Hi Rose Marie - If I were to make a cone such as you are working on,
I'd use much heavier silver, probably 24 gauge. I'd use a wood or
delrin trough and a narrow hammer, such as is used for shell forming.
For years, I used the narrow end of a cross peen hammer. First hammer
the edges into a shallow "c" shape, then do the ditch or center. Then
using the planishing end of a hammer, gently bring the edges together
by tapping on the upturned edges, thus rounding the cone. You can get
a pretty good seam by inserting a thin wire of solder into the cone
at the seam. You can get a much better seam by tinning the metal at
the inside edge of the cone prior to forming.
By using a softer trough or ditch, you wouldn't mess us the
stampings on your sheet.
Incidentally, the way we made lots of these different sized troughs
was to drill many different sized holes into a thick piece of wood,
then saw the wood in half, bisecting the holes. Voila - half round
troughs for shaping shell forms.