Dave Stephens writes
Kenneth: welcome to the group! I had asked a question about porosity in
silver casting and wondered what your experience and solutions are since
you seem to have alot of background in that. Also, why do you prefer
casting over centrifugal? Dave
Art Jewelry for Conscious People
Well Dave I will do my best. As there are no short answers, please stick
with me. Porosity is caused by many, many variable factors. I have been
casting for the last 10 years. As I am self taught this does not
necessarily mean that I have 10 years of quality casting experience, but
rather 10 years of frustration, discovering all of the ways that things can
possibly go wrong.
A couple of years ago I developed a line of link bracelets which used
fairly heavy cast links in combination with constructed wire links. Up to
this point casting silver had always been a hit or miss kind of thing,
pits, porosity, I mean we are talking sponge here. Sometimes however they
were great. Well acceptable. Anyway our production sterling cast pieces
were grueling at best to finish and many never made it to the sales floor.
For this reason I kept our cast sterling designs to a minimum. We had
better success with bronze. I was using traditional sterling casting grain
with a centrifugal casting arm (spring wound) which would hold a 3 1/2"x4"
flask, an oxyacetylene melting torch, and a small stepless kiln which would
hold 4 such flasks at a time.
Over the few years preceding the link bracelet development I thought we
might improve our castings and production by upgrading our setup to vacuum
casting. A friend of mine in the welding gas business gave me an old vacuum
pump which would pull 17 cfm. I built my own chamber and have had
absolutely no complaints about this setup. The advantages of vacuum over
centrifugal are size of flasks, safety, and you are always there with your
metal. I always flux the sprue button after a pour. This helps to keep the
sprue button molten longer, which helps to keep the sprue molten longer.
Proper feeding of the castings from the sprues helps to eliminate porosity.
At first I was skeptical about vacuum versus centrifugal as I thought
vacuum casting could surely never outperform centrifugal. How could it
match the pressure developed as hot metal was slung with such force
developed through such violent slinging. One day it finally dawned on me
that all of that hot metal was being slung into the flask and then hanging
up against dead air pressure. In other words the metal could not enter the
mold cavities until all of the air in the mold was pushed out by the molten
With vacuum casting all of this violent physics takes place in the vacuum
pump far away from me and the melt, and the molten metal can gently, easily
and completely fill the cavity. I love it.
I also bought a melting furnace which I fuel with propane. It will hold up
to a #8 flask, however I use much smaller ones. My reasoning on using a
melting furnace is that if the maximum temperature is about 2500 degrees F
with propane and the temperature of an oxyacetylene torch is around 6000
degrees F, then it would be easier not to overheat the silver, a major
cause of porosity. Melting furnaces are not for the faint of heart. They
are loud (somewhat like a jet engine)hot and can be dangerous if not used
properly. However they do melt metal quickly. If I were not going to use
gas to melt metal I would probably use an electric melter of some sort.
There was one problem. I still had horrible porosity and surface quality.
In fact it was even worse. One of the other things I had changed in the
conversion from centrifugal to vacuum was the flasks. I started using 4X6
perforated flasks. I chose perforated flasks as I cast quite a few thin
pieces and wanted optimal results. I was very frustrated as I had spent a
lot of money and time and all I accomplished is that I could cast a whole
lot more bad castings at one time. Our link bracelet line put a burden on
us because we needed more sterling castings than ever. More castings, more
pits, more problems.
One day a man and his wife came into our store and I started talking to
them. It turns out that they ran a second generation jewelry casting
business in downtown Chicago. I told them of my woes and they suggested
that I use one of the deoxidized sterling casting grains on the market. He
suggested United Precious Metal Refiners in Alden, New York. I think I use
their alloy #57.
Well things started looking up. The deoxidized sterling eliminated a lot of
porosity problems, but I still had some porosity and the surface problems
were just as bad if not worse. I called the folks at PMR, this is great, I
finally have someone to talk to about my problems (casting that is). We
went over everything I was doing and it turns out I had not really changed
the way I did burnout when I changed flask sizes. I was doing burnout
during the day and only burnt out maybe 6 - 8 hrs at most. Remember I was
still using a stepless controller (never again). A 4X6 flask requires
about 12 - 15 hours of burnout.
I decided to solve my problem by throwing money at it. I purchased a step
controller from Swest and attempted to install it in my kiln. When I
enlarged the hole for the pyrometer I cut through one of the elements, I
had one of the furnaces with embedded elements which had burnt out twice
before. These are very expensive to replace and I do not recommend them I
asked Swest if I could return the step controller and purchase a
computerized controller and a new kiln from them. What could they say. The
next week I had a new kiln with replaceable elements and a computerized
I wasn’t quite sure how to act. No more running to the back room to check
the kiln temp. I load the kiln in the afternoon and it is ready for casting
in the morning. Most importantly I could easily control the length of the
burnout and the cool down process. Both of these factors are very important
in eliminating porosity from sterling castings. I might add that I steam
dewax before burnout. This increases the life of the kiln, kiln elements
and the environment.
Now I am not saying that I am a master at casting sterling silver (I still
have a lot to learn), but my castings have improved tremendously. One of
the advantages of deoxidized sterling silver are, it casts bright (no black
castings), reduced porosity, and no fire scale. These castings are a breeze
to polish when properly cast (repeat, no fire scale). The cost of this
casting grain is more than traditional sterling grain, but the payoff in
saved labor is worth it in my book. Porosity can be caused by other things
like sprue size too, but at this point I am learning about proper
Sorry about the lengthy write-up. Sometimes the process is the problem. The
key is to change only one thing at a time.
I would appreciate hearing about how others have solved some of their
casting problems, both casting and molding. Does anyone use metal molds?
Has anyone else tried deoxidized casting grain?