In this installation of BKAB, we’re going to talk about spalling.
Water is an enemy of masonry, and this relentless and ubiquitous foe can can cause bricks to spall. Spalling describes the gradual process of brick erosion stemming from moisture trapped and released during freeze/thaw cycles. This moisture can come from contact with the ground, or it can seep through overlaying layers of plaster, or it can be the result of direct contain with rain. When most people think of porosity, a brick is likely to be among the last things that come to mind, but bricks are indeed porous, with some bricks being much more so than others. When the moisture absorbed by a brick freezes, it expands, creating slight fractures in the brick in the process. During a thaw, the moisture is released, along with the tiny flakes of brick created during the freeze.
Not all bricks are created equal, however. Bricks fired at higher temperatures for longer are generally less porous, so they are less susceptible to spalling. Take the bricks on Eager Street, for instance. The bricks we’ve harvested are not nearly as porous as many others, and for this reason we’ve seen practically no spalling. In fact, the foundations of the houses on Eager Street are made of brick, which some folks see as a no-no because contact with ground moisture could cause deterioration. But the foundation bricks of the Eager Street houses are super hard with very little porosity, fired high and long to that beautiful purple color you’ve seen in some of the photos.
Powdery bricks called “salmon bricks” because of their telltale color were intended for interior wythes and were not designed to withstand the elements. But what happens when a rowhouse is demolished, leaving its neighbor with an exposed party wall that might be composed of weaker bricks? If left exposed to the elements, the bricks would likely crumble to dust. Such walls are generally plastered over with an impervious material, but cracks or leaks in this surface layer might actually create channels for moisture to seep in and become trapped! While salmon and water mix just fine, salmon brick and water don’t. See the photo below for proof.