Other Components
Historic components. In addition to the color of the sand, the texture
of the mortar is of critical importance in duplicating historic mortar. Most
mortars dating from the mid-19th century on--with some exceptions--have a
fairly homogeneous texture and color. Some earlier mortars are not as
uniformly textured and may contain lumps of partially burned lime or "dirty
lime", shell (which often provided a source of lime, particularly in coastal
areas), natural cements, pieces of clay, lampblack or other pigments, or even
animal hair. The visual characteristics of these mortars can be duplicated
through the use of similar materials in the repointing mortar.
Replicating such unique or individual mortars will require writing new
specifications for each project. If possible, suggested sources for special
materials should be included. For example, crushed oyster shells can be
obtained in a variety of sizes from poultry supply dealers.
Pigments. Some historic mortars, particularly in the late 19th
century, were tinted to match or contrast with the brick or stone. Red pigments, sometimes in the form of brick dust, as well as brown, and
black pigments were commonly used. Modern pigments are available which can be
added to the mortar at the job site, but they should not exceed 10 per cent by
weight of the portland cement in the mix, and carbon black should be limited
to 2 per cent. Only synthetic mineral oxides, which are alkali-proof and sun-fast, should be used to prevent bleaching and fading.
Modern components. Admixtures are used to create specific
characteristics in mortar, and whether they should be used will depend upon
the individual project. Air entraining agents, for example, help the
mortar to resist freeze-thaw damage in northern climates. Accelerators are
used to reduce mortar freezing prior to setting while retarders help to
extend the mortar life in hot climates. Selection of admixtures should be
made by the architect or architectural conservator as part of the
specifications, not something routinely added by the masons.
Generally, modern chemical additives are unnecessary and may, in fact, have
detrimental effects in historic masonry projects. The use of antifreeze
compounds is not recommended. They are not very effective with high lime
mortars and may introduce salts, which may cause efflorescence later. A
better practice is to warm the sand and water, and to protect the completed
work from freezing. No definitive study has determined whether air-entraining
additives should be used to resist frost action and enhance plasticity, but in
areas of extreme exposure requiring high-strength mortars with lower
permeability, air-entrainment of 10-16 percent may be desirable (see formula
for "severe weather exposure" in Mortar Type and Mix). Bonding agents
are not a substitute for proper joint preparation, and they should generally
be avoided. If the joint is properly prepared, there will be a good bond
between the new mortar and the adjacent surfaces. In addition, a bonding
agent is difficult to remove if smeared on a masonry surface.
"Repointing Mortar Joints" an Historic Preservation Brief
