 Industrial interiors that are not finely milled may be abrasively cleaned, in some instances. Photo: NPS files. |
For the most part, abrasive cleaning is destructive to historic
building materials. A limited number of special cases have been
explained when it may be appropriate, if supervised by a skilled
conservator, to use a delicate abrasive technique on some historic
building materials. The type of "wet grit" cleaning
which involves a small amount of grit injected into a stream of
low pressure water may be used on small areas of stone masonry
(i.e., rough cut limestone, sandstone or unpolished granite),
where milder cleaning methods have not been totally successful
in removing harmful deposits of dirt and pollutants. Such areas
may include stone window sills, the tops of cornices or column
capitals, or other detailed areas of the facade.
This is still an abrasive technique, and without proper caution
in handling, it can be just as harmful to the building surface
as any other abrasive cleaning method. Thus, the decision to use
this type of "wet grit" process should be made only
after consultation with an experienced building conservator. Remember
that it is very time consuming and expensive to use any abrasive
technique on a historic building in such a manner that it does
not cause harm to the often fragile and friable building materials.
At this time, and only under certain circumstances, abrasive cleaning
methods may he used in the rehabilitation of interior spaces of
warehouse or industrial buildings for contemporary uses.
Interior spaces of factories or warehouse structures in which
the masonry or plaster surfaces do not have significant design,
detailing, tooling or finish, and in which wooden architectural
features are not finished, molded, beaded or worked by hand, may
be cleaned abrasively in order to remove layers of paint and industrial
discolorations such as smoke, soot, etc. It is expected after
such treatment that brick surfaces will be rough and pitted, and
wood will be somewhat frayed or "fuzzy" with raised
wood grain. These nonsignificant surfaces will be damaged and
have a roughened texture, but because they are interior elements,
they will not be subject to further deterioration caused by weathering.
 Decorative wood exterior or interior features should not be cleaned abrasively. Photo: NPS files. |
Those instances (generally industrial and some commercial properties),
when it may be acceptable to use an abrasive treatment on the
interior of historic structures have been described. But for the
majority of historic buildings, the Secretary of the Interior's
Guidelines for Rehabilitation do not recommend "changing
the texture of exposed wooden architectural features (including
structural members) and masonry surfaces through sandblasting
or use of other abrasive techniques to remove paint, discolorations
and plaster
Thus, it is not acceptable to clean abrasively interiors of historic
residential and commercial properties which have finished interior
spaces featuring milled woodwork such as doors, window and door
moldings, wainscoting, stair balustrades and mantelpieces. Even
the most modest historic house interior, although it may not feature
elaborate detailing, contains plaster and woodwork that is architecturally
significant to the original design and function of the house.
Abrasive cleaning of such an interior would be destructive to
the historic integrity of the building.
Abrasive cleaning is also impractical. Rough surfaces of abrasively
cleaned wooden elements are hard to keep clean. It is also difficult
to seal, paint or maintain these surfaces which can be splintery
and a problem to the building's occupants. The force of abrasive
blasting may cause grit particles to lodge in cracks of wooden
elements, which will be a nuisance as the grit is loosened by
vibrations and gradually sifts out. Removal of plaster will reduce
the thermal and insulating value of the walls. Interior brick
is usually softer than exterior brick, and generally of a poorer
quality. Removing surface plaster from such brick by abrasive
means often exposes gaping mortar joints and mismatched or repaired
brickwork which was never intended to show. The resulting bare
brick wall may require repointing, often difficult to match. It
also may be necessary to apply a transparent surface coating (or
sealer) in order to prevent the mortar and brick from "dusting."
However. a sealer may not only change the color of the brick,
but may also compound any existing moisture problems by restricting
the normal evaporation of water vapor from the masonry surface.
"Dangers of Abrasive Cleaning" an Historic Preservation Brief
