Sharing the Mysteries of Mortar

Finding the right recipe to replace crumbling joints in historic buildings can be the key to preserving them. I conducted two recent trainings to show how.

by Catherine Cooper

A man in National Park Service uniform, hat, and N95 mask uses a hammer and chisel to take a mortar sample from an old brick building.
Southern Four Corners Parks Integrated Resource Manager Keith Lyons takes a sample of mortar from the Hubbell Trading Post.

Image credit: NPS / Catherine Cooper

It started with an email on a June evening in 2021 asking, “Catherine, want to go to Missouri?” I’m a research scientist at the National Center for Preservation Technology and Training. It’s in Natchitoches, a city of about 17,000 residents in Louisiana, replete with Native American, Spanish Colonial, and French Colonial history. I help people understand cultural resources like historic buildings and how to protect them through scientific analysis. When I get an email with a question or request about historic preservation, I'm thrilled.

The email was from Sainte Genevieve National Historical Park, which joined the National Park System in 2020. The park was asking for someone to train staff on how to sample and understand materials used in its historic buildings, such as the mortar in the Jean Baptiste Vallé House. A year later, Hubbell Trading Post National Historic Site asked me to visit to take samples and show staff how to analyze mortar. Analyzing mortar is common at the National Center, but getting to teach others how to do it is rare and rewarding. I jumped at the chance to conduct these trainings.

Graphic listing the center’s five subject areas (architecture & engineering, archeology & collections, historic landscapes, materials conservation, and technical services), with three bullet items explaining the NCPTT’s mission. Click to learn more.

Image credit: NPS

Why Study Mortar?

Mortar is a pervasive material in our national parks. You can find it in buildings, cemeteries, walls, and foundations. Builders use it to keep masonry units like bricks or stones together. As with most technology, mortar composition has changed over time. It is important to know how compatible—or incompatible—modern building materials are with historic ones.

In historic buildings, the mortar is sacrificial: the mortar joints divert moisture moving through the masonry, preventing damage to stone or brick building blocks.

In historic buildings, the mortar is sacrificial: the mortar joints divert moisture moving through the masonry, preventing damage to stone or brick building blocks. Over time, builders replaced historic lime mortars with harder and more impermeable mortars like Portland cement, which they began using in the 1800s. Repointing historic buildings with these modern materials could cause the building blocks to absorb moisture and fail, resulting in structural as well as cosmetic damage.

Taking a mortar sample is destructive because we remove the sample from its original context and then digest it into its component parts to understand the composition. This means that we must select and take each sample—whether modern, historic, or archeological— deliberately and carefully. We take samples by carving out small pieces of the mortar with a hammer and chisel while making sure not to damage the surrounding brick or stone masonry. The research question we are trying to answer dictates how we collect samples:

  • Are we trying to match the original mortar? If so, we remove, identify, and analyze a sample of the original. This helps us determine the recipe for a replacement mortar compatible with the masonry units of the historic structure.
  • Are we trying to match the most prevalent mortar? If so, we examine the building to identify the most common mortar and take a sample for analysis.
  • Do we want to know the variety of mortars used on a building? In this case, we choose and analyze multiple samples that represent different repointing campaigns.

Separating the Triad

When mixed, mortars have three main components: an aggregate such as sand to fill space, a binder such as lime or Portland cement to hold the aggregate together, and water to make the mixture workable. As the water evaporates, the mortar hardens and sets, leaving the binder and the aggregate filling the space between the building blocks. The binder affects the hardness and breathability of the mortar. The aggregate contributes to the structure’s stability as well as its texture and color, which are important to the structure’s appearance.

Animation of acid added to crushed mortar in a glass beaker and subsequent foaming.
A lime mortar sample foaming when muriatic (hydrochloric) acid is added. This is the start of acid digestion to determine what the sample was made of. Acid digestion analysis tells us what to use to replace the old mortar.

Image credit: NPS / Jason Church

Acid digestion mortar analysis allows us to break down the sample into its component parts. We grind up the sample with a mortar and pestle, then weigh it. This beginning weight is the whole mortar—binder and aggregate. Common binders such as lime and Portland cement are made from calcium carbonates, which acid will dissolve.

When we add a dilute acid to the ground-up mortar sample, it foams like a baking soda volcano. The reaction helps us determine whether the binder is lime or Portland cement. Lime binders foam violently while Portland cement foams and turns the solution bright yellow.

