Lowering the casting temp of bronze?

Jewelry Discussion
#1

It is a long story WHY I want to do this, which is briefly explained
later on.

I am trying do do a small bronze casting, 200 grams, gravity poured
in a vented plaster mold. My tests have filled the molds OK, but
failed to fully get the details. Seems like the mold or the metal are
not hot enough. I used a 90% Copper 10% tin alloy. The plaster starts
to crack if heated over 500F, so that is my limit for the mold
temperature.

What if I add some pewter to the bronze, maybe 10%. Will that make
the metal more fluid? Enough to make a difference? What about other
elements? Lead or zinc.

I am trying to make a mold that demonstrates how a certain ancient
object was cast. I have made these kinds of plaster molds work with
silver alloys, several hundred degrees lower casting temperature than
the bronze.

If my goal was simply to make the object, I would have no problem
just dropping the mold in the centrifuge. The great dread is that I
am going to have an enormous amount to time invested hand carving the
mold and then fail because the metal does not flow well enough.

A little more explanation. Additives to the plaster, clay, sand,
pumice or talc, all work great to prevent cracking at higher
temperatures. The problem is that they spoil the carving properties,
so I am using straight Pottery #1 plaster of Paris for the part of
the mold that is carved.

If you are really into to it, there is a paper on an earlier project
in my investigation at

http://www.ganoksin.com/gnkurl/ep81g4 [PDF file]

Thanks for any advice and encouragement.

Stephen Walker

#2

There is an extensive literature on the casting of Japanese temple
bells, which are similarly cast into hand incised molds. That might
be a place to look for appropriate techniques.

The molds are fired clay rather than plaster, so stand up to the
heat of the molten bronze better.

Elliot Nesterman

#3

Michaelangelo, or was it Chellini, had the same problem a little
while ago casting his Perseus. He did just what you plan, add some
pewter ie more tin, He threw into his melt all his table are and got
the pour right.

all you can do is try.

To get the same colour you could substitute zinc, up to 30 % but
this brings you into the deep drawing brasses. I dont know the
melting point of brass right now. My engineering books are stored
behind some other stuff.

Best run trials with your various options.

#4

My experience with altering the alloy is the the bronze becomes very
brittle with the addition of more tin to the mix. Your mileage may
vary.

Brenda

#5

Sounds like a cool project.

If the idea is to cast the piece with technology of the period, what
about using steam to force the metal into the details? I’ve never
done it, but I’ve read about a technique where you use layers of
cloth or paper saturated with water, attached to something like an
old jar lid, to force the metal from a reservoir in the top of the
mold. I’m pretty sure it is in The Complete Metalsmith.

Noel

#6

Can you use crystobolite investment? Like Satin Cast? Might still be
carveable…

#7

Adding zinc will both lower the temperature and increase fluidity.

James Binnion

#8 
I am trying do do a small bronze casting, 200 grams, gravity
poured in a vented plaster mold. My tests have filled the molds OK,
but failed to fully get the details. Seems like the mold or the
metal are not hot enough. I used a 90% Copper 10% tin alloy. The
plaster starts to crack if heated over 500F, so that is my limit
for the mold temperature.

200g is marginal for gravity casting. A little assistance from a
vacuum might help here. Failing that, adding a large void attached to
the thickest part of your intended casting would perform two
functions: it would raise the total quantity of metal, which would
help fill the mold by adding to the static pressure of the liquid
metal, and it would also act as a shrinkage reservoir, in case your
problem is due to shrinkage and not lack of pressure or fluidity.

What if I add some pewter to the bronze, maybe 10%. Will that make
the metal more fluid? Enough to make a difference? What about
other elements? Lead or zinc.

That would make it melt at a lower temperature, but I doubt it would
behave much better. Bronze is extremely fluid when it’s at proper
pouring temperature, enough so to fill any tiny cracks in a mold,
especially a hot one.

I am trying to make a mold that demonstrates how a certain ancient
object was cast. I have made these kinds of plaster molds work
with silver alloys, several hundred degrees lower casting
temperature than the bronze.

In that case, it would seem that you should use the same alloy as
was used in the original objects you’re investigating.

If my goal was simply to make the object, I would have no problem
just dropping the mold in the centrifuge. The great dread is that
I am going to have an enormous amount to time invested hand carving
the mold and then fail because the metal does not flow well enough. 

