Hello John, Do you have any suggestions on how to tame the sprue?
Are there any good books on the subject, I am trying to learn how to
improve my models so they cast best and are easy to mold/inject and
sprue. Are there basic rules to keep in mind when creating a model?
Thanks Linda Steinke
The resulting casting, is like a very solid little ingot with
design.And NO porosity! The primary reason , I believe is the "vent"
spruing. When one “molds” the item in the clay, a couple or more vents
are made from the piece itself which exits to the edge of the "flask"
on the same side as the pour.Therefore all (most) the air obstruction
is minimized. My point is, I’ve been wanting to experiment(when time
allows) to put the same principle to work on larger multi or
single-piece vacuum castings. What do you think? Has anyone
experimented with something close to this. I know of wrapping flask
with webbing for better vacuum, and putting wax rods bending back
up(although not to the surface near the pour) the flask to make more
air space. Could this even work theoretically with a vacuum? Or on
single piece castings,(if sprued in the same way), would you even
need a vacuum? Just keeping the ‘Fixing porosity’ thread agoing. Thomas
Blair Island Gold Works Hilton Head SC
J. Pardon me if I sounded like I was condemning all plating, it was
not my intention. I was referring to those who use plating to
deceive. The ring repair I mentioned was a shock. The metal
underneath the plating was actually two tone, part of the item was a
dirty white/gray with the emphasis on grey and part was yellowish
grey. I have been a jeweller for some 25 years and I have never seen
any white or yellow gold in any form in either of these colours. No
doubt I may have been looking at some odd alloy, but this was beyond
my experience and so the alarm bells went off. This incident took
place only last week so it is very fresh in my mind and so maybe I
spoke a bit harshly. Just as I defend the use of laser welders,
burnishers, or plating to deal with casting flaws (fact of life and
common practice) I see no problem with using plating for the purpose
for which it was developed. The use of plating to deceive of course
is reprehensible (hmmm, I wonder where that word came from, probably
TV). B Goodman
“Taming of The Sprue”
Sender: owner-orchid@ganoksin.com
Precedence: bulk
I have to agree with John. The majority of all casting problems come
from improper spruing. My second major concern is metal moving from a
thin area into a thick area. This will cause the metal to spray as if
from a hight pressure nozzel causing porosity. The last, but not the
least important is gas porosity, caused from poor burnout or lack of
venting. I solved the venting problem with the use of Vac-u-liner.
This plastic mesh provides ample exhaust for gases created when the
hot metal hits the cooler investment… I use this for centrifical and
vac casting. it is available at most jewelry suppliers. Instructions
are on the package and I STRONGLY reccommend its use to anyone doing
their own casting. ( usual product association disclaimer goes here). Frank Goss
Disclosure does sanctify the deception of plating...for some. I would opt for getting it right the first time. To each his own.
I am not sure what I could get right the first time when the customer
demands white gold and the white gold isn’t as white as they want.
Sure I try to sell them the design in platinum, but not everyone can
afford it. It is not deception when the options are clearly laid out
for the customer. Deception is when you DELIBERATELY conceal
something.
And, oh yes, your comparing the wearing characteristics of corundum to diamond is not valid. solitaire worn by an active person who does not take any precautions to protect her jewelry can easily destroy a Ruby or Sapphire in a very short period of time.
A person can also destroy a diamond if they don’t take proper
precautions as well. In this, as with all other topics, we keep the
customer properly informed of the limitations of anything they wear.
Daniel R. Spirer, GG
Spirer Somes Jewelers
1794 Massachusetts Ave.
Cambridge, MA 02140
@spirersomes
http://www.spirersomes.com
John
You are absolutely right that improper spruing is the major cause of
porosity. The second major cause is improper burnout if there is
carbon left in the investment from too short a burnout you will lower
the temperature that the investment breaks down or if the burnout is
too hot it will also break down the investment Either way it will
form sulfur dioxide gas as a result of the breakdown and the metal
absorbs it and the result is gas porosity.