Once the binder dissolves in the acid, we rinse it out of the sample with water. We dry the remaining aggregate and weigh it. We can find the binder-to-aggregate ratio by dividing the weight of what dissolves (binder) by the weight of what remains (aggregate). We then separate the aggregate by particle size to show the different grains that make up the texture of the mortar.

When examining historic mortars, we often also look at materials that were available when the work was done, before shipping became so pervasive.

Historically, people used local materials and construction styles to build homes and other structures. When examining historic mortars, we often also look at materials that were available when the work was done, before shipping became so pervasive. What local sand or gravel deposits were around? How did they change the color or texture of the mortar?

The separation by particle size also shows whether there is any coloring agent. Pigments are finely ground powders that impart color. In this separation, they appear with the finest, clay-sized particles of the aggregate. They can be anything from remnants of local manufacturing processes, such as brick dust or coal dust, to purchased pigments. We use the results of the acid digestion analysis to recreate the mortar recipe and mix a matching mortar for repairing the masonry structure.

Two men with masks and NPS uniforms look at chemicals in glassware stored inside a fume hood.
National Park Service staff members working on acid digestion mortar analysis in the portable fume hood at Sainte Genevieve National Historical Park. The fume hood protects them from toxic gases.

Image credit: NPS / Catherine Cooper

Off to Sainte Genevieve!

On July 26, 2021, I went to Missouri to train staff at Sainte Genevieve, which is only a day’s drive from Natchitoches. I loaded up a van with a ductless portable fume hood and boxes of carefully packed sampling tools, grinding tools, glassware, and acid. It was a typically warm and humid summer evening in Missouri when I got to Sainte Genevieve and checked into my hotel for the night. The next morning, park staff helped me set up a temporary lab in an empty office, and the work began!

Wet environments mean that it is important for historic buildings to be able to "breathe"—for moisture from the ground or rain to be able to evaporate.

Missouri is a relatively wet state. Its 30-year average annual precipitation is currently 30–45 inches, although climate change could affect that. Wet environments mean that it is important for historic buildings to be able to “breathe”—for moisture from the ground or rain to be able to evaporate. Missouri also experiences freeze-thaw cycles, which can cause moisture trapped in walls to expand when frozen and shrink when thawed. Depending on where the moisture is in the structure, this can damage or weaken either the mortar or the masonry units.

Sainte Genevieve National Historical Park cares for a handful of buildings scattered throughout the city of Sainte Genevieve, Missouri. These buildings exemplify early local architectural techniques. For example, the Jean Baptiste Vallé House (constructed in 1794) and Green Tree Tavern (1790) have vertical log poteaux-sur-sole (post-on-sill) construction. The Bauvais-Amoureux House (1792) has poteaux-en-terre (post-in-ground) construction, with bousillage earthen infill.

Park staff trainees took samples of mortar from the masonry basement expansion in the Jean Baptiste- Vallé House over the course of an afternoon. They wanted to find out what mortar recipe had originally been used before the Portland cement repairs. We spent the following two days conducting a full acid digestion analysis of the mortar samples. We found that the original mortar used a softer, more vapor-permeable lime binder with a small proportion of Portland cement to strengthen the mix.

Yellow and white historic wooden building with a masonry foundation.
The National Center for Preservation Technology and Training van parked outside the Jean Baptiste Vallé House at Sainte Genevieve National Historical Park. The van functioned as a mobile lab.

Image credit: NPS / Catherine Cooper

"The staff [at Sainte Genevieve] had a blast working with Dr. Cooper,” said Andy Newman-Johnson, the park’s supervisory facilities operations specialist. “Dr. Cooper was able to take the data and provide a comprehensive deliverable that the park will have for years to come.” He said they would use the results of the analysis “to provide direction for park staff when completing any mortar work” on their historic structures.

Our mortar analyses of samples from the basement of the Jean Baptiste Vallé house indicate that a lime mortar was used when the basement expansion was originally built, but Portland cement had already entered the market. Lime was available in Sainte Genevieve at that time, with a mine only a few short miles away from the house. But even if Portland were available, stretching it with a large proportion of lime would have made economic sense.

Our analyses also showed that previous modern Portland cement mortar repairs do not match the original materials. Those repairs are causing the historic bricks in the basement to dissolve as Missouri moisture is forced through them. The original mixtures we uncovered will be used to craft new repairs to stabilize the house’s basement walls.

Uniformed man's hand holding a scale card against a building with old, orange-brown bricks.
A National Park Service staff member holds a scale card next to a mortar joint to document the sampling site before a sample is removed from the Hubbell Trading Post. The scale card shows the width of the mortar joint.

