It’s easy to come to the wrong conclusion here: it certainly looks like the mortar in the joints between the stone is being squeezed out, so that it projects from the stone face. Mortar of any kind doesn’t act like that – it’s not putty – but this is hard mortar with portland cement binder and if there’s anything it doesn’t do once it cures, it doesn’t flow.
The stone here is a good sandstone that is much stronger than most New York brownstone. It is, however, still much weaker and softer than most building stones and even most modern brick. It works just fine in construction, since most traditional masonry buildings are constrained by stability rather than the strength of the stone, but it’s not very good when dealing with conditions other than ideal.
Every construction material deteriorates under some conditions and material exposed to weathering require maintenance to last. Given proper maintenance, masonry can last indefinitely, but what does “proper maintenance” mean? The mortar in a wall serves some important functions, including sealing the joints to keep weather out and evening out load paths from one masonry unit (bricks, or concrete blocks, or stone blocks) to the next. However, the mortar is relatively easily replaced, at least at the wall faces, while the masonry units are not. So, logically, the general scheme of maintenance should be to treat the mortar as a sacrificial element, removed and replaced as needed to keep the masonry units intact.
There are two material properties of the mortar and the stone that play into a maintenance scheme: porosity and strength. If the stone is less porous than the mortar, then environmental water will tend to collect in the mortar and cause deterioration there. If the stone is more porous than the mortar, then water will tend to collect in the stone and cause deterioration there. If the stone is stronger than the mortar, then any accidental stress concentrations will cause cracks and spalls in the mortar. If the stone is weaker than the mortar, then any accidental stress concentrations will cause cracks and spalls in the stone. In the picture above, the mortar is stronger and less porous than the stone. As a result, the face of the stone has spalled where it abuts the mortar, leaving rigid fins of mortar projecting free.
This is not some esoteric theory. One of the basic principles of conserving masonry is that the mortar should be weaker and more porous than the stone around it. The simplest way to do so is to use mortar with lime as binder rather than portland cement. This has been taught in conservation programs for decades, but is unfortunately still considered a novel idea outside of the conservation community. The picture above shows why the use of soft mortar is necessary.
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