Using Lime Putty to Make Mortar. Mortar made with lime putty and sand,
sometimes referred to as roughage or course stuff, should be measured by
volume, and may require slightly different proportions from those used with
hydrated lime. No additional water is usually needed to achieve a
workable consistency because enough water is already contained in the putty.
Sand is proportioned first, followed by the lime putty, then mixed for five
minutes or until all the sand is thoroughly coated with the lime putty. But
mixing, in the familiar sense of turning over with a hoe, sometimes may not be
sufficient if the best possible performance is to be obtained from a lime
putty mortar. Although the old practice of chopping, beating and ramming the
mortar has largely been forgotten, recent field work has confirmed that lime
putty and sand rammed and beaten with a wooden mallet or ax handle,
interspersed by chopping with a hoe, can significantly improve workability and
performance. The intensity of this action increases the overall lime/sand
contact and removes any surplus water by compacting the other ingredients. It
may also be advantageous for larger projects to use a mortar pan mill for
mixing. Mortar pan mills which have a long tradition in Europe produce a
superior lime putty mortar not attainable with today's modern paddle and drum
type mixers.
For larger repointing projects the lime putty and sand can be mixed together
ahead of time and stored indefinitely, on or off site, which eliminates the
need for piles of sand on the job site. This mixture, which resembles damp
brown sugar, must be protected from the air in sealed containers with a wet
piece of burlap over the top or sealed in a large plastic bag to prevent
evaporation and premature carbonation. The lime putty and sand mixture can be
recombined into a workable plastic state months later with no additional
water.
If portland cement is specified in a lime putty and sand mortar--Type O
(1:2:9) or Type K (1:3:11)--the portland cement should first be mixed into a
slurry paste before adding it to the lime putty and sand. Not only will this
ensure that the portland cement is evenly distributed throughout the mixture,
but if dry portland cement is added to wet ingredients it tends to "ball up,"
jeopardizing dispersion. (Usually water must be added to the lime putty and
sand anyway once the portland cement is introduced.) Any color pigments
should be added at this stage and mixed for a full five minutes. The mortar
should be used within 30 minutes to 1½ hours and it should not be retempered.
Once portland cement has been added the mortar can no longer be stored.
Filling the Joint. Where existing mortar has been removed to a depth
of greater than 1 inch, these deeper areas should be filled first, compacting
the new mortar in several layers. The back of the entire joint should be
filled successively by applying approximately 1/4 inch of mortar, packing it
well into the back corners. This application may extend along the wall for
several feet. As soon as the mortar has reached thumb-print hardness, another
1/4 inch layer of mortar--approximately the same thickness--may be applied.
Several layers will be needed to fill the joint flush with the outer surface
of the masonry. It is important to allow each layer time to harden before the
next layer is applied; most of the mortar shrinkage occurs during the
hardening process and layering thus minimizes overall shrinkage.
When the final layer of mortar is thumb-print hard, the joint should be tooled
to match the historic joint. Proper timing of the tooling is
important for uniform color and appearance. If tooled when too soft, the
color will be lighter than expected, and hairline cracks may occur; if tooled
when too hard, there may be dark streaks called "tool burning," and good
closure of the mortar against the masonry units will not be achieved.
If the old bricks or stones have worn, rounded edges, it is best to recess the
final mortar slightly from the face of the masonry. This treatment will help
avoid a joint which is visually wider than the actual joint; it also will
avoid creation of a large, thin featheredge which is easily damaged, thus
admitting water. After tooling, excess mortar can be removed from
the edge of the joint by brushing with a natural bristle or nylon brush.
Metal bristle brushes should never be used on historic masonry.
"Repointing Mortar Joints" an Historic Preservation Brief
