A Portable Lab for Mortar Match Finger-printing of Your Local Geology

“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. Link here to PDF version of this mind-expanding piece.

Rappahannock Virginia Government Buildings, showing the Treasury, Revenue Office and Courthouse
Rappahannock Co VA Government

These several early to late 19th century buildings at the foot of the Blue Ridge Mountains needed a matching mortar made for a series of repairs. To do that, we needed to find the original aggregate source or a close local approximation. Mortars are a fingerprint of sorts for the local geology in most parts of the country through the early 20th century. Sometimes cities brought foreign aggregates via the railroads, but in general, outside cities and for residential construction, looking for an aggregate match is about staying local.


We collected early mortar from the joints in an area of failing mortar and both lime and later cement parging or stucco coats. 

With removal of the cement, water problems at the building can be approached in stepwise fashion until enough water has been redirected. This will likely include grade changes to effectively move surface water away, poulticing deicing salts out of the lower masonry, and then repointing with lime-sand mortar.   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 that source and use it for an exact match.

Portable lab trunk assembled on the truck tailgate for go-anywhere sand sieving and analysis.
Portable lab trunk assembled on the truck tailgate for go-anywhere sand sieving and analysis.

I travel with a small laboratory so mortar analysis can be completed on the go. In this case that meant 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 a recipe fo success. Generally directives come back saying to find a local brown sand and that your original mortar had a certain lime-sand ratio. What is a brown sand? Will a brown sand in your area in any way match your mortar? And just because the original mortar in your building was 1:3 says nothing about the correct ratio for strength of a bagged or trucked-in sand available to you today.

When the mortar analysis checkbox is clicked by sending away for a report that does not give you an exactly matching source (without relying on tweaks from pigments that may not be stable and will change the lime-sand ratio of a mortar and its working properties), your money is being wasted and your building is not getting the best treatment.

So how do you find a matching local aggregate? It’s often surprisingly easy once you make the mental shift to start looking around you.

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 were useless to producing a matching mortar. But by the time all the reports had come back I had spent enough time walking to property and seeing the unique aggregates of the mortar in washouts and tilled fields on the property that I knew what I was looking for was closer at hand. I just needed to stop looking externally for answers.

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. But while waterways provide pre-cleaned sand for early builders, they still required sand to be moved to the building site. (And it should be noted that even when waterways were used, beaches and even river beds do shift with time so the source may also have shifted from one shore to another.)

In fact, far more often, the sand used during constructing a building (and for earlier buildings even the clay for making bricks) 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. There were a total of 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.

On that Georgia project we were lucky to have the neighboring yard to dig up for our mortar, but sometimes we have to go just a bit further afield … but rarely do we need to leave town.

Steps to finding a good local aggregate for your mortar matching needs.

I usually start by crushing mortar through sieves like a #8, 16 and 30. This indicates the aggregate range and type. 

Also — and this is often overlooked — this initially sieving can help indicate if there is a local carbonate component (shell or limestone) that would be lost during acid digestion and in the process completely skew the results.

I cannot tell you how many times I’ve been contacted by people who are trying to make 1:1 lime-sand mortars because they ignored the lime-rich geology beneath them and rushed to acid digestion before accounting for their local conditions. In fact this is one of the most important reasons for considering local aggregate sources to make mortar.

This next step is one I was too arrogant to take seriously when I started.

A wet sample of the crushed mortar helps with visual identification of local sands and the ratio of aggregate and fines
A wet sample of the crushed mortar helps with visual identification of local sands and the ratio of aggregate and fines

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 thought “Boy, he hasn’t learned very much.”

Of course I was wrong. Bill Forsythe has never been short on great tips. 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. Plus, we are often looking at damp soil, not dry samples when out looking for local aggregates. Seeing the mortar separated and wet is a truer representation.

In the case of these government buildings in Rappahannock County though, there was almost no lime remaining in the vial. This is because we had taken samples of what was largely sand remaining in the mortar joints behind the cement parging. In other words the lime binder had largely been dissolved. This aggregate also shows chunks of unfired limestone.

The wet color of the silt (sometimes several silts stratify out) indicates 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. I do not use muriatic acid to dissolve most mortar. This is because even when diluted, hydrochloric acid will react with iron compounds in aggregate to produce greenish yellow stains that will alter the appearance of the sand, complicating any match.

You want to watch the reaction to see if any of the larger aggregate is dissolving. Vinegar or citrus juice will quickly work on the lime putty fraction of a mortar. But it will be much slower to dissolve shell or carbonate stone. These were often part of the aggregate fraction of the mortar, so they should be maintained for that purpose as they definitely affect the mortar’s texture and appearance. In other words, as soon as the lime putty binder portion of the mortar has dissolved, stop the reaction before losing shell and limestone components that belong in the aggregate portion of the mortar and will be a key to locating your local match.

