08/26/2026
Today I’m extracting some samples of mortar to send off for laboratory testing. For historic mass-masonry projects like this, one of the first steps should be identifying the composition of the original mortar so that repairs can be made with a consistent, accurate and—most importantly-compatible replacement mortar.
I’ve found at least seven unique mortars used at the Stone House over its 155-years, almost all of them inappropriate and poorly applied. Most modern mortars rely substantially on Portland cement, which produces a much harder, less permeable mortar than would have been used when the house was built in 1871. The original mortar appears to be predominantly lime and sand, with no Portland cement (which didn’t really go mainstream until around 1900).
In traditional mass masonry, the mortar is intentionally the softer, more sacrificial component of the wall, so that as the wall moves or deals with water or freeze-thaw cycles, the mortar joint can deteriorate instead of the stone. Mortar can be raked out and replaced; 1800s limestone, not so much. A very hard, cement-rich mortar reverses that relationship, leaving the stone as a vulnerable weak link.
Traditional lime mortars, with their high v***r permeability and capillary activity, are much better at managing water, too, allowing moisture to migrate toward the joint and ev***rate. Portland-based mortars resist absorption and inhibit moisture migration, sometimes forcing moisture through the stone itself in order to dry. This means that when water saturates the wall—which is inevitable if left exposed (i.e. not covered with siding or stuccoed)—Portland mortars tend to exacerbate freeze-thaw issues and can even create mold, mildew and humidity problems in the interior if not adequately accounted for. Getting wet isn’t the problem; not being able to dry is.
Lime also has a self-healing superpower: since lime—calcium carbonate—is easily dissolved by water, as it repeatedly gets wet and dries, some of the lime migrates and redeposits in voids missed by the setting and pointing process, or in cracks that developed afterwards—a process called autogenous healing—creating a more monolithic and robust installation over time.
I’d feel confident just specifying a pure lime-and-sand repointing mortar for this project, but a laboratory analysis of the original 1871 mortar will provide a lot more specificity: the type(s), size(s), and color(s) of sand and their ratios to each other; the binder chemistry (likely not simply “pure lime”) and its ratio to aggregates; other geographical or manufacturing peculiarities; and, of course, a recipe for an original color match.
Considering the amount of labor required to completely repoint a structure like this—and to in-fill the old basement windows—as well as how dramatically the mortar will affect the building’s overall appearance, sending off a few chunks of original mortar seems like a first step towards doing it once and doing it right.
(For what it’s worth, these pros and cons apply specifically to this form of historic, mass-masonry construction using relatively soft masonry materials. Most modern building techniques and materials—CMU walls, high fired bricks, or veneer stone—have a different set of pros and cons.)
