I have had a lot of success using some of the same metal I’m
casting, from the sprue if necessary, to fuse into large
pits/etc. Unless the piece is unusually delicate, this method
seems to work quite well.
Curtis Harmon
Dear Dave, I will preface this by saying I have never done any
casting myself, but after seven years of editing articles on
porosity and casting by some true experts on the subject, perhaps
you will not take it amiss if I pose a question? Have you
considered the way you’re sprueing the pieces? This is one of
the most common causes of shrinkage porosity. What happens is the
hot metal fills the cavity at first, the sprue freezes, and then
the metal begins to cool and shrink. Since the sprue has frozen,
no new metal can come in to fill the new spaces, so pits form.
Perhaps you’ve already tried changing the way you sprue the
piece, but you didn’t mention it in your post, so I thought I’d
pose the question.
AJM magazine has done several articles on this topic (each
August issue is dedicated to casting, and since porosity is one
of the major bugaboos of casting, it’s been covered several
times.) Your description of your porosity problem rings a bell:
I’m pretty sure it’s covered in an article published in August
1997 called “Casting Q&A.” (I can’t find my copy of that issue,
or I’d check on it for you!) To get a copy of the article, just
send an e-mail to AJMMagazine@compuserve.com or give them a call
at 800-444-MJSA. Or cruise on over to their Web site at
http://mjsa.polygon.net/ajm.
Hope you don’t mind my two cents worth!
Suzanne Wade
AJM Contributing Editor
Hello Peter,
You must have bumped into a lot of bad castings !!! I started
casting ,mold and model making in 1968 while in Thailand…We
had the 1st lost wax casting business there ( i was quite
young).Often at that time,i had to improvise machinery…
techniques…materials etc… far more so than in the USA due
to lack of materials and technologies.
At that time myself and the goldsmiths who worked for us had to
learn how to burnish pits out of castings and even out of hand
made jewelry.It was a critical part of creating our finished
products…(a lot of fun learning from people who had to make
their own tools to do a job). By the time i moved to theUSA(i’m
American)in 1980…my methods and machinery concepts for casting
had greatly improved to the point that i no longer had to
burnish castings .
When i arrived in the US, i took a job designing Jewelry
machinery and then doing machinery,and supply sales for roughly 6
years(great oportunity to visit on the inside some of thebiggest
factories in the world)This was a great learning experience! What
i had noticed was that porosity seemed to be a great issue in
castings and still is today for many casters and jewelers.
I have found that through great care and diligence, porosity
VISIBLE to the naked eye can be eliminated and that each form of
casting… be it vaccuum casting or centrifuge casting has a
complete set of parameters. Temperatures are different for
both…In 95 % of all cases,vaccuum casting is superior,But in
all cases, to get superior castings, it takes a LOT of
experience.Few casting businesses keep the person doing the
castings for more than a few years, and this can contibute to
lousy quality. Most of what i cast for my customers has to have
an absolute mirror finish on the inside and outside of the
piece…When you have to do thousands of pieces like this, you
cannot burnish the pieces… you have to have an exceptionaly
good casting.
However, i must agree with you that when casting 1 off
pieces,the neccesity to burnish becomes a reality…This porosity
is due primarily to how good or bad the guesswork and experience
of the caster,model maker, mold maker and designer is!!!
So, another few points i have to make as far as castings go
is… the wax carving or model may have a lot of thick/thin
spots… (causes porosity and may be necc.due to customer
design…hence burnishing) the sprue or gate may be inadequate or
improperly positioned The temperature of the mold, metal or both
is improper. The Gods may not be with you on that particular day
! The wax, model and rubber mold may have porosity in them
already and occasionaly the materials used may fail!!!
So my best advice is to throw a few grains of salt over ones
shoulder before pouring your metal for good luck and pray to the
casting gods!!!
Just my 2 cents.
Merry Christmas and a Happy New Year to all !!!
Danel Grandi
racecarjewelry.com
Curtis, I fuse when ever possible too. (Avoiding porosity in the
first place is always best, but being that we’re in the real
world…) Someone name of Issacc --can’t remember his last
name-- gave me a great tip.
Take a small piece of that same sprue-- the “parent sprue” and
lay it down on your soldering pad w/ an equal size snip of the
approriate --carat and color-- medium solder. Completely melt
them together, even cutting through the semisolid ball w/ a clean
soldering pick and scrambling up the pieces. This will yeild a
super welding solder that tarnishes very little, matches
veryclosely in color and melts below the parent metal. Of
course, as w/ most things, this works better on yellow , green
and red golds and not so great on nickel white alloys (the
DEVIL’S METAL)
Good luck and happy holidays,
Andy Cooperman
PS Remember to drill or burr out large pits to remove flux, investment
or shmutz.