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Flask temp for casting gold


#1

There has been some discussion about flask temperature for silver,
but what about gold? I have only cast gold a couple of times, but it
didn’t dawn on me to use another flask temp than I use for silver.
The gold castings (14k) seem to come out ok at 900f, but is there a
better flask temperature?

Thanx,
Gena Wagner
Louisville, KY


#2
There has been some discussion about flask temperature for silver,
but what about gold? I have only cast gold a couple of times, but
it didn't dawn on me to use another flask temp than I use for
silver. The gold castings (14k) seem to come out ok at 900f, but is
there a better flask temperature? 

There really isn’t any one flask temperature for any given metal. It
depends on how you are casting (centrifuge, vacuum with perforated
flask, less aggressive vacuum, etc) the size of the flask, weight of
metal you’re casting, type of metal (karat, color, exact alloy type),
thickness or detail level of the models, how much of a superheat
you’re giving the metal (if the metal is barely above it’s melting
point, the flask will need to be hotter than if the metal is a bit
hotter than it’s melting point, for example)

In general, silver won’t need quite as hot a flask as the same items
in golds, but 14K usually needs cooler than 18K, white golds and
yellow golds differ, and even just within 14K yellow golds, the
difference between silicon deoxidized alloys and zinc deoxidized
alloys will be noticable.

Ideally, you should run some tests for the types of things you cast.
The goal is to use the lowest flask temperature that still gives you
a fully filled casting. Usually, that will give you the best results.
(but not always. Some alloys will give you more porosity if you are
casting too cool, while others will give you more problems if you’re
casting too hot.)

900F is a decent starting point for your 14K yellow castings if
vacuum cast, and items are of typical weight, not very detailed, not
too heavy either. But test it from there to see what changes and
improves with lower or higher flask temps, and more or less
"superheat" (the amount the metal is heated above it’s melting
point). Lower flask temps mean the metal solidifies more quickly,
giving a smaller grain size. This generally is good, giving stronger
metal. Too low a temp though, and you’ll find your castings coming
out glossy and shiny. To an extent, this isn’t bad, but is an
indication that the metal was solidifying before it fully "wetted"
and conformed to the investment surface, which would have left it
matte. And too cool a flask can lead to some shrinkage porosity just
under the skin of the casting. Too hot a flask gives you larger
grain size, more oxidation of the casting surface, and more chance
for porosity caused by breakdown of the investment, or gas absorption
of the metal during the casting process.

You might also consider simply asking your refiner or metals
supplier for the recommended flask temps for their alloys. Usually,
casting alloys are well enough known that the dealers will have an
idea of what temps work best, both for melting/pouring the metal, and
for the flask temps.

Peter


#3

Gena,

What I have found to be the important thing to remember when casting
is flask temp… Getting the molten metal into the mold cavity at the
proper temperature is the first half of the equation. How the metal
cools and solidifies to yield the densest casting possible is the
other half. (The third half is the investment- how it’s mixed, how
high it’s burned out, etc… The fourth half is spruing…) Casting
is a dance of variables.

The flask temperature-- that is, the temp. at the center of the
investment mass-- is key and varies with model size, volume and mass.
In a nut shell, I’ve found that a heavier, “gent’s” signet type of
ring should be cast at a lower temp.-- even as low as 700- 800 F.

A larger mold cavity evacuates air more easily and the lower flask
temp allows for more rapid cooling of the molten mass which, in turn,
means a smaller crystal structure (crystals have no time to grow) and
a more dense casting.

A finer model-- a filigree ring for example-- is different. The
smaller mass means that cooling will happen rapidly even at a higher
flask/mold temp. so the grain growth is already refined (relatively)
Oddly, the small amount of air in the mold cavity has a tougher time
evacuating ahead of the flowing metal- it must move into the porous
investment very quickly. (The larger mold cavity with a larger amount
of air has more time as it fills with molten metal, relatively, to
evacuate trapped air.)

