Sean goes on more.. . "The inside quoins were another matter. At any point up the wall the batter is different, as it curves in or out the radius of the circle and hence the face changes, in addition the curve on the top and bottom of the stone would have different radii. Basically no two quoins were going to be the same. For it to work, I felt the key was to try and mark everything accurately as close to perfect as possible and then the inevitable imperfections would not matter. Several curbs (heritage dressed granite stone, salvaged from the streets of old San Francisco) were cut to give a stock of various thicknesses/lengths, to allow for a choice when fitting (and as they were much thinner than the outside quoins, for me to work out all the possible combinations to get to a level to match the outside for the throughs). Whilst fairly regular, the kerbs were not machined, and so additional ones were also made to order later.
With runners (ie set along the curve) the first step was to offer the quoin up and mark the inside edge so it could be cut to the correct angle allowing for the 2 ends of the face to be just outside the line of the curve for waste to be chiselled off. Stretchers (running into the doorway) were easier as the long edge was just run along the inside face of the doorway/opening. The quoin was placed on the wall, the profile was placed against either end and the curved batter marked on the stone with a pencil. The bottom edge was marked by running the pencil along the stone it was sitting on. The curve along the top length was then marked by eye (ie approximated- the posh version of ‘guessed’) the important thing was that it didn’t scallop sharply or be too straight either - provided it was only slightly imperfect to the eye it would work. I often drew a straight line joining the marks of the curved ends, and marked the centre of this, I could then see a lot easier if my freehand curve line was too asymmetric, since I could see the offset relative to the straight line.
The inside end was far harder to mark than the exposed end with stones getting in the way of the frame, I’m not sure I can explain that fully or even remember why! However I do remember the exposed end…
The key was to get the batter curve very accurate at the exposed end. In reality we realized from the outset that it was unlikely any face was going to be truly perfect with all the curves involved but the only really visible bit that would catch the eye was that exposed end. I also felt it was important that one person…which happened to be me 😊 was responsible for all the marking, as to a degree, it needed a consistent approach and some of it was approximated - if one person was doing it, it was more likely that the imperfections would at least be reasonably consistent. One thing we did not need was imperfect imperfections! The net result was that I sometimes felt I was monopolizing the fitting, but on the flip side if I’d been chiselling we’d probably still be there now.
Fabrication of these quoins prove heroic I think, we had plenty of material. My exhortations were less plaintive just pointing out which was the crucial curved end. <<Wish I could remember how mark described the stone - chiselling sugar or something>> Probably one in six went wrong and a corner blew or the stone cracked. I had meant to make at least one quoin and I sometimes regret that I never got around to making even one of them, subliminally the distant curses probably had an effect."
