Use a 'lean against' support for the open bark structure.
Tree bark can act as a base for mushrooms, frogs, flowers, etc. The bark not only provides a base but allows the sculpture to have some height. These could serve as ornaments for gardens, window boxes or patio displays. They need to be taken inside in winter.
Form a semi circle that is standing.
In order to keep it standing upright, lean it against something. I used a coffee cup.
Shape an interesting old bark shape.
In this case I am planning to place a butterfly on the bark so I made a depression near the top for that purpose. To add interest I made a hole where a branch once existed.
Once you are happy with a shape, smooth the base with a paint brush and water.
The coffee cup gets to multitask, in this demo.
I draw a couple of straight lines from top to bottom. Between them I sketch in jagged marks that go at angles or in a rough V shape. A small knot hole adds to the affect.
Continue in this manner all the way around the outside of the bark. Relax and don't try to be too perfect. This is meant to be more like a sketch than a finished drawing.
Leave the inside of the bark smooth.
Usually I would keep the bark moist while I shaped whatever I plan to place on it.
You can keep the clay bark moist by misting with water and placing it in a plastic bag. A frame of some kind might be needed to keep the bag off the sculpt.
This picture of the bark on its side shows how the bottom is thicker to help support the structure. The bottom edges will be trimmed and made smooth.
Handle carefully. Dried clay sculptures are very fragile and easily broken.
If you do break one, it can be 'glued back together using paperclay 'glue'.
Instructions for making that are in the butterfly instructable.
Refer to the butterfly instructable if you would like to see the step by step.
You can also make much taller and larger pieces of bark. Lean it against a taller object or, better yet, make a frame using a hot glue gun and dowels or chop sticks. The bark leans against the frame until it dries.
Using a buff clay or red clay can give a nice affect for this type of bark. It need not be coated or painted at all, or you can just add a little underglaze colour to the surface texture.
The bark can be a base for all kinds of nature things, and can add a little height to your sculpture.