One of the challenges that architectural conservators face is how identical masonry materials can behave differently depending upon variation in composition, age, and production technology. This is especially true within the context of New Orleans brickmaking tradition, which relied upon two sources of clay to make bricks in the nineteenth century. Soft-red bricks were made using locally sourced Mississippi River clay, whereas hard tan bricks were made using mud from Lake Pontchartrain. Using multiple physical characterization techniques and performance tests, this study compares the behavior of two groups of seven nineteenth-century soft red and hard tan bricks to show that age alone is not a reliable indicator of vulnerability. In doing so, this study produces an index of vulnerability that highlights how composition and production method give hard tans an advantage in performance over soft reds.