I did a quick sketch of an ear in Roma Plastilina, following an ear tutorial in Berit Hildre's book, modelling heads and faces in clay. Her book is very accessible. The best way to learn how to model an ear in oil-clay is by practice, practice, and more practice. Click on any image to enlarge it.
This is a view of the back of the ear. I started out by covering the top of a small block of wood with some oil-clay. Then I formed a general block ear shape and pressed it onto the oil-clay covering. I followed the tutorial from there.
The main parts of the ear are the helix which is the outer rim of the ear, the antihelix, the tragus and antitragus, which define the ear notch, and the lobe.
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