Unrolling Stones

That’s a picture of a rubble wall in the cellar of a building probably constructed in the 1880s or 90s. The wall consists of rubble masonry, mortar, and a coating. Those are facts that I can defend without any difficulty. Everything from this point on is an opinion that comes from my interpretation of visual clues. I’d like to believe my interpretation is always correct but, like literally everyone else, I sometimes come to incorrect conclusions.

The stone is probably schist. It was the most commonly used stone in rubble foundations in the New York area because (a) it was plentiful, since it’s local bedrock, (b) it performs well in wet conditions, and (c) because it is both hard and has irregular bedding planes, it’s a pain to cut into regular blocks, so rubble is a good use for it.

The mortar most likely uses Portland cement as its binder. Portland became very popular in the late 1800s, and the fact that the joints are holding together suggests that they’re not bound by (soluble) lime.

The coating is probably modern – Thoroseal or something similar. It doesn’t look like a lime wash, which is the only original-period coating I’d expect to see, and it doesn’t look like ordinary paint. Depending on why it was applied, it may be useless. If it’s there to brighten the relatively dark cellar with it’s color, then it’s doing just fine; if it was meant to waterproof the wall, then it does nothing.

Finally, the reason I was looking at it: the condition of the joints and coating strongly suggest that there has been no differential settlement. Rubble walls perform fine in compression when they carry the weight of the building above, but they have little shear and tensile strength. If the front of the building had settled relative to the rear, I’d expect to see cracks and even-more-irregular joints. Obviously the joints and coating could have been repaired since movement took place, but you can usually tell when that’s happened and there’s no sign of it here.

So, two short sentences of fact followed by four paragraphs of interpretation. That’s about right for an investigation performed without documents.