Recta or Reversa, Cyma is the Wave
I am not a bowl turner. I’m a spindle turner, first, last and foremost. Oh, sure, I turn bowls. I’ll turn bowls for gifts for my friends and family. Most of the time I’m just not that happy with the final product. You know, the finish is fine; the size appears to be right, but so many of them just seem to be uninteresting. But recently I turned a couple of natural edge bowls that struck me as pretty good. So I tried to determine what it was that made these bowls more pleasing than my usual ones. In short order I realized that the answer was simple. The cyma, the shape of the wave, was staring back at me.
The ancients realized that the curved line caught the human imagination more readily than the straight. Sure, straight lines are great for being the shortest distance between two points. But just walk a couple of miles on a straight path and you’ll find yourself wishing for any diversion you can find. There’s an old Irish saying, “It’s a long road with no turning”. Now, obviously there’s a somewhat more philosophical bent to this old “saw”, but it could certainly apply to our sense of design, as well. Imagine a perfectly straight walk in the woods, enough said.
Craftsmen throughout the ages have used the cyma, or ogee in common parlance, as a decorative device. Look around you at moldings and terminuses (terminii?) and you’ll begin to see them all over the place. Look up at the ceiling of the room you’re sitting in. There’s a very good likelihood that there’s a crown molding running at the intersection of the ceiling and the wall that’s shape is based on the cyma (btw it’s a soft “c”, like sima). The lamb’s tongue is the perfect example of the cyma bringing interest to what could be a absolutely “workmanlike” but very plain chamfered surface.
There are many ways to lay out and the cyma. The classic method is to swing two arcs, one above and one below a common line in such a way that they are posed “sequentially” to create the wave like line. Here’s an example from “The Classical Orders of Architecture” by Robert Chitham:
Another method for laying out the cyma is to determine the length of the surface in question and “halve” it. These will become the arc chords. Then determine the height of the arcs that will work best for the application, i.e. laying out “ogee” bracket feet. Then swing the appropriate radius to create the desired arc. Here’s a simple formula for determining radius to arc height relationships.
We’ll explore other methods of laying out ogees in upcoming posts.
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