Sandra, I’ve worked with delft clay on and off for about two years
now. The “canisters” or mold rings have always been at room
I believe the problem most encounter with the hot metal not getting
through the sprue is caused by making too small a sprue hole
(diameter.) One of the characteristics of the delft process is that
it requires a large diameter sprue hole. About 1/2" seems to work
well for most smaller items (Charms, rings, etc.) Larger for bigger
I tried a smaller sprue hole a couple of times but the result was
exactly what you describe, the metal solidified before reaching the
impression. Didn’t even travel through the top half of the mold
ring. So I don’t believe that heating the “bottom” half of the mold
ring would help as the metal doesn’t travel that far.
As far as calculating the amount of extra metal for the sprue and
button. Well I haven’t as I use silver which is pretty reasonably
priced and once I had cast about five or six items I just got a
"feel" for how much to melt for a particular impression.
If you really wanted to know however, then cast a couple of pieces
until one comes out right, then cut off the sprue and weigh it.
Since the ring molds are constant in size, and since the sprue
diameter is pretty constant in size (To get a good cast piece.), the
sprue/button will be too.
Anyway that’s been my experience. If some of it is useful to you
then great. (BTW: If you’ve not cast with delft yet, have some
ventilation. The smell is pretty nasty, sort of like burning
Battle Ground, WA