Our company’s work is largely focused on historic masonry restoration, and over the past few years we’ve talked a lot about the exploration and understanding the details around the distinctions between historic and modern masonry. There are over a 100 “deep-dive” (thorough analysis) type articles on our website about the technical aspects of historic masonry restoration and historic brick repointing, but today we start one of the most exciting series of all, to-date. One of the main goals of our website and all the information we post is to inform our clients and stakeholders about some of the exciting technical areas of historic masonry restoration. We pride ourselves on being one of the most technically competent and dedicated companies in the entire industry of historic masonry restoration. For example, if you look through the archived past articles on our website you will find posts about historic lime kiln firing, manufacturing processes, methodologies used over a 100 years ago in masonry construction and near limitless information about historic masonry preservation.
Today we actually went out of the United States to see the process, in real life, used over a 100 years ago here in the construction of the historic buildings in Washington, DC, for the manufacturing of historic clay kiln fired bricks. We are very excited to present this new series to our readers.
The outline of the new article follows. Today, we will discuss items #1-3 from the outline below:
- Substrate Materials in Historic Brick
- Substrate Materials in Historic Lime Mortar
- Plug and Brick Molds
- Steps in the Process of Preparing for Historic Brick Firing
- Historic Brick Firing and Vitreousness
- Brick Hardness and why it Matters, Counterintuitively
- Repointing Historic Brick Masonry
The picture below shows a brick yard, using the same methodology as used over 120 years ago here in Washington DC for the initial setting of historic bricks. These bricks were initially cast in a form (nould) but set out from the form while wet directly onto the ground to dry in the sun before being fired.
Substrate Materials in Historic Brick
The substrate materials of bricks are mostly pretty basic, More than anything the main substrate material is clay. Clay is basically a type of mud, that is an oversimplification, but it’s the type of material that’s available and ubiquitous almost anywhere in the world. Some of the main ingredients in bricks, based both on volume and weight are silica, aluminum, and iron oxide. Bricks do not behave, on a physical or chemical level, much like a metal, but there is quite a bit of iron oxide inside of bricks which often imparts the deep red color that we are familiar with with the most common of bricks. Bricks have been foundnused in very old historic times throughout most of the world, even within places that had no interconnection or communication to one another, even in places that were basically isolated from one another. Brick, as a building material, is one of the examples of “Parallel Invention” in the history of mankind.
Now is a good time to also mention that although there are some subtle differences between bricks used all over the world, by and large, bricks from different parts of the world are relatively similar. However, there are a few particular types of bricks that are unique and made, even here in America, both in historic and even in contemporary times. We are mostly referring to the two most common types of brick, in historic instruction, the common brick, and the pressed brick. Both types of bricks were actually pressed into a mold, but the brick we today tefer to as a historic pressed brick was normally put under a mechanical press to squeeze the brick very tightly. Impurities such as rocks or twigs or larger trunks of constituent material were filtered out and you can see pressed brick at many Capital Hill rowhome front facades, for example. You can see that the pressed brick at these rohomes have very consistent shape and planar uniform faces. The lines the mortgage joints of pressed brick are very thin, and this is not possible with a typical common brick because there is great variety, both in the substrate materials and the temperature at which common brick was fired. These differences lead to bricks that have sigmificant inconsistencies from one to the very next.
The concept of Parallel Invention is fascinating. Apparently, through historic records in the anthropological history of Mankind, there are numerous examples of isolated peoples creating and using the same type of things throughout the world, interestingly aroubd the same times. Brick is one of the quintessential examples of parallel invention. Basically, on different continents between people who had no intercommunication, around the same time in human history, these peoples began creating and using bricks in the building of their historic structures. The evidence of this parallel invention goes back thousands of years.
It’s hard to believe that people who had no communication between each other would start building things the same way, but brick is a bit of a Is naturally perfect type of material for humans to use. As discussed above, these substrate materials are available in almost all parts of the world and the use of brick is interestingly highly adaptable and highly effective in the building of both homes and/or larger structures. Even without advanced technologies, humans were able to figure out that brick could be created with substrate materials available right from the dirt around them and that creation of bricks of relatively consistent or similar size was a perfect building block for creating walls around shelters and homes.
The earliest bricks were essentially just dried mud, often reinforced with straw or thatch. The straw or thatch provides a bit of tensile reinforcement, in unfired adobe, But, traditional adobe lacks the very Important characteristic of semi-impermebility. Adobe is often found used, historically, and even through to today in relatively drier or airid climates such as the Southwest part of the United States, but it’s not often found lasting long in wet environments.
In some cases horse hair was also used, just as it was used in plaster in later centuries. Unlike sandy mud, muds that are high in clay or even close to pure clay make better bricks. When heated at a very high temperature, clay becomes vitreous, meaning it becomes a bit monolithic and the individual small particles with any brick will fuse together in a bond similar to the way silica will bond and fuse together in glass.
One of the most fascinating parts of the concept of the invention of bricks, so many thousands of years ago, is that today, the technology, while still understood in industrial settings, has largely been lost to regular people. Particularly the area of historic restoration, while so importantly needed in the historic cities of East Coast of the United States, like Washington DC for example, is very poorly understood. It’s difficult to believe that something so ubiquitous could be barely understood by the majority of humans on Earth, even though it’s still a critical building material and was discovered independently throughout the world, almost like it was a natural fit for human beings and part of our evolution.
