Robertson Chapel was designed by one of Chicago's most distinguished architects, Ed Dart. He was made a Fellow of the American Institute of Architects at age 44 and garnered 18 AIA awards for his work. He developed his personal design style of using natural materials, incorporating a building into its site and free- flowing spaces while at Yale. Primary among those materials was Chicago Common brick.
Much of Chicago is built from Chicago Common bricks. Chicago Commons are the rougher and dirtier bricks on the sides and back of many of Chicago's buildings. They're made from the clay from the Chicago River and when fired they can turn a range of colors, like buff yellow, salmon pink, or deep red. Chicago bricks age beautifully and take on a beautiful patina.
Before the Great Chicago Fire of 1871, Chicago was built mostly of wood. The fire burned down over 17,000 buildings and, after another fire in 1874, citywide building codes were changed to ban new wood buildings. The rebuilders of the city in turn dug into the clay of the river and areas surrounding the city to make bricks.
The clays used to make Chicago Common Bricks are full of lime, iron, and tons of little stones and particulate. Depending on the makeup of the particular batch, the bricks burned to a different range of colors, often colored in spots (called flashpoints, where the bricks touched in the kilns). These Chicago bricks were called Commons because they were rougher and deemed unworthy of gracing the street facing facade of the building. That purpose was left to nicer “face bricks” made with the smoother, cleaner clays from areas like Pennsylvania and St. Louis.
Dart loved the beauty of these bricks and their “common” nature made them perfect in his mind for his religious buildings.