Ah, the beauty of ashtrays. Eliminating ashtrays outcomes takes some
careful preparation in advance. I have focused on a particular culture
for the lesson. Showing slides or other visuals helps students visualize
some of the variety of shapes of pottery. I tell my students that I
don't want to see any ashtrays. If they want to use it for that reason,
that's up to them; just don't let me know by it's design or shape. You
might want to specify certain height requirements and/or the need of the
pot shape to go in or out rather than the typical straight, short shape
of an ashtray.
A project I am doing with my students now are the pueblo storytellers.
My students have finished the coil pot technique and are going to be
using the technique to make a tapering coil pot for the body of the
storyteller. They then will add a rolled and formed head, rolled arms
and legs, and any other details.
A book to consider is "Children, Clay and Sculpture". You can find it in
Davis Publications or in some other catalog.
Hope this helps,