This past week we started a really exciting series on historic brickmaking for which we went and took a look at a traditional type of brick making process, very similar to the historic methodologies used here in the United States over a 120 years ago.

The outline of this article follows.  Today, we will discuss items #4-7 from the outline below:

1. 1. Substrate Materials in Historic Brick
   2. Substrate Materials in Historic Lime Mortar
   3. 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

We start today, by talking about the steps in how bricks are actually created.

Steps in the Process of Preparing for Historic Brick Firing

The historic process of making bricks shares some fundamental steps with contemporary methods, and the steps in the brick making process are relatively simple, to someone, if they’ve done pottery or ceramics in an art class, for example.  Just like in pottery or ceramics, the moist clay is flexible and pliable, it can be moulded and shaped into a desired form. The wet clay In this condition is called plug.  However, before that clay can be formed and moulded it has to be harvested from the Earth.  

The notable differences, between the historic and modern processes are mainly in the power or fuel sources and the tools and technology used.  When you look at all the pictures in the series, you’ll also see many differences in the kiln assemblies, but those differences have a limited effect of the consistency that vary between the historic and contemporary processes.

The typical steps in the historic process of making bricks:

1. Clay Extraction – Raw clay is extracted from natural deposits in the earth, often done manually using simple tools.
2. Preparation of Clay – The extracted clay is prepared, usually by hand, to remove impurities and achieve a workable consistency.
3. Molding/Forming – The prepared clay is shaped into brick forms using molds, which could be wooden frames. This process is often done manually.
4. Drying: The molded bricks are left to air dry naturally, relying on sunlight and open-air environments.
5. Setting and Firing: The dried bricks are stacked in a kiln, which may be a simple clamp or beehive kiln. Firing is typically done using wood or other locally available fuel sources.
6. Cooling:  The fired bricks are allowed to cool naturally after the firing process.
7. Quality Control:. Quality control is limited and relies on the experience of the craftsmen. Visual inspection and simple tests for hardness may be conducted.
8. Packaging and Distribution. The finished bricks are often used locally or traded within nearby communities.

Historic brickmaking relied on manual tools and simple wooden molds. In contrast, contemporary processes often involve mechanized equipment, automated molding machines, and advanced kiln technology for precision and efficiency.  Historic methods involved manual extraction and preparation of clay, whereas contemporary processes may use heavy machinery and automated systems for these steps.  Historic brickmakers relied on natural air drying, which is a slow process. In contemporary methods, controlled drying environments, such as heated chambers, are used to expedite the drying process.  Historic brick kilns typically used wood or other organic materials as fuel. In modern brickmaking, various fuel sources, including natural gas and electricity, are employed for firing.  Historic brickmakers had limited tools for quality control, and the product’s consistency could vary. Contemporary processes involve rigorous quality control measures, standardized testing, and adherence to specific industry standards.

Historic brickmaking was often localized and small-scale, serving immediate community needs, in small markets. Contemporary brick production is typically larger in scale, catering to regional or even global markets.

While the fundamental steps in making bricks remain consistent over time, the shift from historic to contemporary processes reflects advancements in technology, scale of production, and the standardization of quality control measures in the modern construction industry.

Historic Brick Firing and Vitreousness

Vitreousness is a characteristic generally used in relation to homogenity and bond between the smallest units of materials in the brick. Glass for example it’s mostly comprised silica but has been heated to a temperature that has made the substrate materials become vitreous, In that vitreous state the glass is then impermeable.  Ceramic tile is very similar, most types of ceramic tileI have some degree of vitreousness, but they are not always entirely vitreous. Glaze helps tile achieve its level of “waterproof-ness”, to a large extent, especially in the cases of semi-vitreous kiln fired clays.   Essentially, when clay is fired at a relatively or comparatively lower temperature it will reach a degree of vitreousness but it will not necessarily be completely vitreous. Porcelain, by comparison, is made of very similar substrate materials but is fired at a higher temperature and happens to have a comparatively high degree of vitreousness. Porcelain, for example, will be almost impermeable.   Terracotta, on the other hand, is a type of kiln fired clay material that most Americans are relatively familiar with, as it is commonly used for retail goods like flower pots.   Terracotta can even be found in big box store alrernatives like luttle mom-and’pops stores. Whereas if you were to buy a brick In most cases you’d have to go to either a big box store or a commercial or industrial supply brickyard, or a large-scale distribution location. Terracotta though, can be purchased at almost any small mom-and-pop garden store or even a corner hardware store. They even sell flowers in terracotta pots at the local grocery store In Capitol Hill, DC.

Typically, the red color of a fired brick a reaction of the brick’s iron oxide composition.  Hematite is a form of iron oxide found in clay and earthen soils, Fe2O3. Sometime hematite, in mineral form, is rendered to be used a reddish paint in pottery and earthenware.  

