I use a stake and plastic hammer for doming copper shapes for
enamelling. The stake choice and where on the stake you place the
piece controls the amount of curvature, allowing lots of
possibilities for odd shaped pieces. The plastic hammer is
non-marring, unless you WANT the hammered texture. Metal hammers thin
the metal, harden it and increase the size. This can be useful, of
course, but perhaps not if you already have the dimensions or
proportions you want.
Its worth exploring the different textures you can create through
hammering on a stake. Although I don’t use transparents on copper
much, copper is an ideal practice medium for hammering techniques
you might later use on fine silver.
Annealing may be a good idea if the piece is really hard after