Tailgate Lab: Matching Mortars on the Fly

“From an altitude of 36 centimeters – we were lying prone, elbows down, chins in hand – the assembled gravel, like a new Hampshire boulder field, could be seen for what it was: unique sculpture from distinct terrane in widely separated distant worlds.”
 – John McPhee, 1996 New Yorker article “The Gravel Page


The last few days I have been looking at several early to late 19th century buildings at the foot of the Blue Ridge Mountains.

To match the historic mortar, we need to find the aggregate source.



Yesterday I took some mortar from an area of failing mortar and parging. Once the portland materials are removed, grade changes made to effectively move surface water away, plus the salts poulticed out of the lower masonry, then missing mortar will need to be replaced.   Since the sand used to make the mortar as well as the clay for the bricks was almost certainly from this site, it makes sense to find it and use it since it will match exactly.

trunk on tailgate

I travel with a small laboratory so that I can do analysis on the go, in this case conducting a mortar analysis on the tailgate of my truck. The important thing to recognize though is that when your goal is to match an historic mortar, the source is local. Taking samples and shipping them off to a laboratory, sometimes many states away, isn’t really the way to success.

Looking around you is the key.

I first recognized this in the 1990s at Holly Hill in southern Maryland after having a series of experts give me mortar analysis results that couldn’t make a matching mortar. But I realized while walking the property I was seeing washout areas that had the same unique aggregate particles that were in the mortar. Since then I have always matched mortars on projects East of the Mississippi by finding local sand sources. On some projects this is easier than on others. When people think of local sand, they think of rivers and creeks. These certainly could have been an early source (and in the case of beaches, one that has shifted over time), but waterways aren’t the only source of sand.

In fact, far more often, the sand used during constructing a building would have come from a pit … not infrequently from excavation of that very building’s foundations. This was most clearly brought home to us on a project near Warm Springs, Georgia a few years ago when we began trying to match different colored pointing and stucco mortars in the three periods of construction of a home built in stages from the 1830s to 1890s. Six mortar colors but they all shared the same types of aggregate. When we first dug up the soil, we found typical red Georgia clay and couldn’t believe we would ever find any of the same sands, let alone all of them. But washing the clay-rich soil to varying degrees eventually netted us all the aggregate for each period of construction. (We were lucky there to have access to the grounds of the property next door for digging, but sometimes the search is a bit more challenging when you can’t dig nearby.)

So how do we find a good local aggregate mortar match?

crushed mortar

pushing thru sieve

I usually start by crushing mortar through sieves like a #8, 16 and 30. This indicates the aggregate range and type. Also I can see if there may be a carbonate component that would disappear during acid digestion.

This is often overlooked. I’ve often heard of people trying to make 1:1 lime:sand mortars because they think that is what the mortar analysis showed … ignoring the fact the geology beneath them is lime-rich. This step of assessing what components of the mortar might be acid-sensitive before starting digestion is crucial.

wet mortar

The next step is to take the crushed mortar and put it in a straight-sided vial, add water, shake vigorously, and then set it aside for a few minutes. When an older mason first showed me this, I though “Boy, he hasn’t learned very much.” I was wrong. Bill Forsythe has never been short on great tips and every year I become more aware how much valuable information can be gained this way.

The sand, lime, fines, and other materials in the vial stratify. Instantly you can see the general sizes of the aggregate and the amount of silt relative to the sand. Usually there is a distinct band of lime on top which often mirrors the ratio of lime to sand found after digestion.

In this case though,  there is almost no lime visible. This is because we are using what was largely sand that had washed out of the mortar joints from behind the hard cement parging, where the lime binder had largely been dissolved. This mortar also has some chunks of unfired limestone showing up in the aggregate. The wet color of the silt (sometimes several silts stratify out) tells you what to look for in the ground. I usually carry both dry and wet samples of crushed mortar when I start looking nearby for the aggregate.

acid digestion

I take another small sample of crushed mortar and pour some vinegar or lime juice over it. These are acidic but not too strong. Even dilute hydrochloric (muriatic) acid will react with iron componds in the aggregate and produce greenish yellow stains that can completely alter what you see. You want to watch the reaction to see if any of the larger aggregate is dissolving. These acids will quickly work on the lime fraction but are slow to dissolve shell or carbonate stone. It’s important to retain these as they are part of the aggregate fraction and definitely affect the mortar’s texture and appearance.

digested mortarAfter the bubbling stops, pour off the liquid slowly and carefully. Rinse and repeat several times. Pour the aggregate out on a paper towel to pull off excess water and spread the sand on a hot surface. (My black tailgate works great for drying in the sun. Otherwise the engine block, or, when you have electricity, a hot plate or oven.)

Aggregates provide a fingerprint for their locale, highlighting the local geology in miniature. What looked like brick fragments are instead red sandstone.

This aggregate is very fine and mostly made up of clear, white, and tan sharp silica, some feldspar, and a few other minerals I don’t recognize yet. But it is not necessary to be able to name all of the minerals in order to match the mortar. The optical comparison is what is important.

Two quick asides:

* Although this already looks like something we would not think of as structural by modern standards, my experience has been that sometimes the finer mixes make incredibly strong mortar, but they definitely shift the lime:sand ratio and you still need to make sure there is a range of particle sizes that can pack together structurally, even if all towards the finer end.

** Although we are talking about mortar at the moment, I cannot stress enough how important getting the aggregates right is to making a matching stucco. Raking light will not cut you any slack if you don’t have the right mix of particle shapes, sizes, packing and silts when patching into an existing stucco. Get the stucco mortar formula match right and seamlessly patching into the existing stucco is within reach.

