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Porosity in silver casting


#1

Dave Stephens writes

From: Dave Stephens StephensDesign@opendoor.com
To: orchid@ganoksin.com
Subject: Re: (Fwd) Orchid - member’s introduction
Date: Monday, September 30, 1996 7:21 PM

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
vacuum
casting over centrifugal? Dave

Art Jewelry for Conscious People
http://www.opendoor.com/stephensdesign/crystalguy.html

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
metal.

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
controller.

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
temperatures.

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?

Kenneth Gastineau
gastin@mis.net


#2

Kenneth Gastineau wrote:

Dave Stephens writes

From: Dave Stephens StephensDesign@opendoor.com
To: orchid@ganoksin.com
Subject: Re: (Fwd) Orchid - member’s introduction
Date: Monday, September 30, 1996 7:21 PM

====

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
vacuum
casting over centrifugal? Dave

Art Jewelry for Conscious People
http://www.opendoor.com/stephensdesign/crystalguy.html

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
metal.

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
controller.

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
temperatures.

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?

Kenneth Gastineau
gastin@mis.net

orchid@ganoksin.com

Kenneth,
I am new here to the list but have known Dave via email for a couple
of years or more I guess.I gave Dave some details when he started out
and I have always stuck by them myself.I cast in small flasks and just
use several instead of one big one…This would seem to increase the risk
of failure but actually it diminishes the risk of complete failure. 5
small flasks still burnout in the time it takes for one small flask thus
I only run 4 hour burnouts and my electric bill is not so hard to bear
each month.I use an electric gold assay kiln from Vcella Kilns of Calif.
which will run at 2600 all day long if necessary and temperature rise is
very fast due to a special fire brick lining.This kiln is stainless
outside an has replaceable everything and heavy duty elements and runs
on 115 volts instead of 220 which means no special wiring. I can melt in
the Vcella kiln but use my Neycraft spincast machine and melt under a
propane oxygen torch usually just my Hoke from the bench is all I need.I
put the flasks in at closing time and am out of the shop by 11 pm with
the casting done. I don’t have auto or computer controls but the Vcella
is always stable regarding temps.I run it to 300 and turn the knob back
until it clicks off the elements and do repairs or other work for an
hour then step to the casting room and turn it up and wait five minutes
for it to come to 900 degrees then back the knob off till the elements
click off go back to the benchwork for an hour step to the casting room
and turn the kiln up and wait ten minutes for it to reach 1350 degrees
turn it back till the elements click off and go back to the bench for an
hour then go to the casting room and turn the flasks so the sprue
opening is up then close the door and leave for the refrigerator and a
cool drink or eat or work at the computer and wait half an hour then go
open the door and shut the furnace off.I close the door again and then
open it again maybe once till the temp. falls off to 1000 degrees (I
number paper cups with the same number as I scratch in the top of each
flask and place the properly determined weight of metal in the cup
number that corresponds to the matching flask so the metal is ready when
the flasks are)and start casting the first flask at 900 degrees for
silver or 1000 for 14k gold…I always clean ALL the old investment off
all buttons I use from other prior casts in a small ultrasonic
machine…(this is very important since usually it is investment
particles that cause porous castings and they either break off inside
the flask or are added to the melt unknowingly by the operator…If they
come from inside the flask they are often the result of improper
investment preparation ,lumps in the wet investment or contamination
from metal shavings or buffing compound etc…Who left the the
investment bucket open???..I candle each wax for water pockets and
airbubbles in front of a lamp and fill any that show up …(spots in the
wax that just don’t look right).If this all sounds like alot of trouble
it is and I usually take about as long to inspect, sprue and invest the
waxes as I do in casting burnout that being 4 hours…I sprue all waxes
for larger objects directly off the button on the sprue base and tree
smaller items only when necessary…I use mostly 2.25 inch diameter
flasks that run 3 or 4 inches tall but the smaller the better actually
and 1/4 inch is plenty of clearance between the flask wall and wax.Never
have used liners or anything similar and often use automobile tailpipe
for flasks…If you’re lucky the muffler shop will give you short
cutoffs from the stainless steel pipes which are best free of
charge.(heat them under the torch before using them the first time to
burn off any oil)… I begin burnout exactly one hour after pouring the
investment…anyway I don’t have porosity problems and cast up to 100
items in one day this way with no failures…Anyone??? If you have
failures check first that you are cleaning the buttons you re-use
extremely well preferably in an ultrasonic.I also never use anything
over 50 percent old metal and add half new metal to each casting…If
you are using too much water in the investment you will see water marks
as thin lines like spider web on the unfinished casting or flashing like
mold seams in pattern molds sometimes leave on waxes on areas of the
casting where there were no mold seams or flashings …not enough
water or too much heat and the investment smells of sulfur an is crumbly
at the end of the burnout…If you don’t smell something similar to
dirty cotton athletic socks when the furnace first reaches 900 degrees
then you have done something wrong.
In summary I use a four hour burnout on multiple small to medium
flasks and save 4 hours of electricity. This is half the cost of casting
using an electric furnace for burnouts…I don’t stay awake all night
wondering if the computerized furnace has developed a software glitch
and the shop has burned either…LOL…Okay Kenneth I apologize for
being a smart a…Seriously I am trying to help and will be glad to
answer any questions for you if I can…Again I can not stress this
enough CLEAN THE BUTTONS YOU RE-USE or RE-MELT and RESORT TO GRINDING IF
NECESSARY BUT LEAVE NO INVESTMENT or old flux ON THEM WHEN YOU
REMELT… Gavin