Bits of shell and limestone are often part of the aggregate so do not lose them during acid digestion
Bits of shell and limestone are often part of the aggregate so do not lose them during acid digestion

After the bubbles from the reaction of lime and acid subside, pour off the liquid slowly to retain all the clay fines that are crucial to a good color match. Rinse and repeat several times. Paper towels can be used to remove excess water, and then spread the sand to dry. If you are in a lab you can often use a hot plate or oven for drying the mortar. But don’t be deterred in the field. I’ve used everything from my hot engine block on the truck to the black tailgate for quick drying.

Aggregates provide a fingerprint for their locale, highlighting the local geology in miniature, much as McPhee’s quote at the top of this article suggests. Instead of brick fragments, this mortar in the Virginia Blue Ridge Mountains is showing off the local red sandstone underlying everything.

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. No matter.

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.

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. However fines do tend to significantly 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 of the sieve set.

Although we are focused on mortar for brickwork in this article, I cannot stress enough the importance of getting the local aggregates and ratio of fines to match when making repairs into historic stucco. Raking light will be cruel if an exact match for all of the particle shapes, sizes, and packing and variety of silts has not been achieved. There is no way around finding the original aggregate source when patching into existing stucco. Failure to do this has caused too much original stucco to be lost because contractors and conservators have not done their homework. Get the stucco mortar formula matched to the original aggregate however, 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.

Considering 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.

The local pond located via Google Maps confirms the broader area can provide many of the same minerals as our mortar.
The local pond located via Google Maps confirms the broader area can provide many of the same minerals as our mortar.

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.

The pond samples confirm the sand we are looking for is local, and because of the extensive silts, a pit source is needed.

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.

Google Earth, Google Maps and other phone mapping apps with satellite mode on can help identify nearby bodies of water or sand pits that can be a source for your sand needs you might otherwise have missed.
Google Earth, Google Maps and other phone mapping apps with satellite mode on can help identify nearby bodies of water or sand pits that can be a source for your sand needs you might otherwise have missed.

With Google Maps in Satellite View I searched nearby for signs of accessible bare earth. Within a mile I found two construction sites hidden from the road by trees so that without the aerial access they never would have been sites to approach. Sampling the soil from one of these pits and comparing it to the aggregate from our digested 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.

I hope it is becoming clear that a tremendous amount of analysis can be achieved in field with a basic kit of glassware, a balance or scale, sieves, mortar and pestle, sample bags, small microscope, pH meter/pH paper, various test chemicals such as potassium iodide for lead, small containers, a 365- and a 405 uv flashlight, masonry take-apart tools, a labeler, note pad, pens, etc. This is not something that needs to be left to “experts.”

Hoping to inspire you further to take on the exciting challenge of locating your own sand, creating matching mortars, and relearning the techniques of historic masonry repair, I must again recommend you 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. He drew me in with his ability to explain intricate technical aspects and cover a vast amount of historical context in compact treatises. By the early 70’s I was reading his articles in the New Yorker magazine. Then in 1981 I read the McPhee’s book Basin and Range, which for me was a great introduction to geology.

That timing was good because 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 that point I had no experience with masonry. But on this project I was faced with doing woodwork to a wall that needed extensive repointing of the foundations. That meant I 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 did the logical thing: I hired the industry expert in mortars. He 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 mussel shells for 17th and early 18th mortars versus the white oyster shells for mid-18th century coastal mortar replicas in an upcoming post. Collecting all these shells also makes you a great topic of conversation and recognized face at all the local restaurants and firing shells is a great “Tom Sawyer”-type activity for historic sites when they have an Open House, so what’s not to love about replica mortar making!

Undeterred, 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 for the estate 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 (locally available woods for example, each of which has its own best use) and geologic history of the area where they were created. I learned to appreciate the vast knowledge geologists have about the minerals and stone from which these buildings are constructed, even if buildings are not an area of interest for them. Geologists thus became a constant reference point for me as I came to understand the deterioration of these natural materials in the weathering environment.

That first project in 1982 opened my eyes. A larger 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 starts with 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. The rest will flow from there.

For my first two decades in this field I focused on wooden artifacts and finishes conservation. By the mid-1990’s I was no longer working in wood as nearly all my work rapidly shifted to masonry. The need for good masonry repairs was so great. While that happened because I was one of the few people trying to retain original mortars, plasters and stuccoes (which you can see here that the Wyck project was rare in requiring). But equally important because I was determined to learn how to match the remaining original mortars for infill repairs that were effectively invisible. I think there is still a great need for more people to take up this work and continue to improve the quality of masonry repairs.

Again, I can’t recommend enough that you read some of John McPhee work and in particular the attached copy of his January 29, 1996 New Yorker magazine article titled “The Gravel Page.” You will begin to understand how sand forensics has solved so many international cases. I’m sure that as with me it will give you much momentum and inspiration – and a grounding in the knowledge that sand really is as distinct as the human fingerprint.