Pear, egg or "diamond" shape.

Indeed too much cutting of the shoulders will free some of the outer tension.

I find it more of a problem when reshaping hammers than with new, but indeed some hammers that have initially a round shape cannot be made to a Steinway shape without loosing their consistency.

Then the shape does not stay put, anyway.

The first voicing is also creating a basic shape, depending of the way it is done the final hammer will be more or less egg shaped.

The tension of the outer core is often not much and voicing of today hammers is often static, the tension does not move within the hammers, or on a very small zone.
SO if the hammer have been too much heated or too much pressed it will need a huge needling, while if the felt is not so much pressed it can be ready with a quick job, and minimal needling.

but force is yet necessary to have a result.
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Isaac OLEG - http://picasaweb.google.fr/PianoOleg pro