#3

Gavin writes:

Kenneth,
I am new here to the list but have known Dave via email for a couple
of years or more I guess.I gave Dave some details when he started out
and I have always stuck by them myself.I cast in small flasks and just
use several instead of one big one…This would seem to increase the risk
of failure but actually it diminishes the risk of complete failure. 5
small flasks still burnout in the time it takes for one small flask thus
I only run 4 hour burnouts and my electric bill is not so hard to bear
each month.I use an electric gold assay kiln from Vcella Kilns of Calif.
which will run at 2600 all day long if necessary and temperature rise is
very fast due to a special fire brick lining.This kiln is stainless
outside an has replaceable everything and heavy duty elements and runs
on 115 volts instead of 220 which means no special wiring. I can melt in
the Vcella kiln but use my Neycraft spincast machine and melt under a
propane oxygen torch usually just my Hoke from the bench is all I need.I
put the flasks in at closing time and am out of the shop by 11 pm with
the casting done. I don’t have auto or computer controls but the Vcella
is always stable regarding temps.I run it to 300 and turn the knob back
until it clicks off the elements and do repairs or other work for an
hour then step to the casting room and turn it up and wait five minutes
for it to come to 900 degrees then back the knob off till the elements
click off go back to the benchwork for an hour step to the casting room
and turn the kiln up and wait ten minutes for it to reach 1350 degrees
turn it back till the elements click off and go back to the bench for an
hour then go to the casting room and turn the flasks so the sprue
opening is up then close the door and leave for the refrigerator and a
cool drink or eat or work at the computer and wait half an hour then go
open the door and shut the furnace off.I close the door again and then
open it again maybe once till the temp. falls off to 1000 degrees (I
number paper cups with the same number as I scratch in the top of each
flask and place the properly determined weight of metal in the cup
number that corresponds to the matching flask so the metal is ready when
the flasks are)and start casting the first flask at 900 degrees for
silver or 1000 for 14k gold…I always clean ALL the old investment off
all buttons I use from other prior casts in a small ultrasonic
machine…(this is very important since usually it is investment
particles that cause porous castings and they either break off inside
the flask or are added to the melt unknowingly by the operator…If they
come from inside the flask they are often the result of improper
investment preparation ,lumps in the wet investment or contamination
from metal shavings or buffing compound etc…Who left the the
investment bucket open???..I candle each wax for water pockets and
airbubbles in front of a lamp and fill any that show up …(spots in the
wax that just don’t look right).If this all sounds like alot of trouble
it is and I usually take about as long to inspect, sprue and invest the
waxes as I do in casting burnout that being 4 hours…I sprue all waxes
for larger objects directly off the button on the sprue base and tree
smaller items only when necessary…I use mostly 2.25 inch diameter
flasks that run 3 or 4 inches tall but the smaller the better actually
and 1/4 inch is plenty of clearance between the flask wall and wax.Never
have used liners or anything similar and often use automobile tailpipe
for flasks…If you’re lucky the muffler shop will give you short
cutoffs from the stainless steel pipes which are best free of
charge.(heat them under the torch before using them the first time to
burn off any oil)… I begin burnout exactly one hour after pouring the
investment…anyway I don’t have porosity problems and cast up to 100
items in one day this way with no failures…Anyone??? If you have
failures check first that you are cleaning the buttons you re-use
extremely well preferably in an ultrasonic.I also never use anything
over 50 percent old metal and add half new metal to each casting…If
you are using too much water in the investment you will see water marks
as thin lines like spider web on the unfinished casting or flashing like
mold seams in pattern molds sometimes leave on waxes on areas of the
casting where there were no mold seams or flashings …not enough
water or too much heat and the investment smells of sulfur an is crumbly
at the end of the burnout…If you don’t smell something similar to
dirty cotton athletic socks when the furnace first reaches 900 degrees
then you have done something wrong.
In summary I use a four hour burnout on multiple small to medium
flasks and save 4 hours of electricity. This is half the cost of casting
using an electric furnace for burnouts…I don’t stay awake all night
wondering if the computerized furnace has developed a software glitch
and the shop has burned either…LOL…Okay Kenneth I apologize for
being a smart a…Seriously I am trying to help and will be glad to
answer any questions for you if I can…Again I can not stress this
enough CLEAN THE BUTTONS YOU RE-USE or RE-MELT and RESORT TO GRINDING IF
NECESSARY BUT LEAVE NO INVESTMENT or old flux ON THEM WHEN YOU
REMELT… Gavin

