator stands on scaffolding or in a cherry picker,
because of the "line drop" in the distance from the
pressure source to the nozzle. Although technically it may be
possible to prepare cleaning specifications with tight controls
that would eliminate all but a small margin of error, it may not
be easy to find professional cleaning firms willing to work under
such restrictive conditions. The fact is that many professional
building cleaning firms do not really understand the extreme delicacy
of historic building fabric, and how it differs from modern construction
materials. Consequently, they may accept building cleaning projects
for which they have no experience.
 Bronze statuary may be cleaned gently using crushed walnut shells. Photo: NPS files. |
The amount of pressure used in any kind of cleaning treatment
which involves pressure, whether it is dry or wet grit, chemicals
or just plain water, is crucial to the outcome of the cleaning
project. Unfortunately, no standards have been established for
determining the correct pressure for cleaning each of the many
historic building materials which would not cause harm. The considerable
discrepancy between the way the building cleaning industry and
architectural conservators define "high" and "low"
pressure cleaning plays a significant role in the difficulty of
creating standards.
Nonhistoric/Industrial: A representative of the building cleaning
industry might consider "high" pressure water cleaning
to be anything over 5,000 psi, or even as high as 10,000 to 15,000
psi! Water under this much pressure may be necessary to clean
industrial structures or machinery, but would destroy most historic
building materials. Industrial chemical cleaning commonly utilizes
pressures between 1,000 and 2,500 psi.
"Dangers of Abrasive Cleaning" an Historic Preservation Brief