The whole thing is a balancing act, in my eyes. I am always juggling
model/mold cavity mass against temperature. Too low a flask temp.
and you have a no fill. Too high and the item will cast fully, but
the quality may be poor (porosity, brittleness). Way too high and the
investment begins to break down, ruining the cast surface and
eventually releasing sulfur into the casting.

If I have the luxury of time and a rubber mold, I’ll cast a series
of flasks with the same item inside, lowering the flask temp. until I
have a no fill. Then I go back up a bit. This seems like a time waste
and it may indeed be. But if you are producing a line it may make
sense.

While every metal has an ideal melting/casting temp. I find the
flask temp. to be much more important and significant regarding
quality of castings. Another thing that goes into my balancing act is
the specific gravity of metal. Gold, which is heavier than silver,
has more momentum and “packs more punch” when filling the mold.
However, silver seems to be more fluid and fills finer forms a little
more easily. However (again), silver freezes faster than gold…
Casting gold has always seemed easier to me. In so many ways gold is
ore forgiving than silver.

Juggling all this can seem daunting. But learning to drive a car–
now that’s a juggling act. If we learn to do that, everything else
seems easy!

Take care, Andy


#4
How the metal cools and solidifies to yield the densest casting
possible is the other half. 

What do you mean by “densest casting”? I have often heard this term
used in connection with different casting methods but I really don’t
see how two castings without porosity can differ in “density.” As a
casting solidifies, the metal shrinks and molten metal is pulled into
the solidifying mass from the button or “risers.” If the model and
sprue system is designed correctly all porosity should be on the
risers, sprues, or button and the casting should be solid and
non-porous. Under these conditions I don’t see how the “density” can
vary. Faster cooling will result in a smaller crystal size in the
metal, as has been mentioned, but does that result in a measurable
difference in density?

This is a serious question and can anyone point out something I am
missing.

Fred


#5
Under these conditions I don't see how the "density" can vary.
Faster cooling will result in a smaller crystal size in the metal,
as has been mentioned, but does that result in a measurable
difference in density? This is a serious question and can anyone
point out something I am missing. 

If density is defined as weight / volume, then you are correct
(providing no porosity). As a jeweler I define density differently. I
define it as how many times can I bend a prong without breaking it,
or how long the finish would last, or how thin can I make supports,
and etc. From this point of view, the smaller the crystals, the more
"dense" the material. “Effective density” would be a better term.
From this point of view, nothing beats the metal which was forged
and rolled and given shape by manual processes. The effective density
(wearability) of finished piece is far exceeding anything produced by
casting.

Leonid Surpin
www.studioarete.com


#6

Hi Fred,

Yes, size-- crystal size-- does matter. Think about two beaches.
Where I was raised, we had beaches on the north shore and beaches on
the south shore. The north shore beaches were rocky and made of
pebbles. The south shore were fine sand.

Even though the fine sand beach contained many more particles than
its north shore cousin it presented itself as one complete surface.
The grains of sand were so densely packed there were very little open
spaces in the mix.

The pebble beach could never be as solidly–densely packed. There
were voids between the stones. You could trace a winding pathway
across the beach following the boundaries between stones.

This twisting boundary can become a fracture plane when you apply
the metaphor to a casting with larger “unrefined” grains. The result,
in many cases, of too high a flask temp. It is much harder to trace a
line between the tightly packed sand grains of the other beach. You
could trace such a line, but it would be full of many micro twists
and convolutions. There is much more surface area contact between
grains. The metaphor here yields a casting that is dense and much
less prone to brittleness.

In the end, the smaller the grain/crystal size, the more tightly
packed they can be and the more dense a product.

Dense castings produce less porous, less brittle objects that are
less likely to fail and more easy to polish well.

Take care, Andy


#7

fred

Faster cooling will result in a smaller crystal size in the metal,
as has been mentioned, but does that result in a measurable
difference in density? This is a serious question and can anyone
point out something I am missing. 

i dont have a scientific explanation for you but as a working
goldsmith who sizes these rings from asia manufacurers just are not
as dense. my assumeption is that they are cast with vacume machines.
i know when i cast with centurfuge my castings seem more dense.