A little more explanation. Additives to the plaster, clay, sand,
pumice or talc, all work great to prevent cracking at higher
temperatures. The problem is that they spoil the carving
properties, so I am using straight Pottery #1 plaster of Paris for
the part of the mold that is carved.

Have you tried silica flour? Straight plaster doesn’t make a very
good investment material, as you’ve discovered. If you could cast
with the mold hotter, the metal would remain fluid longer and fill
better. I wonder if you’ve even driven off the chemically-bonded
water at 500F.

If you are really into to it, there is a paper on an earlier
project in my investigation at http://www.ganoksin.com/gnkurl/ep81g4
[PDF file]

That’s a nice paper, Stephan, and while I agree with your conclusion
that the pieces you examined were made with a combination of casting
and chasing techniques, I’m skeptical about your conjecture that
gypsum plaster was used to make directly-carved molds for bronze
casting during the period in question. For one thing, it just doesn’t
work that well.

Alternatively, soapstone does carve nicely and it makes a good mold
for bronze, surviving multiple casting attempts. Furthermore,
soapstone molds have been found that show signs of having been used
that way. Here’s a link showing how a modern practitioner has revived
the technique for use with pewter:

http://www.ganoksin.com/gnkurl/ep81g6

Here’s a video of someone using primitive equipment to cast a bronze
axe in a soapstone mold: - YouTube

All the Best

Andrew Werby
Juxtamorph.com

#9

Stegetting yhourven,

When I did large scale bronze castings we had a few tricks to make
the plaster coatings. We first painted the plaster on the wax. Then
we did a second painting and threw sand on it to make a good coating
more like coating chicken with flour and cornmeal to be fried. We did
about ten layers where we would first apply plaster then sand. It got
a thick shell on it. From there you could proceed how ever you were
taught. It is much like the twig casting discussion. You need to seal
in that fine carving you want to capture. Paint the plaster on then
do another quick coat and throw the sand. There are reasons the
silica in the sand is useful.

As for lowering the temp of bronze by adding another metal and
percentage at that, you are creating another alloy. Stick with
getting your molds done properly.

Aggie on the road.

#10

Can you use crystobolite investment? Like Satin Cast? Might still be
carveable…

#11

I’ve lost the original post, so I don’t know who the original poster
was.

Given the subject, I suspect Steven Walker?

Meanwhile, I have three comments:

(A) I’ve always found molten bronze to be more viscous than molten
silver. The way I explain it is to say that molten silver flows like
milk, while molten bronze is more like maple syrup. Without the
power of a centrifuge or vacuum system (Or a lot more weight) to
drive it, you’re going to have trouble with fine pattern details.

(B) At one point in my history, I was very interested in replicating
chip carved pieces like the St. Ninian’s hoard. Given what I know
about period bronze casting, my thought about them has always been
that they were rough cast, and then cleaned up with an engraving
tool, to go back over the chip carving, and really crisp it up.
That’ll give you the fine raised lines, easy layout/carving and
the sharpness.

With a properly sharpened onglette graver, it’s really not that hard
to chip carve directly in silver. Doing a light skim cut over a
pre-cast bronze surface would be very simple.

(Deeply tedious, but not hard.) (Somewhere I think I can find it,
I have the sterling master for a knotwork brooch that I did chip
carve directly into solid silver. With the right graver, you really
don’t see the tool marks. If I can lay hands on it, I can send
pictures.) (No, it wasn’t anything fancy, just a spitstick polished
for bright cutting.) (and it was cut naked eye. No fancy scopes or
powered gravers.)

© If you’re having trouble with cracking and spalling of the clay
mold body, try mixing in a little black iron oxide powder. (The
black stuff that flakes off all over the anvil when blacksmithing.)

I have reason to believe that may have a salutary effect. Grind it
up fine. That may let you get a clay mold that’ll take a higher
casting temp. The ratio you need will be different than what I used,
as I was doing something totally different. Play with it, and see
what happens. What about green firing the mold before doing a
secondary firing to bring it to casting temp?

Regards,
Brian.

