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Casting Science/Art

Dear Atif,

Many of the same concerns found in industry casting are found in
jewelry casting. Whether you’re casting an engine block or an
engagement ring, there are more simularities than not, ie: gates
and risers, cooling rates, mold temperatures, metal temperatures,
casting density, alloys, and equipment. Problems such as porosity
(three types) and strength are directly related to those
considerations. With gold casting, the major consideration by
far, is spruing. If you are talking about a production line of
jewelry or related items you should scrutinize your prototype and
working model for these parameters. Not doing so will doom you
to repeating the same flawed piece endlessly, and as often is the
case, blaming the alloy, crucible, operator, equipment, alignment
of the stars, or any number of other nefarious factors including
endless discussions of Vacuum vs Centrifugal. Here’s 90% of your

When a gold alloy is in a molten state it has significantly
expanded. When you put that expanded metal into a confined space
(investment mold) it begins to cool. As it cools it begins to
shrink or recrystalize, so it is in motion. Generaly, it cools
all over like a skin. If you have an isolated “hot spot” that is
not fed by a sprue it becomes a resevoir to feed adjacent cooling
spots. With no metal to replace it, the result is "hot tear"
commonly known as porosity. On one of a kind pieces you can
lavish attention to spruing. On a production piece lavish the
attention on the model.

Cooling rates and position of the parts in the flask are real
concerns. Assuming you have the correct flask and metal
temperatures upon casting, you can inhance casting performance
with well planned model placement within the flask. Eddie Bell of
Rio Grande has done some interesting research on temperature
zones within jewelry casting flasks. He has given papers at the
Santa Fe Symposium on “Heating and Cooling Characteristics of
Investment Molds”. Check the 1989 Symposium papers. I believe it
is available through the Rio Grande