By Mark Soeth

Mark SoethThe American Society for Testing and Materials (ASTM) defines a brick as "a solid masonry unit of clay or shale, usually formed into a rectangular prism while plastic and burned or fired in a kiln." This article is about bricks, how they are made, their material properties, how they fail and what can be done to ensure maximum life in service.

The Lodi Historical Society building was constructed in 1880. Bricks made during this period of time were primarily clay with the possible addition of sand and water. Sand could have been added to improve the consistency and quality of the finished product. Water would have been used as necessary to ensure workability of the clay.

Two processes are used to shape bricks. In the soft mud process, machines are used to press clay with a water content of 20%-30% into molds. In the stiff mud process, clay with a water content of 12%-15% is extruded through a die and then cut to form bricks. The soft mud process developed with the advent of the Industrial Revolution and was in common use by about 1840. The stiff mud process came later, about 1860. Both processes are in use today, although the stiff mud process accounts for the vast majority of current production.

After bricks are shaped they are burned in a kiln. Burning drives off water, oxidizes combustible impurities and vitrifies remaining material into a homogenous mass. Vitrification is the replacement of an ordered, crystalline structure by an amorphous, disordered glass. Originally this process occurred in field kilns or clamps which were constructed near clay pits. With the advent of the Industrial Revolution periodic kilns and, after 1860, continuous kilns were introduced. These kilns were capable of firing the large quantities of brick made available through mechanized forming processes.

During the last half of the 19th century the brick industry began to provide products of consistent quality, in large volume at low cost. This fact, in combination with the innate fire resistance of the material led to selection of brick as the material of choice by the architects and engineers responsible for building America's cities.

The generic term ‘clay' is actually representative of a complex group of silica minerals characterized by small particle size, the presence of alumina, a layered crystalline structure and the ability to adsorb water. A clay particle is less than 4 microns in diameter, or about 1/50 the size of a human hair. The presence of silica allows vitrification to proceed. The layered structure and absorption capability permit the material to be molded. Clay, when burned at approximately 1800 degrees Fahrenheit for approximately 80 hours, becomes a brick - a homogenous, slightly porous ceramic material.

Bricks are fairly strong in compression. A typical face (exterior) brick of the period is capable of resisting a compressive force of 2,000 lbs/in2 (psi). Multiplying this by the horizontal surface area of the bricks used in the Lodi Historical Society building (8" X 3 ¾") implies that each brick is capable of supporting 60,000 pounds. This ability to resist crushing is contrasted with a much lower ability to resist transverse loading. Transverse loading occurs in the presence of uneven loads. Values at failure under transverse loading typically range from 1/3 to 1/10 of compressive values.

Uneven loading is a key concept; it plays a significant role in the majority of brick failures. These failures have three general causes: 1) Water 2) Salt 3) Use of incompatible adjacent materials. Although each of these will be described separately, it is important to remember that in reality some or all of them act together.

Moisture in the form of rain or fog is usually not important in the context of masonry decay. This type of moisture is too transitory in nature to impact the material. Moisture that results in prolonged soaking of the material is a different story. This type of moisture can originate from a leaky roof or gutter, from subsoil moisture (rising damp) or even from consistently high levels of humidity in the atmosphere. Because brick is permeable and semi-porous, water in a gaseous or liquid state can enter. What happens next depends on the chemical composition of the water, the degree of porosity/permeability of the material and temperature fluctuations, both in the material and the surrounding atmosphere.

In temperate climates like that of Lodi, the most common moisture related decay mechanism is freeze/thaw cycling. Water reaches its maximum density at just over 7 degrees F and expands about 9% as it freezes. This expansion occurs because the freezing process is accompanied by formation of ice crystals requiring more space than the more disorderly liquid or gaseous state. The force exerted by this process in a confined area is significant. It is about 972 psi for every degree of temperature decrease. This force is more than enough to fracture brick. When that fact is added to the approximately 120 freeze/thaw cycles experienced during a typical year in this area, the potential for extensive damage becomes clear.

