A simple in situ comparison will help determine the hardness and condition of
the mortar and the masonry units. Begin by scraping the mortar with a
screwdriver, and gradually tapping harder with a cold chisel and mason's
hammer. Masonry units can be tested in the same way beginning, even more
gently, by scraping with a fingernail. This relative analysis which is
derived from the 10-point hardness scale used to describe minerals, provides a
good starting point for selection of an appropriate mortar. It is described
more fully in "The Russack System for Brick & Mortar Description" referenced
in Selected Reading at the end of this Brief.
Mortar samples should be chosen carefully, and picked from a variety of
locations on the building to find unweathered mortar, if possible. Portions
of the building may have been repointed in the past while other areas may be
subject to conditions causing unusual deterioration. There may be several
colors of mortar dating from different construction periods or sand used from
different sources during the initial construction. Any of these situations
can give false readings to the visual or physical characteristics required for
the new mortar. Variations should be noted which may require developing more
than one mix.
1) Remove with a chisel and hammer three or four unweathered samples of the mortar to be matched from several locations on the building. (Set the largest sample aside--this will be used later for comparison with the repointing mortar). Removing a full representation of samples will allow selection of a "mean" or average mortar sample.
2) Mash the remaining samples with a wooden mallet, or hammer if necessary, until they are separated into their constituent parts. There should be a good handful of the material.
3) Examine the powdered portion--the lime and/or cement matrix of the mortar. Most particularly, note the color. There is a tendency to think of historic mortars as having white binders, but grey portland cement was available by the last quarter of the 19th century, and traditional limes were also sometimes grey. Thus, in some instances, the natural color of the historic binder may be grey, rather than white. The mortar may also have been tinted to create a colored mortar, and this color should be identified at this point.
4) Carefully blow away the powdery material (the lime and/or cement matrix which bound the mortar together).
5) With a low power (10 power) magnifying glass, examine the remaining sand and other materials such as lumps of lime or shell.
6) Note and record the wide range of color as well as the varying sizes of the individual grains of sand, impurities, or other materials.
"Repointing Mortar Joints" an Historic Preservation Brief