Extra points question: Where does the lime go? What happens to the calcium portion of the calcium carbonate when reacted with acidic compound to release carbon dioxide? Find out here.

sand from pondConsidering a local water source that might have provided the builders with a pre-cleaned source, I began with the pond at the bottom of the hill. Rinsing sand from the edge and looking at it with my loupe, sure enough, the right minerals and sizes were there. But it was too clean, with no silts and fewer of the red sandstone fragments. To match an original mortar you need the clays and silts for color that are normally stripped out of modern sand sources. Trying to replace them with pigments gives a monolithic color, but not the nuance that silts provide.

Equally overlooked is how these clays give better working properties. Depending on the local geology, the mortar may have to be aged to stabilize the zeolotic properties of some clays. But, if you want the best working properties, your mortar (with either “clean” or silt-laden fines) should be aged a month before use anyway. If you are planning to cut corners on the aging though, you definitely need to know more about the silt source.


While it is now obvious from the pond sample that my sand is local, how do I find an accessible source? A decade ago, I would have asked a local soil geologist where to look for a particular sand, but I’ve come to learn this is an easy problem to solve.  In fact, I remember once finally pinpointing a particular aggregate with the local soil geologist and driving out to the location only to find the steam rollers compressing twenty acres of fresh asphalt in the parking lot of a Food Lion. Normally I would drive around looking for exposed soil that had the right color for my samples. But this time it had started to rain prematurely, so I was preparing to pack it in for the day when I got to wondering if technology couldn’t help me find a source.

With Google Maps in Satellite View I searched nearby for signs of accessible bare earth. Within a mile I found two construction sites hidden by trees that never would have let me find them just driving down the road. Sampling the soil from one of these pits and comparing it to the aggregate resulting from our mortar showed a good match. The cleaned sand is a bit brighter looking because it was just digested in acid, whereas the sand from the ground is a bit muddier because it has only been rinsed. But clearly the particle distribution and minerals are the same.

aggregate from mortar2 aggregate from hill

I hope it is becoming clear that a tremendous amount of analysis can be achieved in field with a relatively basic kit with glassware, a balance, sieves, mortar and pestle, sample bags, a scale, small microscope, pH meter and/or pH paper, various chemicals for testing such as potassium iodide for lead, flashlights, small containers, a 365- and a 405 uv flashlight, masonry take-apart tools, a labeler, note pad, pens, etc.

Credit where credit is due: An homage to John McPhee’s excellent writing 

In the 1960’s, while I was working as an apprentice in several trades, I read constantly, mostly non-fiction. It was at this point I became interested in McPhee’s writing with books such as Oranges and the Pine Barrens, which drew me in with his ability to explain intricate technical aspects and cover a vast amount of historical context. By the early 70’s I was reading articles by him in the New Yorker magazine and it may have been one of them that got me interested in geology. In 1981 I read the John McPhee book Basin and Range, which for me was a great introduction to geology. 

In 1982 I was asked to carry out extensive repairs to a house that had been built in stages beginning in the 1690’s, with two more additions in the eighteenth century. To this point I had no experience with masonry, but here, for the first time, I was faced with an extensive pointing project and needed to make a compatible matching mortar.  I read everything I could find on historical mortars. None seemed to relate to these early mortars so I hired a mortar expert who visited the site, took a sample and a few weeks later sent me a mortar analysis and a recommended formula. The original mortar in the building was a pale grey brown with many pieces of dark shell, large ironstone aggregate, pieces of charcoal, and smaller clear and white sand. The analysis showed some sieve information that seemed irrelevant, but stated the binder-to-aggregate ratio was 1:3. The formula given was one part S-type lime, one part white portland cement, and six parts of builders sand. Needless to say, when I made a batch, it didn’t look anything like the mortar in the building. In fact, without having those dark blue mussel shells fragments of an early mortar, it never stood a chance of looking right. (More on how to fire these for 17th and early 18th mortars versus the white oyster shells for mid-18th century coastal mortar replicas will be in an upcoming post.)

I began my own analysis by crushing up some sound original mortar and sorting out the sand, the shell, and the lime. I wasn’t really getting anywhere until one day when I was out target shooting with the caretaker and saw a sand wash on the side of a hill. I realized immediately I was looking at the source for the sand I was seeking.

That was the beginning of my interest in historic masonry materials and practices. And of course, it is so obvious that these buildings have always expressed the natural and geologic history of the area where they were created. I learned that geologists know a great deal about the minerals and stone from which these buildings are constructed. Geologists thus became a constant reference point for me as I learned to understand the deterioration of these materials in the weathering environment.  

This one project in 1982 opened my eyes, and a big project at Wyck in Philadelphia in 1994 would fully push open the door for what would become a decades-long journey for me to improve upon masonry restoration techniques. Too often the way historic masonry is treated is embarrassingly inappropriate both in appearance and materials.  Doing better work requires understanding the chemistry of lime, the historical methods of making lime materials, as well as learning about the geology of the area surrounding the building in question. By the mid 90’s I was no longer doing much of the wood and finish conservation in which I had spent my first 20 years in conservation. Now almost all my work was with masonry. McPhee’s article in the January 29, 1996 New Yorker magazine titled “The Gravel Page” gave me so much momentum and inspiration – and grounded me with the knowledge that sand really was as distinct as a fingerprint.

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