Hey, y’all-- I have done sand casting and cuttlefish, but not
delft, so maybe I should butt out, but what about, in addition to all
the great suggestions, you preheat the mold on a hotplate, or, better
yet, a small open-top pan-shaped kiln such as is used for hobby
enameling, or for reticulation? With the latter, the mold could stay
on the heat during the pour. Rio has these kilns–item #702-016, page
94 in the new tool catalog, though I suspect there might be a cheaper
source. For what it’s worth, I find this kiln a godsend for some
soldering jobs, such as soldering a large but thin, low bezel onto a
heavy piece, where it is difficult to keep the whole piece hot. The
kiln does the main heating, the torch just the final touches (you can
use the torch right on the kiln). I use it the same way for
reticulation.
Sorry if this is too rambling. I teach, and I seem to have a
compulsion to offer more info than was ever asked for. Just ask my
kids!
–Noel