Method For Identifying A Brickmaker’s Brick
By Dan L. Mosier
Among the artifacts that may be found at an archaeological site are bricks. They are durable manmade building materials made of fired clay that were probably used in the foundations and walls of a building, a chimney, or a garden walkway or patio, or even in a driveway or street. Yet, little is known about the bricks found at such sites. This is partly due to the lack of historic information as regards to bricks and its brickmaker. Its not enough, and can be misleading, to just assume that the found brick came from a local brickyard with no supporting evidence. Proximity alone is not evidence for the provenance of a brick. Also, if there are multiple local brickyards, then which one made the brick? A method for identifying a probable brickmaker’s brick is discussed in this article that has been used to identify the provenance of bricks found in California. I emphasis the word “probable” because the evidence is not absolute proof that the named brickmaker made the brick. It only suggests that the named brickmaker could have made the brick in question, as we did not witness the brickmaker making the brick and we have no documents to support it. The implementation of this method requires some work, but it provides a scientific solution to a complex problem that would otherwise remain unanswered.
The method developed is called the clay body components method. The clay body components can help to identify the probable brickmaker, provided there is a match with certain components in a brick of a known brickmaker. The unknown brick in question exhumed from an archaeological site will be compared with bricks from all of the possible brickmakers who could have made that brick. Certain features in the bricks are compared to verify that the same materials and the same manufacturing process were used. The rocks in the brick are verified by checking the geology of the probable brickyard site, providing geologic evidence for the provenance of the unknown brick.
In a sense, this is an attempt to figure out the brickmaker’s recipe. The brickmaker experimented with materials to make the best consistent brick as possible. Everything was tested from the type of clay used to whether to crush the material, to screen the material, to add sand, to add grog, to add water, and so on. Once the brickmaker was satisfied with the brick, all bricks made followed that recipe. This consistency allows us to record and identify the bricks made by that brickmaker. There are certain features that are common to all bricks made by different brickmakers. But the features that are different or unique are what we look for to identify a brickmaker. Some of these unique features can be seen in the clay body components, which will be described.
The clay body components are the visible features in the interior of the brick. This includes the clay texture, the rock inclusions (clasts), and the percentage of pores, if any, in the clay body. I say if any, because in some clay bodies, clasts and the pores may be absent. It is these components that are examined. The choice of these clay body components used in this method is a result of examining hundreds of bricks in California to determine what features were the most useful in identifying the probable brickmaker. Although bricks are manmade objects, they share similar characteristics as sedimentary rocks, in that they are composed of clastic materials. Therefore, some sedimentary terminology have been used to describe the components in the clay body (Pettijohn, 1975).
This method accounts for the alterations or changes made by the brickmaker to the natural clastic materials. The clay texture reflects the manufacturing process of the brick, by the addition of water and sand in the tempering process, the amount of compaction in the mold or brick press, the drying time, the removal of water and air with vacuum extraction if done, and the degree of firing in the kiln. The clasts or rock inclusions may have been naturally occurring in the clay or may have been added to the clay by the brickmaker. The size of the clasts also may have been its original size in the clay or may have been reduced by crushing and screening of the clay material. The percentage of pores in the clay reflects the degree of compaction and the degree of firing of the clay. This is not the same as porosity as applied to sedimentary rocks. For example, highly compressed and highly fired clay will result in compact clay without visible pores and grade to vitrified texture, whereas lightly compressed and moderately fired clay will result in a fine porous clay or a porous sandy clay. It was found when examining the clasts and pores that recording only the visible sized clasts and pores was sufficient enough to differentiate the brickmaking process as evidence for the brickmaker. Brick examinations can be done with the naked eye or with the aid of a 10-power hand lens to help identify the rocks. Detailed microscopic work is not necessary.
