Stone arches... without mortar (Fireplace Part III)

The weather outside has turned nasty, with alternating rain, snow, and sleet, so the stone mason and I have moved indoors to work on the fireplace.

I had started on the fireplace months ago (link to that blog entry), but other projects called me away. Pausing the fireplace progress was serendipitous, because now I have a real stone mason to help me. He shows up on alternate days to advise and to help me glue together the dry laid stone puzzle that I have been piecing together.

Recently, I tackled two arches. The first arch (over the kitchen wood storage alcove) is pictured here. After cutting the stones, I decided to test fit them. I placed them on an arched wooden form and wondered if they would stay even without mortar. Curiosity got the best of me, so rather than wait for the stone mason I slowly removed the wooden form. The stones didn't budge! To bask in my miniature victory, I stacked 6 pop cans (full and unopened) on the keystone. I couldn't wait to show the kids and my wife. My 9 year old son's response "Uhh, big deal Dad, I do that with Lincoln Logs all the time." My 7 year old son's response "I'd like to knock that down."

In any case, I took pictures as I made the second stone arch (over the great room wood storage alcove) so I could explain the process. If you ever decide to try this yourself please keep in mind that there are many possible paths to complete any task, I just happen to be particularly adept at finding the slowest path! First I used a piece of flexible Pex pipe to define the shape of the arch. Yes, you can draw a curve (spline) on the computer, but it's nice to see your curve at full scale and bending a piece of pipe is more intuitive than dragging a mouse. After I traced the curve on to a piece of plywood (later to become my wooden form), I entered the dimensions of the curve into the computer and drew the stones that I would need.

I printed out a drawing of the stones complete with dimensions and headed to the back yard with the tools you see here. Each of these tools has an analog in the craft of timber framing... only the raw material has changed. I was surprised that the large rock beneath the framing square provided me with enough stones for the first and second arches. In this picture, the stone is half of its original size because the first arch has already been cut. It was still difficult to maneuver... this is not something you try on saw horses!

The first picture in the collage shows the profile of one small stone, laid out with pencil on the large doner rock. The second picture was taken after I scored the rock along the lay-out lines with a chop saw. The rock was about 9" thick, and the chop saw cut only about 4" into it. In the third picture, I carried the score marks all the way around the big rock. Even though I cut all four sides of the rock, I was not able to cut all of the way through it. I used a chisel and hammer to liberate the smaller piece of rock containing my stone from the larger piece of rock (4th picture). With a more manageable chunk of rock, I was able to carry the chop saw cuts all the way through the small rock to get the stone profile I needed (center picture). Patiently, I tapped the 9" thick stone all the way around its perimeter with the chisel until it split into two stones that were each 4.5" thick (6th picture). In the 7th picture, you see the result - two stones, each a mirror image of the other. I could have used a chop saw to more predictably cut the stone into two usable stones, but the resulting texture would not have been as pleasing as a "split face." After cutting the keystone, the two stones to its left and the two stones to its right, I laid them all out on my plywood form guide only to discover that my method of cutting the stones had introduced enough error that I needed to modify the design of the remaining two stones. Fortunately I had not cut them yet. As I had anticipated, the rocks that the arch would rest upon also dictated some minor modifications to the two stones at the base of the arch. (notice the scribe marks on the base stones in the 8th picture). Finally, I traced the profile of the assembled stones on to the plywood. The shape of my actual arch deviated a bit from my theoretical arch, and I wanted my form to support the real arch, not the theoretical arch, so I cut the plywood to match the real arch! In the final picture, you can see the arch assembled! This great room arch has 7 stones whereas my kitchen arch had 5, so I was not so bold as to remove the form before adding mortar and surrounding stones!

If you've read this far, you might be interested in one bit of minutia that may or may not matter. I've decided that it does matter, so I took great pains to pay attention to this detail... Every stone laid in our house so far (outside and inside) is laid just as it came out of the ground, with the "bedding plane" or "grain" of the stone parallel to the earth's surface. (Well OK, 50% of the stones are probably upside down, but that doesn't matter!) Based on reading and experimentation, I decided this was important because sedimentary stone like ours splits most easily along its bedding plane. It was deposited millions of years ago one microscopic layer at a time, so if it ever does crack, it likes to crack along these layers. Cracks can weaken the stone, and admit water, which can freeze, further weakening the stone, perhaps causing flakes to spall off. If the cracks are horizontal, they don't matter too much, so we lay the stones horizontal.

Now for the twist... Which way should the grain run for the stones in the arch? Each stone in the arch is pressing radially, toward the base of the arch. For instance, the primary direction of force transfer in the keystone is to the left and right, not downward. If the keystone were to crack and the crack ran horizontally, I would think the bottom of the keystone could fall downward (granted, this is an extreme example). By thinking about which cracks would least affect the structural integrity of the arch, I decided that the grain in the arch stones should be arranged liked spokes on a bicycle wheel, and that's what I did! It meant passing up lots of doner rocks before I found one from which I could cut the suitably sized arch stones pictured here. I might very well abandon this sort of rigor when I have to cut the nine stone arches for our tower!