#12 
Failing that, adding a large void attached to the thickest part of
your intended casting would perform two functions: it would raise
the total quantity of metal, which would help fill the mold by
adding to the static pressure of the liquid metal, and it would
also act as a shrinkage reservoir, in case your problem is due to
shrinkage and not lack of pressure or fluidity

The volume of the reservoir void is not directly related to the
static pressure, the height of the reservoir is what creates more
pressure. A taller pouring cup or riser will give an increase in
pressure but volume alone will not. I agree that the mass of metal
is a bit small to cary the heat for filling the mold.

James Binnion

#13

I really liked your paper! But why lower the temperature of the
bronze in question? Was medieval bronze of a lower melt temperature?
I think you need to reexamine the production of clay molds for this
process. You tested various clays for carving but did you test them
in various fired states? What about an extremely green fire of the
clay: I’m thinking sub bisque temperatures to leave the mold material
soft enough to carve and scrape.

You could also mix in powered charcoal to the clay and fiber
(herbivore dung) to act as a fibrous binder. This would burn out
easily or carbonize at low temp. See Ashanti casting for
sophisticated casting techniques preformed in modern times
without (mostly) our sophisticated technologies.

http://www.ganoksin.com/gnkurl/ep81gd

While Ashanti casting molds are strictly lost wax and not directly
carved, the mold making technique might provide you with some
alternate clues.

Good luck and post any new findings!
Michael Edwards

#14

Thanks for all the replies!

Aggie, I am backing my plaster with a sand/plaster mix or with
sand/pumice mix. How hot were you firing the burnout for the
sculpture? Did you have much trouble with cracking or flash? I
suspect I could push the temperature up a bit without too much risk.
Maybe a little cracking and the resulting flash will be easy enough
to deal with.

Andrew C., I have used Satincast as a core and for backup in my
trials. It is not hard enough to stand up to the type of carving I am
doing. The test is to make parallel lines, 2 vertical and two
horizontal about 3 mm apart (#)with a “V” graver about 60 degree
angle. If this will consistently leave a sharp pyramid, it works.
Doesn’t work with Satincast, clay or any plaster tempered with more
than just the smallest amount of an additive.

Andrew W., Unfortunately we don’t know the original alloy. The piece
in question, the Ardagh Chalice, has been extensively studied. There
have been assays of the silver and gold bits, but the bronze has not
been tested as far as I can learn. I am trying to reproduce the
cylindrical stem section. Not especially a good project for a
multi-part mold in soapstone. The four bronze cast pieces are all
gilded and all show some casting porosity. I don’t have to make a
perfect casting for this to be a success. Moisture can be driven off
OK at 500F if it is done long enough.

I have done often enough to have solved the problem.

Noel, steam might be a solution to get better pressure into the mold
but I am not going to try it. It seems like there is too much to
learn about how to make that work. I have had pretty good luck
gravity casting silver in these kinds of molds and I am almost there
with bronze.

Brenda, Brittle will not be a problem. Not planning to bend or forge
the piece after it is cast.

Ted, That was Cellini who tossed in all the household pewter in the
pot while stressing about getting his bronze to melt. It is a truly
great chapter in his autobiography. You can really feel his anxiety
as he tells about this over-the-top project that looks like it is
going to fail because he runs out of fuel.

Jim, I think what I will do is add some zinc to the mix by measuring
out some brazing spelter. I will do the math and add enough along
with some more tin by way of pewter, so that I have about 4 or 5%
zinc and around 15- 18% tin. There will be a small amount of antimony
also. I will pour a test into a sand mold first just to make sure it
isn’t to weird.

Elliot, Clay was what I first assumed the metal was when I first
twigged it that this style was done in carved mold. I tried many
kinds of clay and clay with dung as recommended by Theopholis. Tried
them all all different levels of semi-dry, low fired, etc. It is
great stuff for lost wax or making bivalve impression molds,
especially the dung/clay mix. Could not get it to carve on the level
I needed as described above in the reply to Andrew C.

Here is the plan for the next step. I have made three test molds,
two are carved in haste as tests to see if the detail will fill. The
third will be fully carved. Probably on Wednesday next week I will
try the bronze with some zinc and pewter. 1st I will pour one of the
test molds with gravity, getting the metal especially hot. (Just
getting it hotter might be all it takes) If it works I will pour the
fully carved mold the same way. If the test fails I will put the
second test in my centrifuge. If that goes OK I will finish off the
last one that way. Moving on to “plan C” if all else fails, I will
pour it with a lower melting silver alloy of around 60% silver so
that I can save my effort

Stephen Walker

#15 
I really liked your paper! But why lower the temperature of the
bronze in question?

