Repointing gets treated like routine maintenance, the masonry equivalent of caulking a window. Somebody notices the joints between the stone or brick look chalky and crumbling, calls a contractor, and a few days later the wall looks tight and fresh. Six or seven years on, though, the face of the brick itself starts flaking off in sheets, or the stone develops soft, pitted spots right where the new mortar meets it. That's not age catching up. That's almost always the wrong mortar doing exactly what the wrong mortar does.
Old masonry was never built for modern mortar
Anything built before roughly the 1930s in this area, stone farmhouses, brick twins, fieldstone foundations, was laid up with lime mortar, not the portland cement mixes sold in bags at the hardware store today. Lime mortar is simple: lime, sand, water. It's soft, porous, and slightly flexible, and the brick or stone was quarried and fired to work in partnership with exactly that kind of mortar.
Portland cement wasn't in common use in the U.S. until the early 20th century, and even then it usually showed up as an additive rather than the main ingredient until the 1930s. So a house built in 1890 and one built in 1950 are, from a masonry standpoint, entirely different animals, even if they're two doors apart on the same block.
The rule that actually matters: mortar should always be softer than what it's holding
Here's the physics of it, stripped down. A masonry wall moves, constantly, in small ways: thermal expansion in summer, contraction in winter, minor settlement, vibration from the street. Something in that wall has to absorb that movement, and it's supposed to be the mortar. Lime mortar is soft and slightly water-soluble, which means it can flex with the wall and even reseal its own hairline cracks over time.
Cement-heavy mortar can't do that. It's dense, rigid, and far harder than the historic brick or stone around it. When that wall moves, the movement has nowhere to go, so the stress transfers into the masonry unit instead. And because dense cement mortar doesn't let moisture pass through it easily, water that gets into the wall stays trapped, along with the salts naturally present in brick and stone. Come the first hard freeze, that trapped moisture expands, and the face of the brick, being softer than the mortar around it, is what gives way. Masons call this spalling, and once it starts on a wall, it doesn't stop on its own.
The rule preservation masons work from is blunt: the mortar should always be the sacrificial part. It's supposed to wear out and get replaced every few generations. The brick and stone are supposed to last for centuries. Repoint with something harder than the masonry, and that relationship flips, and the part of the building that was actually meant to last starts eroding instead.
How to tell what you're dealing with
Rough rule of thumb by construction date gets you most of the way there. Pre-1930 masonry in Delco, and especially the stone farmhouses scattered through the county's older townships, almost always started life with a lime-based mortar. But renovation history complicates this constantly. A house from 1880 may have already been repointed once or twice with cement mortar by a well-meaning previous owner decades ago, and that patch job is often the thing actively damaging the wall right now.
Short of lab testing a mortar sample, the practical move is to get someone who specializes in historic masonry to look at it before assuming either direction. A general contractor with a bag of standard mortar mix isn't the same as a mason who's worked specifically with lime repointing. It's worth asking the question directly when hiring: have you repointed pre-1930 masonry with a lime-based mix before, and can you show examples.
Repointing isn't glamorous work, and done right, it mostly just disappears back into the wall. That's sort of the point. The failures show up years later, quietly, in the form of brick faces that were never supposed to wear out in the first place.