Like clinker bricks, which are different, but also culled from a firing process, the failed bricks in the photo above which are either cracked, deformed, or broken in some way, are set aside but not necessarily discarded in all cases. In some cases those rejected or culled bricks can be used again or recycled into future bricks.  A significant portion of bricks are also used in the brick making process to stack and separate the drying bricks from the soil below.

Brick Hardness and why it Matters, Counterintuitively

Intuitively, there is a common perception that harder and stronger materials are inherently better, especially in the context of construction and building materials. This expectation aligns with the logical assumption that stronger materials would generally offer superior performance. However, this principle doesn’t perfectly apply to bricks.

For both historic and contemporary bricks, the firing process plays a crucial role in determining several of their key performance properties. When bricks are fired at higher temperatures, they exhibit a higher degree of vitrification and increased compressive strength. Vitrification refers to the transformation of the brick’s composition into a glassy or non-porous state. This results in bricks with reduced permeability compared to those fired at lower temperatures. Additionally, higher vitrification contributes to a more solid structure, yielding a slightly higher tensile strength.

Despite their intrinsic advantages in compressive strength and vitrification, bricks, along with other silica-based earthen materials such as concrete, stone, and stucco, share a common limitation—they have relatively weak tensile strength. Tensile strength is the ability of a material to resist a force trying to bend it or pull it apart. In the case of bricks, their tensile strength is notably low.

This inherent weakness in tensile strength poses challenges in applications where materials may be subjected to pulling or stretching forces. While bricks intrinsically have very high compressive force resistance, bricks are intrinsically weak in tensile strenghth.  They are prone to cracking or breaking when subjected to tensile forces. This characteristic is a fundamental aspect of the nature of silica-based earthen materials and is not unique to bricks alone.

However when an assembly is properly designed, brick masonry is not exposed to extreme tensile forces and this issue is very limited.  To overcome this limitation, builders often employ design strategies that distribute loads more evenly or incorporate materials with higher tensile strength in conjunction with bricks. Reinforcement with materials like steel or the use of structural elements that bear tension, such as arches or lintels, helps compensate for the weak tensile strength of bricks. Understanding the nuanced properties of building materials allows architects and engineers to make informed decisions, balancing the desirable attributes of hardness and compressive strength with the need to address tensile forces in a comprehensive and effective manner.

Compressive strength, in contrast to tensile strength, as it applies to bricks and masonry, representing the material’s ability to withstand axial loads or forces applied perpendicular to its surface. In the context of bricks, compressive strength is a measure of their resistance to crushing or deformation under pressure. When bricks are subjected to vertical loads, such as the weight of a structure or additional loads from above, their compressive strength determines their capacity to withstand these forces without collapsing or breaking. Higher firing temperatures during the brick manufacturing process often result in increased compressive strength, making the bricks more resistant to crushing. This property is vital in ensuring the structural integrity of masonry constructions, as it influences the load-bearing capacity and durability of the entire assembly. Engineers and architects carefully consider compressive strength when selecting bricks for various applications, aiming to match the material’s capacity with the anticipated loads the structure will bear. Additionally, the understanding of compressive strength guides the design and construction of masonry elements, ensuring they meet safety standards and contribute to the stability and longevity of the built environment.

Repointing Historic Brick Masonry

Repointing historic brickwork with new lime mortar is a meticulous process that prioritizes the preservation of structural integrity and authenticity in heritage buildings. Lime mortar, traditionally used in historic masonry, is specifically made for its compatibility with old bricks, allowing for modulus of elasticity and permeability to prevent damage for natural micro-movement and greater movement and pressures during freeze-thaw cycles. Modern mortar can be too rigid and lead to long-term damage to the bricks of a building. Before repointing, a contractor who ks skilled and knowledgeable on the extensive requirements of this work must conduct an assessment of the existing mortar. Correcting previous mistakes, such as the use of inappropriate materials, is extremely complicated in some cases but may be necessary. The old deteriorated mortar is then removed at the wall surface and new lime mortar is applied through a repointing process.  This work must be conducted under proper environmental conditions with attention to pre-hydration and proper mixture of the mortar.  Repointing us needed to preserve the historic facades of the building, with the application of ongoing maintenance without compromising authenticity. Regular inspection and maintenance are crucial to monitor and address signs of deterioration promptly. Overall, repointing with lime mortar contributes to the sustained beauty and structural stability of the historic brick buildings of Washington, DC.

Overall, brick repointing is a central part of the historic masonry preservation process, but repointing a brick facade alone is not the only element required in full-scale facade restoration.  Other types of stone and facade restoration elements are required to keep a building both weatherproof and in good shape for the long-term. Deterioration of historic buildings happens in a gradual but non linear form. This nonlinear relationship is reflected in the fact that as elements of a building go without proper care and or restoration or preservation, their respective deterioration continues but increases at an increasing rate. So not only does it get worse, but it declines faster and faster  an increasing rate.   We often, in our blogs on restoration and preservation mention the fact that the curve of deterioration, particularly with a focus on historic masonry elements, increases faster and faster, without proper ongoing care.

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.