After topping out and dry fitting the key bricks, the next step was to carve the granite key stones for each side. I had brought back some scrap granite from a project in MA last year. ( Clark, I owe you a post about that one..it was granite steps.) Click on the pictures to enlarge them if you wish.
The candidates. I just so happened to have two pieces that are the same thickness ( 6.5″ ) and large enough for the template I made.
Then I used a template to put the shape of the keystone on the granite.
Next I used my Carbide Tracer to remove large stock from the granite. *Read about the technique here* The rest of the stock was sawed off because it was too close to the pattern to use a chisel. This is the best way to prevent a bad break…remember, these are reclaimed and have been handled many times. There may be a stress fissure that cannot be seen..until it breaks off of your chisel line and ruins the piece. None of the saw marks with be visible when the keystone is done.
After sawing off as much as possible, I relief cut to my pattern line and used assorted carbide chisels to take off the stock.
The only part that will be seen on the keystone besides the face will be the intrados ( inner curve ). For shaping this out I used my pneumatic carver made by Trow and Holden and I have an upcoming post about these great tools. I used a carbide 4 tooth chisel to remove stock and add texture to the intrados to take away that manufactured look that you get from saw cuts. After all signs of the saw were removed, I softened the edges with a 32 point bush hammer. I like to do this because it brings back a hand-made quality and kind of ages the piece.
After a couple dry fits to check how the lines transition the brick arch, I mortared in the row of brick keys and the granite key stones. This is the rear facing key…
Front facing key.
All mortared in. The mortar between the keys is about 1.5″ at the point because the brick keys had a flat top. This was because cutting diamond shape keys would have wasted a lot of the remaining whole brick and I may need them for finishing on the oven.
In the next post the form gets removed and the brick is cleaned and pointed.
I would of stacked bricks up inside, put clay over the top to shape, but maybe wood forms is faster and cheaper for you.
Where is the center of the circles?
I assume you are loading the vaults on the sides, so i bet there will be almost 0 horizontal thrust but all straight down!
Matt Sevigny says
Yup, wood forms are the only way to go for me.
Center of the circles? You lost me there buddy..I’m not sure what you mean.
The most of the weight will be roughly center/forward on the vault and the mass of the sides is designed to counter any horizontal thrust. Essentially, the sides serve as buttresses (not telling you anything you don’t know ) and if my calculations are correct then I have more than I need to keep this thing from collapsing!
The two sides of the arch are part of a circle, where was the centers? hope that makes sense. Well for that matter, are the semi circular?
Matt Sevigny says
Ok gotcha. It ended up one point on the outside of the intrados.
Very cool. Thanks for the thorough step-by-step. I can’t imagine that granite is an easy stone to shape. Although maybe the strength gives you a little more control, albeit at a slower rate. (Slow can be good).
I missed your #50 the other day, so belated congrats on #51!
Matt Sevigny says
True about granite, its hardness can be both a strength and a weakness…and yes slower is good!