If you burn out over night and keep the oven temp below 1200F you
will improve your castings. In fact you do not need to exceed 1100F
if you have enough oxygen/air flow in the kiln environment to react
with the carbon in the flasks. A major advantage for gas burnout
furnaces as they always have air flowing through them which makes for
cleaner burnout because there is more oxygen present to react with
the carbon in the flask.
Along this line Rio has a new experimental convection electric
burnout furnace that produces beautiful castings. I saw it in action
this summer. It will be great for production shops but it is too big
for small shops.
Jim – http://www.mokume-gane.com jbin@well.com James Binnion Metal Arts
4701 San Leandro St #18 Oakland, CA 94601 510-533-5108
The way that I “fix” porosity is by welding in fresh metal using 24
ga wire as welding rod, then filing and “power burnishing” with a
home made rotary burnisher, works pretty well, not for every piece,
every time, but I have salvaged several pieces in this manner
recently. Better yet, figure out why your castings are coming out "spongy."
Hope this wll help, A.J.
Boy,What a thread…I am new to this ,so have heart… I learned
casting to have heavier findings,( i cut stones and had to put
them some where )…I did so much recasting trying to get rid of
little holes ( now i know porosity)…that i had to learn mold
making and wax injection…then to wax carving to really get the
findings right…IF I had of known all these methods for
fixing …well you say what you want…but I still try to FIX
(porosity) in the casting stage…but with all these
ideas…I ll will just try to be a bit more careful…with
investing…burn out,flame control…ect.ect…had to comment…clyde
in south east texas …expecting cold front of 90
tomorrow…OH…This all started 32 yrs ago
And bully for you too Ron. Expecting the consumer to have even a
tiny bit of responsibility for caring for their jewelry seems to have
gone by the wayside. It is definitely our responsibility, however,
as the experts to educate and inform our customers, giving them the
ability to make the right choices. A.J. at Bremer Jewelry in
Peoria, IL, where Autumn has definitely landed with a thud!
…Thank you J.Morley for this nice article which Is very well
explained and pointing to the fact that porosity can occure under the
surface.This is the kind of porosity which can not be repared that
easily and this kind of jewelry is subject of decieving
customers.Your comment to this subject clears the reason why I not
like this treatment using a burnisher for hidding the flaws of
porosity.You’re really not sure if your items doesn’t have porosity
tears inside the metal and therefor -again- you should try to avoid
them. By all means,my answer to this article was not an attack to the
originator of it.But,I would like to point to the craftsmen amongst
us to spend more time to avoid castings with porosity imperfection
instead of hidding the flaws.
Regards Pedro
Palonso@t-online.de
I realize you addressed this to John but as a model maker and jeweler
of 25 years here is my two cents. In all model work consistency in wax
thickness is a must.The more consistent the thickness and the smoother
the surface the cleaner the casting. This does not mean that a model
cannot have areas that are of different thickness. It means that the
thickness of an area should be consistent and that any changes in
thickness should be from thick to thin in relation to metal flow. If
you have a thin area moving into a thicker area compensate for the
volume change by auxiliary spruing. Add a small sprue to bridge the
thin area so that total metal volume is equalized. In spruing make sure
all sprues do not have the rolled fillet at the attachment points, but
instead make a smooth transition to the sprue base as well as to the
model. A slight thickening at the two joints is preferable. A rolled
fillet created by the soft wax used to attach the sprue only creates
turbulence in the casting. I have also found that larger castings, 150
grs or more, create turbulence in a round central sprue. I have
started using a square sprue for the center of multiple castings to
help eliminate this problem.
All, It never ceases to amaze me how we all seem to have the
"ultimate" answer to problems in technique. After thirty five years
of trying to figure out what went wrong, I have come to the
realization that many jewelry making functions are definitely
controlled by supernatural forces and that we are not worthy ! (sic!)