First, examine a two-dimensional view of the interior clay body, preferably a broken surface of a cross-section of the brick, to record all of the essential clay body components. It is easier to see the clay body components on a broken surface of a brick than on a cut surface, which can destroy certain features that we want to examine. Also, examining the exterior surface of a brick for clay body components will not work, unless it is highly eroded. Assign the clay texture, using a 10-power hand lens, to one of these categories:
- compact clay (clay with no pores, includes vitrified clay)
- fine clay with pores
- granular (equant clay or mineral aggregate, more common in firebrick)
- sandy clay (clay with sand mixed in)
Next, identify all of the visible rocks, if any, in the clay body with the naked eye or a 10-power hand lens. Visible rocks are those larger than 1/16 inch in diameter. It is not necessary to identify the sand grains in the clay. Make a list of all of the rocks and minerals and note the range of sizes of each. List the rocks in the order of most abundant to least abundant. For example:
- quartz 1/16” – 1/2”
- shale 1/8” – 1”
- sandstone 1/8” – 3/4”
- iron oxides 1/16” – 1/8”
This list will indicate the most abundant rocks in the brick, which are usually in the top two or three. The rest are considered minor or rarer occurrences, which may vary from brick to brick at a single brickyard. Usually there are not many different types of visible rocks in a brick to identify, so the list is usually short.
Next, if rocks are present, estimate the percentage of the rocks relative to the clay body. This is the total percentage of all of the rocks combined. I use the visual chart for percentage composition by Terry and Chilingar (1955) to help determine the percentage. The percentage for rocks will usually be less than 50%, such as 20% rocks and 80% clay.
Finally, estimate the percentage of the pores, if any, in the clay body, using the same percentage comparison chart. A 10-power hand lens may help to see the tiny pores in the clay body. These are the voids and tiny holes that remained in the clay body after the manufacturing process. Do not include the holes left by eroded rocks; those appear as imprints of rock or may have remnants of rock around the sides of the hole. Assign the percentage to one of these broad categories:
- none: 0%
- low: <10%
- moderate: 10 – 30%
- high: >30%
Now arrange the clay body components data for the unknown brick into a table, as follows:
Table 1. Clay Body Components For Unknown Brick.
Rocks (decreasing order)
quartz, shale, sandstone, iron oxides
Note that the volume % is for all of the clasts combined. This indicates how much rock was permitted in the clay mixture. Rock Size is the minimum and maximum size of all of the rocks and minerals listed under Rocks. This indicates if the rocks were crushed or screened or not. The roundness of the rocks is an indicator of crushing, so that may also be noted, although that component was ignored in this example because there were obviously no crushing or screening of the large-sized rocks seen in the unknown brick. For a more complete description, the rocks could have been prefixed with roundness terms, such as: subangular quartz, well-rounded shale, rounded sandstone, and rounded iron oxides.
Make another table for the exterior features of the unknown brick, by measuring the brick, noting its color, and noting if it has a sand-coating on the surfaces, as follows:
Table 2. Exterior Features of the Unknown Brick.
Other exterior features may be added to the table if necessary. The above tables can now be used to compare data with the bricks of known brickmakers to find a match. If similar tables for the possible brickmakers are not already available, which in this case they are not, they need to be compiled.
Repeat the above for each of the brick samples from each of the local brickyards. This usually requires searching for brick samples from each of the brickyards, either by finding brick samples from the brickyard sites or conducting historic research to locate the bricks in buildings or structures, so that they could be examined. Collect a whole brick, if possible, for recording the exterior features and measuring the brick size, and a bat for recording the interior clay body components. On buildings, look for already broken brick surfaces or eroded (skin removed) surfaces to examine the interior clay body; these may be difficult to find. Also note the exterior features and measure the brick size. If suitable bricks cannot be found, then this method cannot be applied. Historical research of the operating years of the brickyards should be compiled for dating purposes, or this could be done after the probable brickmaker has been identified just for that brickyard. Operating years for brickyards can be found in city or county directories, newspapers, history books, and trade journals.
Tables 3 and 4 display the clay body components and exterior features of bricks, respectively, from three local brickyards for comparison with the unknown brick
Table 3. Clay Body Components of Bricks from Local Brickyards.
