Matching Color and Texture of the Repointing Mortar
New mortar should match the unweathered interior portions of
the historic mortar. The simplest way to check the match is to
make a small sample of the proposed mix and allow it to cure at
a temperature of approximately 70 degrees F for about a week,
or it can be baked in an oven to speed up the curing; this sample
is then broken open and the surface is compared with the surface
of the largest "saved" sample of historic mortar.
If a proper color match cannot be achieved through the use of
natural sand or colored aggregates like crushed marble or brick
dust, it may be necessary to use a modern mortar pigment.
During the early stages of the project, it should be determined
how closely the new mortar should match the historic mortar. Will
"quite close" be sufficient, or is "exactly"
expected? The specifications should state this clearly so that
the contractor has a reasonable idea how much time and expense
will be required to develop an acceptable match.
The same judgment will be necessary in matching replacement terra
cotta, stone or brick. If there is a known source for replacements,
this should be included in the specifications. If a source cannot
be determined prior to the bidding process, the specifications
should include an estimated price for the replacement materials
with the final price based on the actual cost to the contractor.