All kidding aside, I would like to suggest that we all keep an open
mind when trying to figure out such complex things as casting. Any
book on casting is sure to tell you that there are at least a hundred
variables that can bedevil you.When we say that porosity is caused by
this or that, we should bear in mind that it can be caused by any
number of factors and that no one approach is going to garantee
freedom from defect. Nonetheless, I am going to let you in on an
observation that I have made since using Miracle Cast. I do a lot of
gold recycling wherein I use the customer’s gold. Since old gold is
often of dubious origin and, since it has not been refined of
impurities such as oxides formed in the original castings, it is
imperative that you make every effort to eliminate these oxides by
using the aforementioned additive…why? because oxides create
gases which cause porosity. I RARELY have any porosity and I also use
the Miracle Cast when casting new gold. Why? Because most of us do
not have a perfect flame, often overheat the melt, don’t use the
right kind or amount of flux, screw up on calculations , etc. etc.
All of the foregoing can contribute to the formation of oxides which
cause porosity. I suggest that you use a de-oxidising additive for
all yellow gold casting. ( Don’t forget to adjust your metal ratios)
Ron at Mills Gem, Los Osos, CA.
Hi Linda, I have been threatening to write a book for some time!, but
who has any?! The basic thing to understand is this: (1) Metal is
greater in volume when it is molten, this expanded mass is injected
into a confined space, ie: an investment mold. (2) Generally speaking,
metal cools from the outside in, like layers of an onion. (3) As metal
cools (re-crystalizes) it is in motion. Now…measure your wax pattern
or your metal model before molding. Literally draw a picture and
record the mesurments in mm’s on your “map” Think of where you would
put the sprue (did you ever try looking up “sprue” in the
dictionary!?) Count how many thick spots you have isolated by thinner
spots. Thats how many sprues you need and where you need them. The
sprue should be slightly thicker than the spot you are spruing. This
is the only way to feed the cooling metal and prevent porosity. When
you do not feed theese “Hot Spots” they have nothing to do but pull
themselves apart. OK…now you have 14 sprues! Be critical…can you
make the shank thicker on the bottom and slightly thinner on the
sides? Can you take out some material from the inside to make a smooth
transition from thin to thick? When an experienced modelmaker (I’ve
seen the work of extremely gifted and practiced artists who didn’t
understand these principles) sets up a model in this fashion it is
called “progressive solidification” One more thing…when you sprue a
mutiple sprue piece, try to use short pieces, about 3/4", gather them
to one spot to a trunk sprue that is as large as their combined
circumfrence of about 3/8" in lenghth, then attach that to your sprue
base. Think of it as casting a complete system, not just your piece.
The system should weigh arround 3-4 times the weight of your piece.
All of this will be in great detail with lots of photographs and
illustrations in the book…or should it be on CD? John, J.A.Henkel
Co.,Inc. Moldmaking Casting Finishing
Several have said, rightly I think, that improper spruing is a cause
of most casting porosities, but they didn’t clarify what to do about
it. I could easily be repeating something already offered, but I don’t
recall it and I’m surprised that it hasn’t been elaborated.
The first rules for “taming the sprue” (love that, thanks) are to
attach them to the heaviest sections of the wax models whenever
possible; and when that isn’t possible (because of design surface
details, perhaps) try to make the transitions from thin to thick more
gradual, and use thicker sprues with heavier castings – these will
help prevent the majority of uneven shrinkage porositys. (more later)
. . . If only it was all really that simple. . .
Regarding various casting techniques and skills, I think we
metalsmiths and artists have a common tendency to “reinvent the
wheel,” which is a waste of time compared to research, of course, but
then, to top it off, we frequently have the audacity to not only think
we did it first, but to steadfastly thereafter avoid the learning that
predates us, and to believe ours is the best “wheel” ever invented. .
. when damn few of us have invented much and we may even be (gasp)
wrong about what we think we understand. Moreover, some of the most
renowned authors and teachers of the art seem to have also avoided
learning little more than the above first rules from their teachers,
and so it goes on.