Rocks (decreasing order)
quartz, shale, iron oxides
chert, quartz, shale
Table 4. Exterior Features of Bricks From Local Brickyards.
orange red, pale red
Use these comparison tables to see if the unknown brick matches with one of the possible brickmakers. From the tables, we can see that the unknown brick matches closely with the Smith brickyard, in that it is a sand-coated brick of similar size and color with matching clay body components. In the clay body components, the clay texture, pores, rock size, and rock volume percentage match perfectly. The only difference is in the rocks, where Smith is missing sandstone. Because this is the third rock listed, that’s not too serious. There will usually be some variations or omissions with the minor rocks. It is more important that the first two rocks match which we have with quartz and shale, plus black iron oxides. So, we can conclude that the unknown brick could have come from the Smith brickyard and not from the other brickyards on the list. If there is no match with the listed brickyards, then more brickyards could be added to the list from more distant points. If still no match is found, then the brick must remain unknown. It could have come from a forgotten brickyard from which the bricks are not available for this study or it could have been an imported brick from another state or country.
Next, check the geology around the Smith brickyard to verify that the rocks seen in the bricks did indeed come from that brickyard. The geology of the Smith brickyard can be found by checking a geologic map or a geologic report of the brickyard area. For example, lets suppose that the geologic map shows that the brickyard area is underlain by gray shales with sandstone and siltstone lenses of Jurassic age and there are Cretaceous age granitic intrusions in the area. The unknown brick has gray shale that matches with the Jurassic gray shale at the brickyard site. The sandstone could have come from one of the lenses in the gray shale. The quartz in the unknown brick could have come from the erosion of the nearby granitic intrusion, which contains quartz. So all of the rocks seen in the unknown brick occurs at the brickyard site. This provides geologic evidence that Smith could have made the unknown brick.
If more than one brickmaker operated at different periods at the same brickyard site, a sample of their bricks should be acquired as well and data added to the tables. Although the rocks in the bricks may be similar, the recipe should be different and that should be evident in the data of the clay body components. Some differences may be seen in the volume percentage of the clasts or the pores, or in the size of the clasts. The manufacturing process may be different enough to change the exterior appearance of the brick as well. So all of these features need to be compared to differentiate the different brickmakers who operated at or near the same site.
If the rocks at the brickyard site do not match with the rocks in the unknown brick, there may be a reasonable explanation for that which should be explored before being rejected. Some brickmakers, such as face brick manufacturers, may have acquired their clays from other sites with different geology. If so, that information may have been mentioned in geologic or mining reports for clay deposits or in newspaper or trade journal articles that described the brickyard operations. Again, verify the rocks at the other site to see if they match the rocks in the unknown brick. If such information cannot be found or the rocks cannot be verified, then the brickmaker for the brick remains unknown.
Now that we have a probable brickmaker for the unknown brick, we can see in table 4 that the Smith brickyard operated from 1861 to 1872, giving us a probable date range for the brick. So, this brick can provide a probable construction date at the archaeological site.
In conclusion, the clay body components method can be used to identify the probable brickmaker of an unknown brick provided there is a match with a brick of a known brickmaker. Without a brick of a known brickmaker, this method cannot be applied and the unknown brick remains unknown. The above example demonstrates how the unknown brick, with matching features and geologic evidence, probably came from the Smith brickyard, using this method. The brick’s exterior features and the clay body components matched well with those in the Smith brick and we verified that the rocks in the unknown brick matched with the rocks found at the Smith brickyard. The unknown brick, therefore, can provide a probable date range that can be useful for archaeologists at the found site.
This also points to the significance of having a brick archive along with the brickyard information so that the provenance of a found brick can be determined more rapidly. Too often when a brick structure is demolished, the bricks are discarded and the information is lost. Saving a sample brick and a sample bat, along with any information about the brick, can be useful for this purpose and for future study. But if samples cannot be saved, recording the data mentioned in this article, along with detailed photographs of the brick, can still be useful for determining the brickmaker.
I have spent many years searching for and documenting bricks from brickyards in California in an effort to build a brick archive and database for the purpose of identifying the probable brickmakers of unknown bricks. This has allowed me to identify a brickmaker’s brick in historic buildings and to verify the period of construction which, in a few instances, contributed to saving the buildings.
Pettijohn, F. J. Sedimentary Rocks. Harper & Row, Publishers, Inc., third edition, 1975.
Terry, Richard D., and Chilingar, George V. Comparison chart for estimating percentage composition. Journal of Sedimentary Petrography, v. 25, no. 3, p. 229-234.
Citation: Mosier, Dan L. Method For Identifying a Brickmaker’s Brick. California Bricks, https://californiabricks.com, 2022.