As I said before (Load Bearing Arch, Yikes!) there seems to be way more information on how to set the skewbacks than on how to figure out the arch. And having done it (the arch and the skewbacks) I’m afraid I will do the same thing… sorry. But the thing with the arch is, it really is about playing with it until it ‘looks right’.
So, I laid down a piece of plywood that had the required span marked on it (remembering to leave room for the skewback!) and got to it. Once I was ‘happy’ with it (there really aren’t more concrete parameters) I set an edge along the bottom of it, and a soft brick on top of the first arch brick, and figured out the angle for the skewback. Like so:
Those measurements were transferred to another piece of wood, to make the forms to cast the skewbacks. This is where the mysterious yellow liquid comes in: old veggie oil to use as a resist. Notice that the seams of the forms were filled with thin coils of clay (before applying the oil). This not only prevents things from oozing out, but it also makes nice (less chippy) rounded edges.