Crystallization also plays an important role in decay stemming from the presence of salts. Salts are neutrally balanced ionic compounds that form crystals. This is a broad definition including not only common table salt (NaCl) but gypsum (Ca SO4 2H2O) and Silver Chloride (AgCl, used in film). Salts can be present as an impurity in the brick, created through chemical reaction of the brick with acid rain (ion stripping) or can be transported into the brick during the course of routine maintenance. De-icing of walkways adjacent to masonry walls falls into the latter category. In this case halite (NaCl) or more recently calcium chloride (CaCl2) is spread on walkways, liquefies in solution with water and is splashed on the building where it moves into the bricks and crystallizes. Halite at typical concentration levels and 32 degrees F is capable of creating crystallization pressures in excess of 8,000 psi.

The final source of brick decay is use of incompatible materials. In the case of masonry buildings constructed prior to 1895 the most common related failure is improper repointing. That date is important because it marks the approximate time when Portland cement started to come into wide use as a replacement for lime in mortar. Portland cement can be as much as 40 times stronger in compression than lime. Mortars that rely on it as a binder are not only harder than the original lime based material, but often harder than the bricks they surround. This fact reverses the intended relationship between mortar and brick in a masonry wall. Instead of mortar failing preferentially to brick the opposite occurs. In this situation bricks become subject to flaking, spalling and particulate erosion. Once disaggregation begins and the face of the brick is compromised, further damage cannot be stopped, only delayed. In reality material failure tends to accelerate over time as additional surfaces are exposed to decay processes.

The best way to prolong the life of a masonry wall is to institute a good program of preventative maintenance. Ensure that roofs, gutters and drainage systems are in good repair and acting together in order to direct water away from vertical surfaces. Do not allow water to pool in the vicinity of building foundations. Use de-icing salts only when necessary and supplement them with sand to provide good traction. When mortar repairs are necessary always use mortar that is softer than adjacent bricks. Avoid use of caulking compounds as a substitute for mortar. These bonds tend to separate over time and channel water into the structure. Finally, all repointing should be to an adequate depth at least two times joint height.

Visual observation of the Historical Society Building exterior allows tentative conclusions to be drawn about the bricks and their decay over time.

The clay used in the bricks contained im- purities that burned off during firing. Evidence of this can be seen in the number of voids visible throughout the material.

The bricks themselves are fairly soft. The thin facing (exterior red color) and the salmon colored biscuit (interior) are the result of low firing temperatures, perhaps in combination with a limited period of time in the kiln. Softness is also indicated by the pervasive rounded aspect of bricks that exceeds normal weathering patterns. Finally, the finish of the brick units indicates they were probably made by the soft mud process in waterstruck molds. This process results in softer bricks than the stiff mud process.

The combination of impure clay, soft brick and substantial dimensional tolerances leads to the conclusion that the brick, if not made locally, was obtained in the region. Examination of an individual brick may reveal a frog (indentation) marked with the manufacturer's name. This was a common practice at the time.

Maintenance in terms of moisture control is well in hand. The roof is in good condition, flashings and external gutters are in good shape and the structure is sited to allow adequate drainage. Prior repairs to mortar joints are causing problems. These repairs have incorporated cement, resulting in a hard mortar that has accelerated brick decay. In addition, the cementitious mortar may have introduced salts, observable as thin white films, which could be acting to exacerbate damage. A possible solution to this would be initiation of a selective repair program using lime-based mortar and application of a breathable coating to temporarily protect exposed surfaces.

The Lodi Historical Society building is a significant structure, one we are fortunate to have and challenged to preserve. Understanding materials used to construct the building allows us to ensure its availability for future generations.

(Mark Soeth has an undergraduate degree in Engineering from the U.S. Military Academy and graduate degrees in Business from M.I.T., Public Policy from Johns Hopkins and Historic Preservation from Columbia. He is the owner of a preservation planning and architectural conservation firm, as well as a member of the Lodi Historical Society. Mark can be reached via his website at www.abconserv.com.)