Aging the Mortar
Even with the best efforts at matching the existing mortar color,
texture, and materials, there will usually be a visible difference
between the old and new work, partly because the new mortar has
been matched to the unweathered portions of the historic mortar.
Another reason for a slight mismatch may be that the sand is
more exposed in old mortar due to the slight erosion of the lime
or cement. Although spot repointing is generally preferable and
some color difference should be acceptable, if the difference
between old and new mortar is too extreme, it may be advisable
in some instances to repoint an entire area of a wall, or an entire
feature such as a bay, to minimize the difference between the
old and the new mortar.
If the mortars have been properly matched, usually the best way
to deal with surface color differences is to let the mortars age
naturally. Other treatments to overcome these differences, including
cleaning the non-repointed areas or staining the new mortar,
should be carefully tested prior to implementation.
Staining the new mortar to achieve a better color match is generally
not recommended, but it may be appropriate in some instances.
Although staining may provide an initial match, the old and new
mortars may weather at different rates, leading to visual differences
after a few seasons. In addition, the mixtures used to stain the
mortar may be harmful to the masonry; for example, they may introduce
salts into the masonry which can lead to efflorescence.