My main problem is that I don’t have much time left to make this
work before I present this at a conference in July. With enough trail
and error I am sure I could make it work with 10% tin bronze.
Probably just shortening the risers and superheating the metal would
do it, or heating the mold higher. I am going to run some tests on
that tomorrow. But it will take something like 10 to 12 hours to
carve the mold to reproduce the entire pattern of the Ardagh Chalice
stem. My motive to lower the melting temperature is to give myself
some insurance that I will have something to show for it. My thesis
is mainly to show that this is the best explanation for how the piece
was sculpted and cast. It would be better if I can work out all the
other practical details. But I will cheat on the alloy or put my
fully carved mold in the centrifuge rather than sacrifice the time
invested carving.

Was medieval bronze of a lower melt temperature?

Medieval bronze was not especially consistent from what I can learn,
but I cannot find out what the actual alloys were on the specific
pieces I am interested in. The silver alloys used in the St, Ninian’s
Isle brooches have been assayed. They are similar to silver-solder
alloys. I suspect that highly detailed small jewelry type castings
were done in bronze with more of the alloy elements that would cause
them to cast better under those conditions.

I think you need to reexamine the production of clay molds for
this process. You tested various clays for carving but did you test
them in various fired states? What about an extremely green fire of
the material soft enough to carve and scrape.

Good idea. That actually was my original theory. You can read my
first paper on the subject (2006) at
http://www.ganoksin.com/gnkurl/ep81gk Alas!I was wrong. Plaster turns
out to be a much more likely material. I tried all that with clay in
2004 & 2005, including the dung binder. I didn’t try the sub-bisque
temperatures, but there wasn’t much success at any of the other
stages.

Clay can be an excellent casting material for molds and it can be
carved, but it won’t hold the details for kerbschnitt style
chip-carving on a small scale. Of everything I tried in clay,
semi-dry “leather hard” was the best carving. But not even close to
good enough.

Thanks for the suggestions. I will report back on the tests I am
running tomorrow, (Monday) There are 8 molds in the kiln, each made
with a carved flat slab of straight plaster. They are backed with a
flat slab of plaster with 1/3 pumice. These are then enveloped in a
wad of mixed plaster and sand 50/50. It is set to slowly rise to 500F
and hold for the first casting tests. I will try several alloys at
that temp. Then I will raise the temperature by 100F and try again at
600F, 700F, up to 1,100F and see what happens. At some point I expect
the plaster will be so cracked it will be useless, but I will find
out.

Stephen Walker

#16

This reply By Chris is a good start to solving your enamelling
problems.

Id like to add the following.

You do need to have a very clear idea what you want to achieve with
enamel.

just experimenting is great fun but without a proper plan in place
to establish a technique to achieve the idea, your not going to get
very far.

Let me give you an example from my enamelling experience.

I decided to go for results just like Faberge got on his easter
eggs.

That was clear, transparent colours on all enamelling metals.

I did the research, visited his enamel makers Schauer Et Cie in
Vienna, and ran controlled trials with their enamels as well as every
other enamel maker in the UK.

I documented the results of each test, ie enamel type, hard medium
or soft metal, temp, duration with torch firing then kiln likewise.

As Ive mentioned here on numerous occasions, if its been done
before, it can be done again. you just have to reinvent the
technique. that takes patience and time.

Torch firing with small hand held ones is ok to start, but dont be
afraid to think bigger like 1 in to 3in dia. Flame speed also
effects the way the torch works. but thats another story.

Kiln firing is different to torch work, torch firing is effectively
using an atmosphere at the right temperature that needs to be
oxidising not reducing. do you follow? Did I get the results I
wanted? yes and did enamelling for 7 yrs before moving on to bigger
work in the silversmithing world.

#17 
The volume of the reservoir void is not directly related to the
static pressure, the height of the reservoir is what creates more
pressure. A taller pouring cup or riser will give an increase in
pressure but volume alone will not. I agree that the mass of metal
is a bit small to cary the heat for filling the mold.

