Take a Closer Look at the Foundation of Oakland: Bricks (Part 2)
By Ashley Shares, Director of Preservation
In part one of this series, we talked about the history of bricks at Oakland Cemetery and some examples of their uses on the grounds. Today, we are going to go deeper and discuss what bricks are made of and how they were manufactured.
Bricks at Oakland with fingerprints visible
Bricks have been around for centuries, and until the late 19th century, the process for making them remained relatively unchanged. Locally sourced clay, sand, and potentially other additives like ash or shale would be dug up and blended together in a mill and then pressed by hand into rectangular molds. (Hand-pressing is why you can see fingerprints in many of the older bricks at Oakland). The soft bricks would then be removed from the molds and set out in the sun to bake. Until 3500 BCE, that was it. Sun-baked bricks would have been the end-product. But in the 19th century, the next step would be to fire the bricks at high temperatures inside a kiln for upwards of 1-2 weeks. By the late 19th century, many brick companies began to switch from hand-pressing to extruding and wire cutting the bricks.
Most of the bricks in Atlanta are some shade of red, which is indeed the color we most often associate with bricks. However, bricks come in a variety of colors: from yellow to brown to red to purple-black. Bricks get their color from two main sources: clay and firing conditions. Clay that contains a large amount of iron will produce a red brick.
Even if the clay not red when harvested, the firing process will cause oxidization, transforming a dull brown clay slab into a lovely red brick. Yellow bricks contain a clay with more calcium or lime. The temperature in the kiln plays a role as well. Much like a batch of cookies in your oven, higher temperatures will produce a darker brick. Under certain extreme conditions, it can produce a nearly-black brick.
Kiln at the Chattahoochee Brick Company
Clay and kiln conditions effect more than color, though. They also play a role in influencing how well the brick will stand the test of time. In terms of mineral composition, clay with lots of lime will be more susceptible to dissolution when exposed to damp conditions. Clays also vary in their density and porosity (how much water they can soak up). Low-temperature fired bricks are more porous than those fired at a high temperature. Even within the same “batch”, brick vary because of their distance away from the heat source. We refer to low-temperature bricks as Salmon bricks and the crispy-looking ones as clinker bricks.
Clinker bricks tend to have a deformed, warped appearance but can withstand the elements pretty well. Salmon bricks, on the other hand, tend to perform very poorly because of their relative weakness and porosity. The freeze-thaw cycle often causes the faces of these bricks to spall off, a process exacerbated when bricks were set in a rigid, Portland Cement mortar. Overall, bricks at Oakland have held up pretty well over time, aside from a few areas where salmon bricks are slowly eroding away. Historically, bricks at Oakland were set in a mortar blend that contained both lime and Portland cement, meaning that the mortar was relatively flexible compared to something more modern. We try to continue using a “middle of the road” mortar when we repair our pathways as well.
