Thomas, The porosity you are seeing at the bottom of the shank in your
white gold is probably shrinkage porosity. The sprue that went into the
ring when it was cast was probably eiter too big for the piece or not
flared out enough..
Marc: I always thought the biggest problem was a sprue that wasn’t big
enough for the piece. Can you enlighten about “too big for the piece”.
Also, which way does the flare go?? Should the sprue narrow as it goes
toward the piece??
Roy and all, It is very possible to have too big of a sprue on any piece.
When you are spruing the general rule is not to have a sprue bigger than
the thickest part of any piece. It is very hard for me to explain what I
mean without actually showing you. But in general if your shank is only 3
millimeters wide, than your sprue should be at the most 3mm wide, I
personally flare my sprues out at the point of connection when I am casting
most pieces. I take a piece of spruing wax, take a pair of flat pliers and
sort of just put enough pressure at one end the sprue to flatten it a bit
and at the same time widen it (thus you get a bit of flaring outward) Then
I attach that part to the shank of my ring or whatever I am casting. And
remmeber the size of piece will and should determine the size of the sprue.
If a sprue is too big the “rush” of gold entering the piece as it is
casting will cause that shrinkage porosity. It has to do with the rate at
which the gold will cool before solidifying…I know that Elaine from
gesswein could explain this much better than I can, in words…If you are
really not understanding what I mean, email me and I will send you some
pictures of what I mean…I hope I explained this somewhat to the point
where it is understandable… Marc Williams
Roy and all, It is very possible to have too big of a sprue
on any piece. When you are spruing the general rule is
not to have a sprue bigger than the thickest part of any
Roy, Marc and all, This sounds backwards to me. There was a
very good article in the June 1992 American Jewelry Manufacturer
magazine titled Improving your silver casting. In his article
Richard V. Carrano states “To minimize the chance of shrinkage
porosity, pay careful attention to the sprue size and location.
Choose a sprue that’s AT LEAST as thick as the thickest portion
of the casting. If it is thinner, it will solidify before the
casting and cut off the flow of molten metal into the casting,
resulting in porosity in the areas the molten metal didn’t
reach.” The casting needs to be able to draw molten metal from
the sprue until it is solid. This applies to gold as well.
Timothy A. Hansen
TAH Handcrafted Jewelry
Tim, I didn’t mean to give anyone the wrong idea. The sprue should be
about 30% thicker than the thickest part of what is being cast. But
when the sprue is grossly oversized for the piece, that is going to
cause problems. I myself have not had any problems to speak of and my
sprues are fairly light. I never use heavy sprues for anything. In
fact if I think there might be a problem I just put 2 or 3 sprues on
the piece in strategic areas.All of my casting is done in either 14kt
yellow or 14kt white. I don’t cast much silver. And believe me, 95% of
what I cast is one of a kind pieces that I just spent hours carving
myself. So to sum it up I never, over sprue with too big a sprue, but
then I never under sprue either. This spruing thing is just something
one will learn thru trial and error. Marc Williams
Here’s my 2 cents on spruing.
Buy the pink spru wax in 10 ga 8 ga and 6 ga. One of these three
sizes will work well for most cast pieces. Heavier for the mens
lighter for the womens and pendants. All attachments should look like
tree trunks going into the earth. Both on the piece and connecting to
the spru base. The flow of the metal will work better if there are no
sharp corners. The moten metal can breeak any sharp corners of the
plaster off. This broken off plaster can travel into your object
making it junk. Keep away from the pinched straw syndrom and sprus too
long. The length should be about 1/2 inch from the button or tree.
A note for casting temperatures. If you have a piece in production,
set up four or five flasks of exactly the same items. Now guess at the
proper temperature you think will work. Cast the first flask at that
temperature. Keep track of this flask. For the following flasks drop
the temperature 100 degrees each. Make sure the temperature stablized
at the next temperature before casting. Cast until one of the flasks
doesn’t fill. Try to get the molten metal to freeze by having the
flask too cold. Now if you add 100 degrees to the flask that failed
you have the perfect temperature to cast that item. A lot of your
porosity problems will go away. You may think this is expensive to do.
But you can do it in silver and gold temperatures are very close.
Flask temperature is critical for flow, fill and porosity.
TR the Teacher & Student