• Matching and patching bricks and mortar A quick introduction to a complex subject.
• Burning off old paint Pros and cons of a commonly-used method of removing failed exterior paint.

MATCHING AND PATCHING BRICKS AND MORTAR

The Office recently completed repairs to a turn-of-the-century brick concert hall that involved matching existing brick and mortar. That seemingly simple task is actually more difficult than it first appears, because the match involves both different times and places.

The source of the clay from which the brick is made can make a great difference in the appearance of the finished product. Different pits yield different clays that fire into brick of differing appearances; even different parts of the same pit can yield variations in color and texture.

The older the brick to be matched, the more localized the pit from which its clay was dug is likely to have been. Particularly in the eighteenth century, the clay for chimney bricks was often dug from an on-site claybank. Many old clay pits have been consumed. Even where they haven't been, modern production requires a larger scale of operations, which means that relatively large pits are used. Matching the exact composition of a clay is therefore unlikely.

Not only is the composition of the clay difficult to match, so is the process of firing it. Brick has been made by firing clay biscuits ever since ancient times, but the size, construction, and arrangement of brick kilns; how long they are burned; and what fuel is used--charcoal, wood, coal, oil, gas, or combinations--all affect the ultimate surface color and appearance of the finished brick. Taken together, these factors mean that getting an acceptable brick match can be a time-consuming, exhaustive procedure involving more than a little luck. Even then, one commonly has to settle for a near miss. Weathering and aging will help to blend the new and old brick over time, but it will always be possible to distinguish between them.

The mortar between the bricks is almost as important to the appearance of a brick wall as the bricks themselves. Since mortar is a site-mixed product, rather than a manufactured one, it seems logical that it should be possible to obtain a very close match to an existing mortar.

Mortar has three principal components: cement, lime, and sand. Cement and lime colors can be matched fairly accurately without too much trouble. The sand is the most difficult component to match, but even here it is usually possible to come quite close (It is also possible to add pigment to the mortar to help color it. This approach is secondary to getting the sand color right, however, since as the mortar weathers, cement and lime will be worn off the face of the joint, exposing the sand granules).

But, there is a more fundamental, and vexing, problem: What mortar color do you match? Customarily, mortar is matched to a clean sample of mortar taken from inside the wall (not as difficult as it may at first sound). The reason for this is that the exposed mortar surface, being rough and somewhat porous, picks up dirt over time and changes color, sometimes quite dramatically.

This still leaves us with the question: "Do we try to match the mortar as it actually is, knowing that it will not match the exposed mortar until it has been in place to age and discolor for a decade or so, or do we match the surface appearance of the exposed mortar, knowing that our new work will change and discolor, leading in time to a mismatch?"

This is not a question that can be answered once for all, although normally we try to match the true color of the mortar (or an intermediate compromise). When repointing the entire wall is appropriate, that can solve the problem nicely, since all the mortar will look the same. In any event, only rarely do we try to match exactly the discolored surface of the existing mortar, if only because we know that the match will surely be lost as the new mortar ages.

Technical assistance for this article was provided by Preservation Technology Associates, Inc.