Rick Roger's two-story Georgian has stood for more than 80 years in the prosperous suburb of Evanston, Illinois, north of Chicago. With its columned portico, brick walls and ivy-covered façade, his house is the image of prewar solidity. Up close, the walls tell another story. Between the brick, slapdash patches of gray mortar clash with the original white. In some places, the brick face has begun to flake off — a sign that water is getting in, freezing and slowly turning the hard red clay into dust. A dozen 3-foot-long cracks radiate from the windows on the north side, leaving the wall open to water infiltration. Anxious to find out what has gone wrong, Rogers called Mario Machnicki, a mason who specializes in fixing brick and stone walls. Several weeks later, after a visit to examine the walls, Mario and his younger brother, John, arrive in their red pickup, ready to work. "Old buildings are like people," Mario Machnicki says reassuringly as he takes out his chisels. "If you know how to listen, they will tell you why they are cracking." The message at the Rogers house is loud and clear: The mortar is killing it. Not the original mortar, a relatively soft mix of lime and sand, but the previous patch job, which used masonry cement. The old lime-based mortar had been a perfect partner for the soft, porous brick, flexing to accommodate the brick's slight expansion and contraction. Like all mortars, however, it slowly eroded, and after 60 or 70 years the weathered portion was chiseled out and replaced, a process called repointing (or pointing). Unfortunately for this wall, masonry practices underwent a tectonic shift in the 1930s. Brick became harder and more rigid, as did mortar. With the ready availability of portland cement, a material so hard and so impervious to water that it is used to plug leaks in basements, masons abandoned time-consuming lime-based mortars, which set so slowly that no more than seven courses could be done in a day. Instead, bricklayers adopted fast-setting masonry cement: sand and ground limestone blended together with as much as 65 percent portland cement. That modern mix was the unyielding cement the previous masons had slapped on the Rogers house joints. Once it cured, the delicate give-and-take of brick and mortar was replaced by a protracted battle-which the old brick was losing. The cement dammed the joints, trapping moisture inside the brick. In winter, the waterlogged walls froze and cracked, allowing still more water to penetrate. In summer, as the brick tried to expand, its protective fire-skin literally popped off. "The cement mortar is not helping the wall; it is actually hurting the wall," says Mario Machnicki. Americans' reliance on masonry cement surprised him when he arrived here from Poland in 1977. He grew up building brick houses without a single grain of cement. The mortar he used contained simply three parts sand to one part lime putty, the ratio established in 10 B.C. by the great Roman military engineer Vitruvius. These days, he buys 5-gallon buckets of custom-blended lime mortar, which cost about $8 and hold enough to repoint the 3/8-inch joints on 60 square feet of wall. For the Rogers house, he had the original mortar analyzed so he could order the same recipe. The assay revealed the proportion of lime to sand, the size and color of the sand grains and the compressive strength of the brick. The analysis is expensive ($500), but he says he prefers to "know exactly what's in it, rather than guess." Even without the test, he gets a good idea of mortar type just from knowing the year a house was built. He double-checks his hunches by chiseling out a small piece of mortar and dropping it on the sidewalk. A piece containing a lot of cement makes a high-pitched ring; a chunk containing mostly lime makes a muffled thud.