Preliminary research is necessary to ensure that the proposed repointing work
is both physically and visually appropriate to the building. Analysis of
unweathered portions of the historic mortar to which the new mortar will be
matched can suggest appropriate mixes for the repointing mortar so that it
will not damage the building because it is excessively strong or vapor
impermeable.
 This late 19th century granite has recently been repointed with the joint profile and mortar color carefully matched to the original. Photo: NPS files. |
Examination and analysis of the masonry units--brick, stone or
terra cotta--and the techniques used in the original construction will assist
in maintaining the building's historic appearance. A simple, non-
technical, evaluation of the masonry units and mortar can provide information
concerning the relative strength and permeability of each--critical factors in
selecting the repointing mortar--while a visual analysis of the historic
mortar can provide the information necessary for developing the new mortar mix
and application techniques.
Although not crucial to a successful repointing project, for projects
involving properties of special historic significance, a mortar analysis by a
qualified laboratory can be useful by providing information on the original
ingredients. However, there are limitations with such an analysis, and
replacement mortar specifications should not be based solely on laboratory
analysis. Analysis requires interpretation, and there are important factors
which affect the condition and performance of the mortar that cannot be
established through laboratory analysis. These may include: the original
water content, rate of curing, weather conditions during original
construction, the method of mixing and placing the mortar, and the cleanliness
and condition of the sand. The most useful information that can come out
of laboratory analysis is the identification of sand by gradation and
color. This allows the color and the texture of the mortar to be matched
with some accuracy because sand is the largest ingredient by volume.
In creating a repointing mortar that is compatible with the masonry units,
the objective is to achieve one that matches the historic mortar as closely as
possible, so that the new material can coexist with the old in a sympathetic,
supportive and, if necessary, sacrificial capacity. The exact physical and
chemical properties of the historic mortar are not of major significance as
long as the new mortar conforms to the following criteria:
-
-- The new mortar must match the historic mortar in color, texture and tooling. (If a laboratory analysis is undertaken, it may be possible to match the binder components and their proportions with the historic mortar, if those materials are available.)
- The sand must match the sand in the historic mortar. (The color and texture of the new mortar will usually fall into place if the sand is matched successfully.)
- The new mortar must have greater vapor permeability and be softer (measured in compressive strength) than the masonry units.
- The new mortar must be as vapor permeable and as soft or softer (measured in compressive strength) than the historic mortar. (Softness or hardness is not necessarily an indication of permeability; old, hard lime mortars can still retain high permeability.)
"Repointing Mortar Joints" an Historic Preservation Brief