Anyone can learn about casting porosity from dental technology and
metalurgy sources – libraries, moderately difficult reading,
humbling, and it’s been understood since the late1930’s. I’ve known
the following for ten years and I still have occasional
problems that I consider “normal” with very difficult projects; there
are simply too many variables to always attain perfection. The truth,
in spite of some our tender egos here, is that ALL castings have
porosities at some level of microscopic scale. By and large, but not
always, they can be controlled, once understood.
My previous description of electroforming and burnishing as a repair
method (a week ago) was a technique I’ve never seen used before. I
reinvented the wheel (but I knew it, at least); it’s similar to repair
techniques in industry. It’s value is that it allows for the
completely successful repair of gas porosity or microporosity – the
smallest kinds – in a surface that could not be compressed lower
than its surroundings with simple burnishing or “beating” alone, and
it could not be soldered with heat at all because it involved a watch
case with an installed glass crystal. This was a disheartening flaw
rightfully small enough to be excusable in the first place, if I’d
cared less, but instead I healed it. I was able to maintain the exact
contour and level of the surface by repeatedly adding metal and
compressing it into the pores.
Some of this has been described here previously, but for the sake of
putting it all on one page. . . .There are four basic catagorys of
the most common porosity causes:
1.Mold failure (large porosities filled with investment) Mold
failures release pieces of investment loose in the cavity. this can be
thin weak sections breaking from the force of onrushing metal; or
spalling, parts of the mold walls falling off from incomplete mixing
of the investment; or contaminated investment with already cured
particles mixed in (drops of water splashed into the dry investment
container); or overheating in the burnout; or the result of "painting"
on investment and having various thin layers of the mold curing or
drying before other layers; or cavitys in the wax model that fill with
investment during vacuum investing.
-
Shrinkage (irregular porosities located near sprue inlets or at
the fast transitions from thin to thick sections) This is caused by
uneven cooling and shrinkage, a thinner section solidifies before
another thicker section farther in, cutting off the supply of liquid
metal as the thick section shrinks within itself, rather than the
ideal situation in which the button remains a liquid reservoir for the
whole casting. Shrinkage of this sort may have a tiny tree-like
appearance because the portions of the alloy that solidify first grow
thin crystals, dendrites, and the lower solidifing constituents shrink
away from those dendritic structures as the rest of the metal cools. -
Trapped gas (pin hole gas bubbles, spongy areas) These are
generally roundish and smaller than shrinkage porosity. They can
extend throughout large sections but are often associated with fast
changing transitions and may be the result of excessive turbulence and
the venturi effect – spraying through constrictions, then trapping
bubbles; another frequent cause, though, is a result of the release of
gasses from the alloy as it cools during its liquid stage. Silver and
copper (both in sterling and gold alloys) can absorb excess oxygen
from an oxidizing torch flame and release it during the cooling, when
it forms bubbles. I believe platinum and palladium tend to absorb and
release hydrogen from either hydrocarbon fuels or a hydrogen/oxygen
torch. -
Micro-porosity shrinkage (microscopic porous layer) These are
smaller than the previous gas bubble variety, and tend to be found in
heavier castings, located in a layer beneath a solid skin and can
extend around large areas or even the whole casting. Often they are
not discovered until the final polishing after filing down through the
apparently perfect skin. The cause is a mold that is too “cold,”
(under 900 degrees Farenheit) or too much difference in temperature
between the molten metal and the mold (Particularly insidious, this,
because it seems so reasonable to cast a heavier item into a
relatively cold mold); The metal enters the cavity and slams into a
relatively cold mold wall, the first metal solidifies instantly and
half a millimeter back the metal shrinks within itself causing
microscopic pores – no gas, tiny vacuums – because of rapid cooling
in that thin layer all around the casting. The inner metal isn’t hot
enough to remelt the already solidified outer metal, but it cools
slower and draws in metal from the button normally, so the core of
this casting will be solid. A solution is a hotter mold, probably 1000
degrees Farenheit.
(5.) Combinations of all these causes, and more.
Alan Heugh
John, What a great idea, a CD on Casting by you, would be a treasure.
