calculate the amount of metal needed when casting from wooden,
plastic models or any type of material for that matter.
Not so hard, Chris… First you need to understand that it’s only an
estimate - any casting process needs an excess of metal for spruing,
button, what have you. It doesn’t need to be exact, just good
There are a limited number of things you are going to be casting
FROM - wood, plastic, metal - and it’s all about specific gravity.
Water has an SG of 1 by definition. In the casting business we
consider things that float in water to be 1, just like water. That
means waxes of all kinds and wood, too. If you get really into it,
you might have to adjust for things like styrofoam, which is
especially light weight, or a large piece of lignum vitae, which is
especially heavy. But for jewelry scale items made in wax, plastic
or common woods you are looking at tenth pennyweight differences per
piece, which are absorbed by the button. That’s why it’s justan
For metals, you use the ratio of the SGs of the metals. SS is 10.4,
14kt is 13.4, 18kt is 15.5, platinum is 21.5 but we use 22. Brasses
run around 8.5. Offhand I don’t know steel(s), but that is easily
So: 22 (platinum)/10.4 (sterling) equals 2.12, which means a
platinum piece will weigh 2.12 times as much as the sterling piece.
Same thing with whatever else - if it’s tin, use the SG of tin and
do the ratio… Not so hard…