Contributors: Stephanie Croatt
Throughout time, the base material for brickmaking has been clay. In most historical instances of brick production, the clay had to be mined and prepared for molding. The mining, or “winning,” process was as simple as locating a clay bed, removing the topsoil, and then digging out the clay (Gurcke 1987:4-5). The winning process was traditionally a seasonal procedure, as the clay had to “weather” for a period before it could be used. This weathering and mixing process allowed soluble salts to be washed out of the clay, and for the harder lumps of clay to break down.Once the clay weathered over the winter, the brickmaker would temper the clay. This involved adding water and other ingredients to get the plasticity and color just right (Gurcke 1987:7).
After the clay was mixed, it was ready to be molded. The box mold has been the most popular means of molding bricks. The box mold consisted of four pieces of wood fastened together to create a mold that would produce clay bricks of slightly larger size than the final product (Figure 1). Figure 1. Box molds (Hand-Made Fire Brick 2011).
Clay was packed into the mold, the excess was struck off the top, and the green brick would be knocked out of the mold and carried off to be readied for burning. This particular process creates diagnostic “strike marks” on one of the large sides that are essentially scratch marks from where the excess clay was scraped off. In the mid- to late 19th century, box molding was supplanted by machine molding. Machine molds had their beginning in manually driven contraptions that shaped one to five bricks (Pulice 2006:56).
After the bricks were molded, they were set aside to be dried. The purpose of drying the bricks was to remove excess moisture that might destroy the bricks while they were burned in the kiln. That said, too much drying was equally as deleterious for the bricks. Most bricks were dried for about twenty-four hours. After the bricks dried sufficiently, they were “hacked,” or stacked on edge with about a finger’s width between them, in preparation for burning (Gurcke 1987:24). This special stacking allowed the hot air in the kiln to move between the bricks, thus making the heating more even.
Next, the bricks were fired in the kiln. This was the most important process in the brickmaking sequence because it determined the size, color, and strength of the bricks. The burning process occured in three stages. The first stage, water-smoking, prevented the bricks from shrinking too rapidly. During this stage, the kiln temperature was raised slowly to around 250°-350°F (Gurcke 1987:28). The dehydration stage required that the kiln be heated to 1,400°-1,800°F. This stage removed the molecular water (Gurcke 1987:28). The vitrification stage was the final stage in which the “..clay softens, the pore spaces fill up, and the larger grains begin to adhere or melt to each other” (Gurcke 1987:28). During this stage, the kiln was sealed off and the bricks heated at around 1,600° to 2,200°F (Gurcke 1987:28). After vitrification was finished, the heat was turned off and the bricks were left to cool in the kiln between 48 and 72 hours (Gurcke 1987:28). If the bricks were allowed to cool too quickly, they would be prone to cracking and weakening.
After the bricks were heated and cooled sufficiently, they were graded and sorted according to their quality. Because early field kilns were constructed of the green bricks that were meant to be fired, the outer bricks would be exposed to much less heat than the inner bricks. This meant that the hardness and shape of bricks varied widely after they were burned (Gurcke 1987:35). For video of a historical group recreating small-scale brickmaking in 19th century Pennsylvania, see: .
Although bricks may seem like stable objects, they are in fact susceptible to a number of environmental factors that cause decay. Water is a major agent of deterioration of brick. Whether simply eroding the outer surface of brick, causing soluble salts to percolate into the brick, or causing damaging expansion and contraction in the freeze/thaw cycle, water is very hard on bricks. Wind causes abrasive erosion, soluble salts crystalize and cause pressure in the brick’s pores, and acidic pollutants in the air contribute to the further erosion of the brick’s surface (Warren 1999: 148-163). Damage from biological growth and from humans and animals also causes a substantial amount of breakage and slow deterioration over time (Warren, 1999: 170-178).
When subject to a variety of environmental factors, such as those discussed above, that contribute to the brick’s deterioration, there is a number of ways the deterioration manifests itself in the bricks. Cracking happens when internal or external pressures on the brick cause cracks that may be small fractures, or may result in portions of the brick breaking off the main body (Grimmer 1984: 7). Crumbling occurs when the constituent parts of the brick (i.e. the binder) begins dissolving, causing a loss of cohesion of the brick (Grimmer 1984: 8). Efflorescense is a whitish haze that forms as a result of soluble salts that were absorbed into the brick recrystallizing and forming crystal blooms on the surface of the brick (Grimmer 1984: 11). Spalling occurs when layers of the brick begin to flake off (Grimmer 1984: 20).
- Elemental analysis
- X-ray diffraction analysis
- Infrared spectroscopy
- Porosity and absorption tests
Cleaning is a popular and simple way to remove some unappealing staining or marking caused by deterioration. Cleaning, however, should be done carefully and the abrasive action of some cleaning is minimized. In order to do this, the conservator must always test their cleaning methods on an inconspicuous area to be certain that discoloration or damage will not occur. The materials used should be mechanically and chemically weaker than the brick. For example, pressure washers, highly abrasive brushes or scrubbing tools, and strong detergents all have the ability to damage the brick by contributing to erosion. Instead, low pressure, soft-bristled brushes, and mild detergents should be used. Furthermore, cleaning with water should be avoided if there is the threat of frost or freezing temperatures that may freeze the water after it has absorbed into the brick (Warren, 1999: 193-198).
- Aquasorb F poultice
- Electrochemical desalination
- Clay poultice
- Sand/Lime sacrificial render (sacrificial plaster)
After damaged bricks have been cleaned and completely desalinated, they might benefit from consolidation using acrylic resins, alkylaryl-polysiloxanes, alkyl-trialkoxy-silanes, or methyl and ethyl silicates (Warren, 1999: 208). While all of these different consolidants are viable options, their effectiveness varies. While some have a fairly deep penetration of 50mm that has proven to be useful in not only adhering the brick’s parts together, it also can help deter water seepage through the brick (Warren, 1999: 208).
For bricks outdoor, or in situ, waterproof and water resistant coatings might be a useful tool for protecting the bricks from deterioration agents. Painting is a widely used option for protecting brick, but aesthetics and historical accuracy must be primary considerations when considering this option. If painting is the way to go, the conservator must be aware of the toll taken by primers that etch the surface of the brick to allow the paint to adhere better to the surface (Warren 1999: 204). Other protective coatings that are meant to protect without obscuring the original surface of the brick include silicones and fluoro-polyethers. These coatings have proved to be useful in the short run, but some research suggests that long term use of these coatings, especially coatings in the silicone family, sometimes result in brownish staining (Warren 1999: 207).
Grimmer, A. 1984. A Glossary of Historic Masonry Deterioration Problems and Preservation Treatments. Washington, D.C.: Department of the Interior National Park Service Preservation Assistance Division.
Gurcke, K. 1987. Bricks and Brickmaking: A Handbook for Historical Archaeology. Moscow, ID: University of Idaho Press.
Hand-made Fire Brick. July 2, 2011. Old Canal Pottery.  (Accessed 04/15/13).
Pulice, M. 2006. Machines for Making Bricks in America, 1800-1850. The Chronicle of the Early American Industries Association, Inc. 59(2): 53-58.
Warren, J. 1999. Conservation of Brick. Oxford: Butterworth Heinemann.