Blue Ridge Mortarmaking

It’s about time for a change of subject (to something really different like carbon fiber), but before we do, I wanted to put the awesomeness of fractal geology into perspective as we finish up this segment on making mortar.

Blue Ridge Mountains

Now that we have located a matching aggregate, it’s time to make some mortar. In the process we will also try and understand more about the geology of this area as well as explore the fractal nature of geological deposits. We are digging in the foothills of the Blue Ridge mountains. They are part of the Appalachian mountain chain which stretches form Newfoundland to Alabama. These are one of the oldest mountain ranges on earth. At one time, these were as high as the Rockies or the Himalayas are now, but they have been weathered down over hundreds of millions of years. This Piedmont province is primarily underlain with granitic rock, quartzite, phyllite, and acrostic sandstone which for me is an interesting and unfamiliar geology for building materials, very different that what I encounter along the coastal plains.

granite outcroppingsThe underlying granite peaks out from the side of roadways here, but few in the area may recognize the rocks that populate their backyards are this same granite worn down.

Once you digest a mortar, you may not be able to identify or name the minerals, but you can begin recognizing the same aggregate under your feet.

photo 2As we began rinsing the aggregate from this site, we realized we had seen these same minerals in DC. But there they were more rounded from tumbling river action as they  passed over the fall line, with the more soluble components dissolving, before being deposited where the Potomac River fans out into the marshes on which the Capitol city is built.


Acrostic sandstone – which comes from the breakdown of granitic rocks – is found throughout. Typically it contains loosely clay-bound stream bed depositions of quartz, feldspar, potash and granitic rock fragments that can be worn down into the components by crushing or river action and dissolution.

3     first-sieve

I pushed the soil through a 1/4 inch screen.

After initial sieving, but before crushing, there is a lot of larger pieces. I crushed them as best I could and further screened them. Many were too hard to crush this way so I set them aside in case we later decided to use some mechanical means to break them up.

6    7

My low-tech version of mechanical crushing for this small batch.

In the second round of sieving, I further crushed the soil while trying to push and grind the aggregate across the screen.

8    9

This is the aggregate retained on the screen after sieving … and the sieved aggregate, now ready for washing.

When you are looking at a pile of rocks and dirt, it is easy to loose perspective on what makes up those aggregates. I decided to take a handful of the rocks that had not gone through screen and put them in a rock tumbler so I could polish them up for easier identification at a scale I could see with the naked eye. Of course I’m now thinking of a side career in jewelry making these things are so beautiful.


You will also notice that the shapes and percentages or particle distribution are similar, regardless of the scale you are at, whether looking at the mountains or soil beneath your feet. This is the fractal quality of geology. Whether in the distribution of big boulders seen along Skyline Drive or the size of rocks people dig out of their gardens which match those in this picture, or the grains of “sand” in the mortar in historic buildings of the area, the same proportions of the same minerals are showing up, just in different stages of the weathering process from mountains down to soil.

What we are looking at here is what geologists would refer to as poorly sorted. In other words, there is a wide variability in the size of particles next to one another.

11     12

Silty mortar below the watertable … versus the upper wall mortar which is cleaner or more free of silt.

There is such care given in the joints up high, that I now wonder whether the lower wall was always parged and only more recently the failing softer early lime parging was replaced with cements.

So now, it is time to begin rinsing the aggregate to determine how much washing are needed to match the mortar both above and below the watertable.

13     14

Again, the mortars do look very different, but the aggregate is the same, only much more of the silts have been washed from the mortar above the watertable.

First you will notice just in beginning to shake or disturb the water that the silts are lifted as the aggregate falls. We will use this tumbling of water to wash the silts away, pouring off the silts from the top, before adding and rinsing with fresh water. The question is how many rinses will be needed.

After three washes, the volume of the aggregate is shrinking as the silt washes out. Individual aggregate grains are now visible. It is time to make a 1:3 lime:aggregate batch of mortar to see if this matches the dark mortar below the watertable.


Sure enough, the wet and cured mortar samples at three washings are a good match.


It ends up taking ten washings to get most of the silt out to make a good mortar match to the upper wall mortar.

Now I think it is time for a change of subject. Coming up next: an introduction to uses of modern carbon fiber technology for repair and reinforcement in historic structures, and then a little bit on woodwork.

This entry was posted in Mortars and tagged , , , . Bookmark the permalink.