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How Clay Bricks Were Made from the Mid 1800s to the Early 1900s in USA-Ⅲ
The bricks were arranged into arches and the fire built inside the arch -- the bricks themselves were the kiln. "If fired bricks were on hand, they were used to construct the outer walls of the kiln and the surface was daubed with mud to contain the heat. If no fired bricks were available, the kiln was constructed entirely of green or raw bricks which were stacked in such a way as to act as their own kiln. These kilns were called clamps or scove kilns. Some coal dust was added because it made them burn better, and some companies added red oxide for coloring, some didn't. The walls and top were plastered with a mixture of sand, clay, and water to retain the heat; at the top the bricks were placed close together and vented for circulation to pull the heat up through the bricks. The kilns were originally fired with wood, then some used anthracite coal, and some eventually used oil.
Even after drying in air the green bricks contained 9-15% water. For this reason the fires were kept low for 24-48 hours to finish the drying process and during this time steam could be seen coming from the top of the kiln. This was called "water smoke". Once the gases cleared this was the sign to increase the intensity of the fires. If it was done too soon the steam created in the bricks would cause them to explode. Intense fires were maintained in the fire holes around the clock for a week until temperatures of 1800 degrees F were reached.The knowledge and experience of the brickmaker dictated when the fireholes would be bricked over and the heat was allowed to slowly dissipate over another week. The brick were baked from eight to 12 days. Each finished brick weighed approximately eight pounds. Salt was added in the kiln "eyes" while the brick baked. This changed their color and made them waterproof. When the bricks were sufficiently fired, the heat was reduced, and they were allowed to cool gradually before removal from the kiln.
Then the entire kiln was usually disassembled and the bricks were sorted. If only raw bricks were used, the bricks from the outermost walls were kept to be burned again in the next kiln. Some bricks which were closest to the fire received a natural wood ash glaze from the sand that fell into the fires and became vaporized and deposited on the bricks. These bricks were used in the interior courses of the walls. Bricks that became severely over-burned and cracked or warped were called clinkers and were occasionally used for garden walls or garden paths.
The best bricks were chosen for use on the exterior walls of the building. Those that were only slightly underfired had a salmon color and early bricklayers knew that the porosity of these bricks would help to insulate the structure and they were placed on the innermost courses of the wall.
