First, a little Mortar 101 is in order. Mortar is typically made up of three dry components: a binder, an aggregate, and lime. This is usually Portland cement, sand and hydrated lime. It's the ratio that determines the strength, or ASTM classification, of the mortar. The pre-mixed bags found at home centers are usually ASTM type "S" mortars, similar to that used on commercial construction sites to lay modern brick and block walls. It has lots and lots of Portland cement in it and probably differs wildly from the mortar found in the average historic home. Before 1872 in the United States, there was no such thing as Portland cement. Mortar was generally lime and sand mixed, or lime, sand and natural cement (discovered in the 1820's during construction of the Erie Canal in upstate New York.)
The paradox of a masonry structure is that it's strength comes from it's ability to fail. Well, what the heck does that mean you're wondering. And rightly so. Here's how an old friend best explained it to a class of preservation students: Masonry units, be it brick, stone or block, are laid in mortar. That mortar absorbs and expels moisture. Moisture is water, and water freezes. When it freezes it expands, increasing volume by as much as 12% in the case of an ice cube. So, in a sense, the mortar expands, even minutely. Something has to give: the brick or the mortar. If the mortar is ‘harder' (meaning a high cement content) than the bricks laid in it, the bricks will spall and pop, their faces crumbling and falling off. But if the bricks are ‘harder' the mortar will give, often without cracking or falling apart or leaving any visible record of the strength through failure. And, if the mortar joints do fail, it's FAR less expensive to repoint masonry than it is to rebuild it!
A good mason will be able to mix up a repointing mortar that will not jeopardize the historic masonry fabric of your home or building. If the color or texture are more challenging, there are firms available on-line that will custom match mortar samples for under $200. That's right folks, you can cry Foul! the next time a mason says "I can repoint your brick wall but I can't match the old joints where they meet," "It'll take a couple years for it to blend in, if ever," or, worse, "It'll never match." This is the same guy who buys bags of pre-mix mortar at Home Depot and repoints old, soft-brick chimneys. After a couple of winters, the chimney is crumbling and falling apart. If he's really slick, they unwittingly call him back to ultimately rebuild the chimney that he destroyed!
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