orchid@ganoksin.com

procedures

Gavin:
Thank you for sharing you casting experience. Your advice on cleanliness
is certainly correct. I pressure blast, wire brush, pickle, sand blast and
ultrasonic clean my sprues. This is to prevent mostly what I call pits and
inclusions. Porosity to me is a completely different matter. The porosity I
am talking about is the absorbtions of gases which creates a sort of
micro-porosity. This is barely noticeable to the naked eye, but prevents
the surface from coming to a high luster due to tiny bubbles trapped in the
solidified metal. This type of porosity also causes discoloration, not the
purple fire scale, but kind of a greyish looking surface that does not take
a good polish.

This type of porosity is caused primarily by insufficient burnout,
overheating the metal, and or flask temperature being too high. By using a
kiln controller I am assured of consistent and variably controlled burnout
and flask temperature independent of what my oven would do left to its own
inherent tendencies. As far as the safety of unattended burnout and losing
sleep wondering if my program is working properly I can say I trust it as
much as my central heating system, my kitchen stove, my hot water heater,
and the cruise control on my vehicles. I steam dewax to eliminate burning
wax and my casting room is constructed of block with a cement floor, and a
two hour fire rated ceiling. I also have a smoke alarm system wired into my
building. Frankly I’m much more worried about tornado’s, we were nearly
taken out by one of those this last April. So far I’ve had one failure due
to a burnt out element. I replaced the element the next day and cycled it
again, everything went fine. The kiln controller allows me to cast early in
the morning when I am fresh, alert, and before the daily calamity starts.
Also I do not have to work until 11 p.m. I would be interested to know if
anyone has experienced any dangerous problems with burnout controllers?

I use large flasks because I can pack a kiln load of small flasks into one
large flask and pack the kiln full of large flasks and get about 400 pieces
of jewelry into 5 4x6 flasks. Also many of our designs do not pack well
into small flasks. This saves me time, energy and money. Of course I use
smaller flasks for smaller jobs. I also use a melting furnace because I
don’t like being tied down to holding a torch all the time. Not that
attending to a melting furnace is any picnic, but I find it easier to do
all of the other attendant jobs without paying constant attention to torch
melting. To each their own.

Does anyone else use large flasks? Do you have problems with them? I am
particularly interested in knowing if there are any querks to using a 9"
flask length versus 6".

Has anyone else tried the deoxidized sterling alloys available? I would
like to hear about your experiences with it. Mine have been very positive.

Kenneth Gastineau
gastin@mis.net