All shrinkage rates are not alike. Shrinkage depends on many factors,
one of these being the mold rubber used. The type of wax used and the
wax temperature, pressure, and cool down time, the type of metal
used, melt temperature, and cool down time, all affect shrinkage.
There is also the casting surface, that you will abrade away or
polish, that you must consider.
If you really want to know EXACTLY what the shrinkage rate is, try
this: take four equal lengths of square wire (I used silver). One
piece is 1 mm X 1 mm, one piece is 2 mm X 2 mm, one piece is 3 mm X 3
mm, and one is 4 mm X 4 mm solder these to a bar and sprue them. What
you now have looks like a fork, with four different sized tines.
Now, make a mold. Inject a wax, and measure the four square rods. How
close are they to your master? Now, cast the wax, and measure the
casting. How much have you lost?
You can now experiment with various mold rubbers, waxes, wax
temperatures and pressures, casting alloys, melt temperatures, etc.
Record your results. You may be surprised by how much variation there
is. You will also find that the four square rods do not shrink at
the same rate.
With this simple tool, you can now accurately predict shrinkage rates
for a variety of castings, and adjust the size of your master models
so that stones will fit perfectly every time.
Now, if I could only find perfectly sized stones…