I agree that placing any extra volume higher in the mold will
maximize the static pressure exerted by the molten metal. But sheer
volume, without extra height, does also contribute. Imagine, for
instance, gravity-casting a simple horizontal plate 1/4" thick and an
inch square, with a small sprue. The pressure in that case would
barely be sufficient to overcome surface tension and force it into
the corners of the mold.

Now imagine that it was adjacent to a larger plate in the same mold,
at the same height and of the same thickness but 6 inches square.

Don’t you think the extra metal would exert more pressure? A small
amount of metal will certainly freeze faster, but the pressure of a
larger volume of metal is what makes gravity casting work, and is why
it’s necessary to use vacuum, steam, or centrifugal force to cast
small amounts of metal into molds.

Andrew Werby
Juxtamorph.com

#18 
Don't you think the extra metal would exert more pressure? A small
amount of metal will certainly freeze faster, but the pressure of
a larger volume of metal is what makes gravity casting work, and is
why it's necessary to use vacuum, steam, or centrifugal force to
cast small amounts of metal into molds.

Sorry, it is basic physics, two columns of liquid of different
diameters but the same height will exert exactly the same pressure
at the bottom of the column it doesn’t matter what the difference in
diameters is. Gravity castings work because they are vented and if
properly done they have a tall enough sprue to apply enough pressure
to fill the mold. The only thing you will gain from more volume is
more heat which may or may not aid in the mold filling depending on
the gating system and the cross sectional areas of the mold.

James Binnion
James Binnion Metal Arts

#19

Test results encouraging:

The main thing I learned is that there are gasses pushing back
against the molten metal. Obviously steam for the first test at 500F
and the shortest heat soak, but then even at the higher mold
temperatures. My first test was with a “bell metal” alloy that I made
by adding some brass, pewter and silver to classic bronze. very fluid
and sharp details at the thinner cross sections. Poured with the mold
at 500F there was some steam blow-back that actually pushed metal
back out of the thicker cross sections. The metal stayed molten for a
few seconds after the cup was filled and splattered as the steam
escaped.

Three hours later I poured two more at 500F mold temp. None with the
obvious steam reaction like before. The molds fully filled but the
crisp raised detail cut in the plaster was rounded, especially near
the top of the mold.

Higher mold temps gradually got better results, but all of the
casting showed some push back in the detail, especially where the
cross section was thicker. My interpretation is that the mold is
filling but then gasses are pushing back before the metal freezes.
The thinner sections freeze before the gasses push back. I have known
from fairly early on that straight P1 plaster does not breath well.

Raising the temperature did not crack the plaster until the 900F
mold trial. Then there was just a hairline that could be seen in the
metal. My best casting was this one at 900F mold temp with classic
bronze. Didn’t do any at higher temps as I ran out of test molds.

My conclusion (at this point) is that the historical castings of
these pieces were cast in molds that were made with some kind of
additives that made them breath better. Unfortunately for my
demonstration piece I am already about 90% done with a carving in
straight P1 plaster that has taken too many hours to start over. I
expect to have that finished tomorrow (Wednesday). Will cast it on
Saturday, most likely.

Regarding clay molds. The traditional formulas with dung mixed in
the clay make a mold that is so porous that the small medieval mold
fragments that have been recovered by archaeologists show no venting.
Apparently it was not needed if the material could vent the gasses
through the pores of the mold.

Thanks for the suggestions. I will report back on the tests I am
running tomorrow, (Monday) There are 8 molds in the kiln, each made
with a carved flat slab of straight plaster. They are backed with a
flat slab of plaster with 1/3 pumice. These are then enveloped in a
wad of mixed plaster and sand 50/50. It is set to slowly rise to
500F and hold for the first casting tests. I will try several
alloys at that temp. Then I will raise the temperature by 100F and
try again at 600F, 700F, up to 1,100F and see what happens. At some
point I expect the plaster will be so cracked it will be useless,
but I will find out.

Stephen Walker

#20

Success! The plaster mold replicating the stem of the Ardagh
Chalice, gravity cast with classic 90/10 bronze worked. Raising the
mold temperature to 1,200F (after 24 hr long drying time at 450E) and
super-heating the metal did the trick.

Photos attached. Thanks to everyone who offered suggestions and
encouragement.

Stephen Walker

Andover, NY

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