Substrate Materials in Historic Lime Mortar
Similar to historic brick, the materials used in making historic lime or also basically ubiquitous and available through most parts of the world. It’s very likely that lime was discovered by accident, but, humans throughout the world began using it around the same time in the history of mankind. The earliest lime basically as simple as a mixture of chemicals rich in hydrated calcium hydroxide.
The fascinating thing about lime, as a constituent component of mortar, is that lime was also disvovered, like bricks, and used throughout the world in historic times, even between disparate civilizations where there was no intercommunication. People discovered tabby lime, probably at the earliest point of the use of lime, by avvident when seashells end or limestone were burned in or next to a fire pit. When water was added back to that material, in the ash, it would have a relatively violent chemical reaction with bubbles, pops, steam, and small explosions. The water, added to large amounts of fired lime would become extremely hot, even when the lime itself had cooled, after firing, to ambient temperature. In historic times, people notice this and then later realized that the lime, once hydrated by adding water would become rock hard, it’s like they had created a rock out of nothing more than old pulverized stone or seashells and ash.
People then mixed this lime together with sand and they were basically creating a mortar that would be strong enough, similar to rock, which could lock masonry into place, similar to the modern mortar we are most familiar with today. Historic lime mortar like this was used for thousands of years. On the coast of the Carolinas, for example, from hundreds of years ago there are still remaining standing ruins of old Tabby construction walls. Tabby construction was a mixture of seashells which have been cooked in a fire and sand and other type aggregate materials with stone, set as a binder to hold the stone in place.
Plug and Brick Molds
Historic bricks were larger and therefore heavier than modern bricks, even in the wet stages. Modern bricks are dried more extensively than historic bricks so that’s an additional differentiation which also makes modern or contemporary bricks actually weigh a little bit less than they would otherwise.
Similar yet different than the contemporary clay plug conveyor process, where the mass of clay (called: plug) is extruded from industrial hydraulic equipment, in historic times, the clay mass, also referred to, in historic times as a “loave” for each brick was hand set in a wooden frame or mould like the one shown below. In some cases the historic brick molds were lined at the bottom with a metal ring. The process is a relatively wet process, even though the play is somewhat stiff and it needs to be pounded into the mould, the clay comes from the Earth and is generally at least partially hydrated to the point that it can be formed, a plastic material in nature. These loaves were hand-shaped, essentially squished onto the surface of a tabletop or a board made for this purpose and then forced into the shape of the mold. The general wetness of the material caused the mould to deteriorate and or weaken in use at a relatively accelerated pace. The mould In the picture below is from relatively modern times, but still replicates the historic process and can fit as much as 4 bricks at a time. The edges of the inside of that mould are covered in a plastic laminate which helps decouple or debond the clay from the wood. Without a decoupling material like that, much more sand has to be used to essentially provide a dry release from the mould, in the brick forming process.
Clay relatively close to the surface of the Earth can have a variety of impurities in the clay and must be filtered or culled to an extent to prevent things like organic materials, twigs, sticks, vines, etc. from being formed into the clay brick. In some cases, if those organic materials are left in the clay loaf, when the brick is formed, dried, and then fired, during the firing process The organic material can burn, even within the inside of a brick and create a void which can lead to breakage of the brick or a future passageway or a window / pathway of moisture to run through a brick from the exterior to the interior of a building.
When the clay loaf is pressed into the mould and then released from the mould, the clay brick is generally set direvtly on in flat area of unshaded ground. If the area is in the woods or under treetops, it will be partially or fully shaded and will not dry nearly as fast. Optimal areas for brick drying yards are relatively large and do not have much shade.
Historic masonry upkeep and preservation
To properly maintain, repair, and care for these historic buildings, a knowledge, interest and understanding of historic building principles is required. Here in Washington DC, historic masonry buildings are extremely expensive and the amount of financial loss caused by improper repointing and low quality construction is staggering. However, in addition to the direct financial value of the property, there is also a cultural loss when historic buildings are damaged. By comparison, consider neighboring poor cities, when historic buildings are damaged, it’s not just the loss of value to the property owner, there’s also a loss to all inhabitants and visitors of a city, present and future, who care about architecture, history, and culture.
We encourage all of our clients, and all readers of this article and to our blog in general, to prioritize the historic built environment of Washington DC and neighborhoods such as Capitol Hill, Dupont Circle, and Georgetown and become educated on on the difference between proper historic preservation versus improper work which leads to significant damage to the historic fabric of a building.
From a conservation and preservation perspective, several approaches can be taken to improve conditions related to deteriorated historic brick masonry. Primarily, lime mortar brick joints and low temperature fired soft red clay bricks should be inspected and checked on a routine maintenance schedule, either seasonally or at least annually. If brick masonry is kept in good condition, the life of embedded wood elements can be significantly extended. Hire a professional contractor which specializes, understands and appreciates historic construction elements and buildings.
You can learn a lot more on our blog. Feel free to check it out. If you have questions about the historic masonry of your building in Washington DC, contact us or fill out the webform below and drop us a line. We will be in touch if we can help.