I hope you do find the time. Thanks for an excellent (as usual) answer.
Teresa
One other consideration I haven’t yet read on this thread is this. I
was teaching a friend wax work and then spruing. I would then cast her
pieces. So home she went to spru her meticulous little pieces she had
created. She had done an adequate job with the placement of sprus and
all but hadn’t used good closeup lenses to look over the condition of
the spru wax and the connections to her pieces. If I had cast the
pieces from the trees she had brought over, the metal would most
likely been full of bits of investment. She had left all sorts of
nooks and crannies in the spru wax and the places they connected to
the sprus and main trunk. These would have filled up with investment
and the metal rushing in would surely have sent them into the pieces
she had modeled. I get out my magnifying glasses and go over the whole
tree just as if it were a wax model making sure all surfaces are
smooth, free of any pits and with smooth rounded areas at all
corners and connections. NET
Hello thomas,
The vacuum is needed to force the metal into the entired mold since
metal formes a blub instead of flowing out like water does.Now, the
investment by itself is very porous and that is the reason why you
can literaly suck a vacuum trough the investment pulling the metal
into the mold.More spruing not always results in better castings
!!!It’s obvious isn’t it.The strengtness of the vacuum stream
tremendously decrease the bigger the hole is.In other words your are
reducing the force the more sprue’s you make ending towards the
casting opening. If you would make a opening on the side of the flask
you have to consider the loss of metal in one way and in the other
way more work in order to finish your item. A technic is used a lot
with people who are casting big statue’s or other big items.They make
little needle like long (approx 2" long)pins all over the model in
order to capture the gasses in these veins if I may call them so.Of
course some bronze-or whatever the casting metal is-will fill those
little channels but the casting by itself will not be damaged or
misformed by airbubbles or gasses. I’ve been in Jacksons hole
(Wyoming) and I’ve seen extreem nice bronze casted tables and other
bronze art work.Maybe you should go a head and try to contact one of
those craftsmen for more knowledge about this. I’ve seen good books in
the museum of “buffalo bill” but I’m sure that you could find books
about this in any larger casting art stores.I think that other
members know more about where to find this kind of info in America. To
end this subject,it takes some experience to find out how to sprue up
your items to finish with a good casting.A good rule is to keep the
heaviest point away from the sprue entrance,pointing to the outside
of your flask.If you have an unregular item make one sprue which is
big enough to provide the whole casting with metal and make smallers
sub-sprue’s to provide this critical areas with metal.This may sound
odd but try to imagine how you would act if you where in place of the
hot melted metal!!!
Regards Pedro
Palonso@t-online.de
Hi John, Thanks for clarifying “taming of the sprue”, this will help
me greatly. Lots of photos is always a good thing, don’t forget to
give verbals with each photo. Definately both illustrated and CD,
after all not everyone is equipped with plastic and silicon chips yet,
besides why limit yourself. Hoping that Time will
find you…Thanks again, Linda S.
I just came across what to me at least is a set of tools from
Gesswein for dealing with porosity. They are called “Porosity
Removers.” What follows is from the Gesswein description of the
tools.
"Rotary Burnishers for platinum, gold and silver
The fastest and most effective tools for removing porosity and
scratches. Patented creations, these rotary burnishers are made of
highly polished tungsten carbide and mounted on 3/32 shanks. Each
burnisher has a 12-, 18-, or 36-flute sidewall that produces its own
unique effect on metals. The 12- flute burnisher leaves a coarse
finish and covers large pinholes on gold and silver; the 18-flute
burnisher leaves a medium finish and covers moderate pinholes on all
metals; and the 36-flute burnishers leave a fine finish and covers
small pinholes on all metals."
There are 12, 18, & 36 flute burnshers that measure 5x13 mm and an 18
and 36 flute burnisher that measures 10x7 mm. Prices range from $45
to $69.95 each (US $).
Rather pricey, but I thought I would mention it as there has been
some considerable discussion on tools and methods for dealing with
porosity.
B Goodman in Beautiful BC