The difficulty in turning a natural edged bowl is mostly in convincing yourself that you can do it. While it looks oval it is really a round bowl, turnd in the same manner as a regular bowl except that the blank is oriented at 180° to a regular blank. I addition, a large part of time you end up turning air so there is a feeling of hitting the edge of the bowl twice in every revolution. Very quickly you get used to it and turning proceeds as usual.
Not surprisingly, a natural edged bowl starts with a log. Here is a fairly dry piece of cherry, complete with a radial split along which I plan to cut, leaving me with two half logs to choose from for bowls.
With a chain saw, cut the log to length and then through its pith. I like each half to be longer than the diameter of the log. Each half can make a bowl
I first mount the blank between centers and arrange it so that the top edges of the bowl are in the same plane. This is not necessary but usually I feel that the bowl will look better this way. I also begin with the top of the bowl toward the head stock.
Rough out the shape beginning with the top of the bowl and working towards the bottom. This goes against the grain of the wood but has a better chance of keeping the bark on as the cuts push the bark towards the wood. Much of the time we are turning air and it is difficult to see how close we are to the edge of the bowl without stopping the lathe. On the other hand it is good to stop the lathe from time to time and move the tool rest closer to the work. It balances out.
Continue to refine the shape as you turn dpwn the sides. As the wood spins on the lathe, the image of the turning wood or "ghost" as it is usually called, reveals the shape of a bowl but the still object looks to be becoming more and more oval.
As the cuts define the bottom the contrast between heart and sap wood becomes more and more obvious. Although the spwood here is only a little less than an inch thick, the long sloping cuts make it seem like two or three inches.