Here are the top 5 ways to tell if brick mortar is historic or contemporary
Last week, we looked at 5 of the best ways to determine if a brick is historic or modern. You can take a look back at that article at this link. Today we look at a similar but arguably even more important topic: the top 5 ways to determine if mortar is historic or modern.
Selecting the correct mortar for a pointing or facade restoration project is crucial. By using an improper mortar, it can cause significant detrimental physical damage to the condition of the wall and cause accelerated deterioration and premature failure. This type of failure manifests through bricks cracking at the face, called spalling, and internal breakdown into small subcomponents and dust.
In several past articles, we have discussed the damage caused by improper tuck pointing, also known as pointing or repointing. The article at this link, for example, on our company’s website goes into detail related to damage and destruction to masonry and brick facades caused by improper brick pointing. Basically, using the wrong mortar can cause a wall to crumble.
Proper historic brick pointing requires understanding the age and characteristics of the brick wall assembly. In this case, we’re taking a closer look at the brick mortar. Modern brick mortar is made with a Portland cement binder. Historic brick mortar, the original cohesive element in Washington, DC historic brick masonry buildings, by comparison, was made with a lime cement.
There are 5 easy signs to distinguish historic lime mortar from modern portland mortar. They are NOT as solid and certain as simply taking a mortar sample to a laboratory for analysis. However, in the field, while analyzing in situ conditions, it is very helpful to understand these concepts:
- Mortar Color
- Mortar Consistency
- Mortar Density
- Lime Clumps
- Absorption Rate
*Also, it isn’t included in the list above, but the age of the building can give us a lot of insight into solving this puzzle.
Portland cement is typically much darker than lime cement. There are more components in mortar than just cement, but generally the mortar colors have the same distinction. Portland is interestingly named after the color of the rocks of the islands off the coast of Portland, Maine. At the time that it was invented it was entirely new to the world. Since it had this interesting darker color than traditional cements, it was considered to be a unique property.
By comparison, lime mortar is a very light color. Builder’s sand in particular, is generally tan or a light brown color. The sand is an important ingredient in both types of mortar. However, the color of the binder itself is significantly different between the two types of mortar. Portland cement, as described above, is very dark, whereas; lime is extremely light in color, almost pure white in some cases. Therefore, lime mortar is a very light color. Often, overtime, for architectural and aesthetic reasons, colorants are added to lime mortar to achieve a particular visual appearance. However, at the non-front facade walls, often common bricks were used historically.
Often in cases of scam pointing, the mortar color alone can be an instant indicator that improper pointing has taken place. Once closer observation identifies a shallow embedment of pointing mortar, often it is indicative of scam pointing.
The image below shows a comparison between Portland mortar, improperly installed in a historic double wythe brick wall, on the left and traditional lime mortar on the right. You can see that the lime mortar is a lighter color. On the right side, you can see the historic common bond.
- Here in this article, we are talking about characteristics which can be identified from the field without sophisticated equipment and or laboratory testing. Fascinatingly, Portland cement is made from very similar materials to lime cement. One of the biggest differences though is that Portland cement is fired twice and lime mortar, depending on the exact process, is generally fired just once. Similar to historic brick kilns it was difficult for historic processes to reach the high temperatures of modern methods. By comparison, Portland mortar is much harder, and has a higher PSI, higher resistance to failure under the pressure of a weight load. PSI stands for pounds per square inch but it really means that a mortar of this type can resist a rating up to the stated capacity, per square inch.
- From a perspective of simple perception, mortar is hard in either case, but particularly in the case of historic brick masonry, lime mortar is technically much softer on a scientific level. It can be crushed much easier than Portland mortar. When using a scratch-awl or probe, for example, a mortar joint can be tested in the field to gauge density, and a probe will penetrate deeper into a lime mortar. A typical probe might be able to be inserted into a lime mortar joint, aged over 80 years, up to a depth of half inch with only 40 to 50 pounds of pressure applied. The same amount of pressure with the same technique may not penetrate more than one millimeter at the surface of a Portland mortar joint.
In the field, even a toughened old fingernail can also be used to tell the difference between old lime mortar density, by gouging the surface. Don’t attempt this method if you have soft hands. The topic of this particular article is focused on methods that can be executed in the field without specialized tools or equipment. However, it’s also possible to remove a portion of mortar from a brick wall and that portion of the mortar can be crushed with a simple tool such as a hammerhead. Without even striking or hitting the sample, just by hand the hammerhead can be used to smush and press down against the mortar sample. From that alone, you can get a perceptive idea of the amount of resistance within the sample. The comparison between samples will alone identify a comparative level of hardness.
Homogeneity is similar to consistency. Portland mortar has fine materials in small sizes which are consolidated and mixed by modern hydraulic and electric equipment, on a large scale, the finer materials and use of large equipment in modern times allows for great consistency and homogeneity in modern portland mortar. Such modern equipment did not exist in the period of history in America between the 1880’s and the early 1930’s, the predominant time of lime mortar masonry construction here in the USA.
The pictures below show examples of a historic lime mortar in two different examples of brick masonry walls with great variation and contrast. By comparison, Portland mortar often looks consistently homogeneous. This facade shows significant signs of deterioration.
In contrast to modern mortars, traditional, historic lime mortars often had large clumps of lime in the mortar. Take a look back at a recent article on the IDS site to see photos of an example of lime mortar with large clumps of lime present at the worn and exposed outer edge of the historic lime mortar joints.
Portland mortar absorbs very little water. It still absorbs a small amount of water, but compared to historic lime mortar, Portland mortar has a very low absorption rate. This can be simply tested by applying a wet sponge to the wall. In the same conditions of exposure to sunlight, the temperature and relative humidity the historic mortar will absorb water much faster. After application of water to the wall with the sponge, you can see the water absorb and dissipate faster at a historic lime mortar brick wall.
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.
In this article we talked about the following terminology and concepts, follow the links below for more related information from the IDS website:
- Portland Cement
- Lime Mortar
These concepts are part of the fundamentals of historic masonry restoration, tuckpointing, and brick repair.
The links in the list above will take you to other articles with more information on defects, failures, preservation and repair of historic masonry. 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, fill out the webform below and drop us a line. We will be in touch if we can help.