Image credit: NPS / Catherine Cooper

A Drier Climate and Different Challenges

Sainte Genevieve was one of two recent mortar trainings I did. The other was at Hubbell Trading Post National Historic Site on June 13, 2022. It was for staff from national parks in the Southern Four Corners Group: Navajo National Monument, Canyon de Chelly National Monument, and Hubbell Trading Post. All three parks are within the Navajo Nation in Arizona.

Unlike Missouri, Arizona is an arid state. Its annual average precipitation is currently from less than 4 inches in the southwest to 40 inches in the mountains, and the state is in a drought cycle over 20 years long. The locally available materials also differ significantly from those in Missouri, as do the traditional building techniques. Instead of bousillage, historic earthen architecture in the American Southwest centered around adobe. In adobe buildings, mud is used as a mortar between dried earthen blocks. Many people built with sandstone rather than brick when not using adobe. There was no lime mine nearby. Water in the desert southwest is a precious and scarce resource, and the soils are drier, with a higher proportion of clay.

I used the "Julia Child" method. That iconic chef taught America—in a half-hour show—how to make lengthy, complicated dishes by preparing stages of the dish in advance.

The format of this course was different from Sainte Genevieve because driving the mobile lab to the Hubble Trading Post was impossible with our budgets and schedules. Instead of using a van, I packed sampling tool kits and example materials for each stage of mortar analysis along with example reports into my suitcase to fly out to Arizona. At Hubbell, I trained staff members from the three parks over the course of one morning. I used the "Julia Child" method. That iconic chef taught America—in a half-hour show—how to make lengthy, complicated dishes. She did this by preparing stages of the dish in advance and swapping them during the show. Similarly, I presented my students with the materials I had prepared in advance.

Four Corners staff wanted to find out what mortars have been used over time in the buildings at this historic site and how they varied. Hubbell Trading Post has a small collection of buildings that have changed and grown over its history. The original trading post was expanded multiple times. Buildings were added as needs changed, including a stone hogan guesthouse. We focused on mortar sampling, each person taking a turn identifying where to take a sample and collecting it. I guided trainees through the mortar analysis using a sample report. I used previously prepared samples of mortar, ground mortar, digested mortar, and separated aggregate to show the various stages of acid digestion.

Some of the samples from the structures at Hubbell Trading Post were incredibly hard and took time and much effort to remove. Some were so soft they turned to powder in our hands. As each individual only took one or two samples, the type of situation they were faced with varied. I found that wrapping up the morning with a question-and-answer session helped tie together the various interests, experiences, and goals of the participants.

Those who participated in the full acid digestion analysis were better able to describe and discuss the technical aspects of the report.

The two different training regimens—Sainte Genevieve and Hubbell—were both effective in teaching participants how to take mortar samples for analysis and how to use a mortar analysis report. But those who participated in the full acid digestion analysis at Sainte Genevieve were better able to describe and discuss the technical aspects of the report. This will help them more effectively communicate with staff and visitors about what we are doing to preserve these buildings.

As I packed up my gear to leave Arizona, I found that I had to mail all my tools back to Natchitoches to make room for fourteen mortar samples in my luggage. The drive across the desert from Hubbell to the airport in Albuquerque, New Mexico, showed a completely different landscape—stark and dry with views for miles—compared to the lush green of Missouri. I was struck by the similarity and differences of these two trainings. Both showed the intersection of traditional building materials and techniques with modern materials and care regimens, despite how different the sites were climatically and culturally.

Preserving Our Past Enriches the Present

Historic structures like buildings, cemeteries, and foundations in our national parks need care and maintenance. Scientific research in mortar and other materials is important for taking care of them. The National Center for Preservation Technology and Training is proud to contribute in this unique way to the National Park Service’s mission to protect our cultural heritage.

Many visitors are keenly interested in and emotionally connected to our historic buildings. They care that the National Park Service preserves them. Sampling and analyzing mortar tells us the appropriate materials to use to maintain them. It also shows how they were constructed before the National Park Service managed them and how they are faring under our stewardship. Applying scientific research and training staff help us provide visitors to historic sites with more enjoyable, meaningful, and safer experiences.

Catherine Cooper

About the author
Catherine G. Cooper, PhD, is a research scientist in the Technical Services division of the National Center for Preservation Technology and Training. Image credit: NPS.

Hubbell Trading Post National Historic Site, Ste. Geneviève National Historical Park

Last updated: January 1, 2023