Robert L. Martin
Goldsmith/Platinumsmith
Diamond Setter
since 1976


#8
i dont have a scientific explanation for you but as a working
goldsmith who sizes these rings from asia manufacurers just are
not as dense. my assumeption is that they are cast with vacume
machines. i know when i cast with centurfuge my castings seem more
dense. 

For all practical purposes there is no difference in the density of
cast items whether cast with centrifuge or vacuum or static pour.
The work you are referring to may use a different alloy than you are
used to and that alloy may indeed be slightly less dense but you
cannot alter the density of a metal with the casting technique to any
significant degree. All castings contain some porosity some more
than others but even on a very porous casting you would not be able
to note a difference in density without very sensitive analytical
equipment. All casting methods that are used for jewelry yield
densities greater than 99% of theoretical alloy density.

You may indeed see differences in these castings to those you are
used to working with but it is not the density as long as it is the
same alloy in use.

James Binnion
@James_Binnion
James Binnion Metal Arts


360-756-6550


#9

Hi Andy,

Your analogy of the beach w sand and pebbles doesn’t translate to
metals real well. While there are definite advantages to small
crystals in the metal matrix density is not one of them. You analogy
suggests large voids between crystals that would be vacant but in
reality there are not voids between metal crystals. They form and
grow till they encounter another crystal then stop but they do grow
to fill all the void between adjacent crystals as long as there is
molten metal to supply the growth process. So whether the crystal is
small or large the density is the same.

Also given the typical temperature of the flasks and the poor
thermal conductivity of the investment the crystal size will not vary
that much with a few hundred degrees of difference in flask temps.
What will change is the time that the metal stays liquid and how much
metal mold reaction there is. This action stops when the metal
solidifies at the mold face.

James Binnion
@James_Binnion
James Binnion Metal Arts


360-756-6550


#10
If density is defined as weight / volume, then you are correct
(providing no porosity). 

When referring to materials (metals or alloys) density is the ratio
of its mass to its volume.

As a jeweler I define density differently. 

no once again you make up meanings

I define it as how many times can I bend a prong without breaking
it, 

No this is ductility, a materials ability to be deformed without
losing toughness.

or how long the finish would last, 

No this is abrasion resistance

or how thin can I make supports, 

This is Youngs modulus or modulus of elasticity, the tendency of an
material to deform along an axis when opposing forces are applied
along that axis. It is defined as the ratio of tensile stress to
tensile strain.

From this point of view, the smaller the crystals, the more "dense"
the material. "Effective density" would be a better term. From this
point of view, nothing beats the metal which was forged and rolled
and given shape by manual processes. The effective density
(wearability) of finished piece is far exceeding anything produced
by casting. 

Again there is no real difference in density between wrought and cast
material but worlds of difference in their microstructure.

As virtually all metals and alloys we work with are cast some where
in their journey to our bench. You need to understand what happened
to get them there. To quote Dr Chris Corti

The properties of materials are dependent on their microstructure
and processing history. They are all interrelated. Whilst used
primarily for decoration, jewelry alloys are engineering
materials and follow the same 'rules' of science as aerospace and
construction materials! To make good jewellery, a goldsmith
should understand the materials he uses. 

There are castings that are many times better than wrought materials
could be. As an example the single crystal or directionally
solidified

turbine blades used in jet engines, or super cooled cast gold alloys
that have nanometer size crystals, many orders of magnitude tinier
than can be produced by cold work. But those processes will not be
economically viable for jewelry work. It is also possible to ruin
wrought material by over annealing or annealing too often, too much
work between annealing and in other ways to produce material that is
sub-standard and not the equivalent of well cast material.

The trick is to choose the right processes to produce a material
that will perform well in its intended use. In some cases it is die
struck others it is cast or in others it is fabricated wrought
material.

James Binnion
@James_Binnion
James Binnion Metal Arts


360-756-6550


#11

James:

Thanks for chiming in on this discussion. We needed your background
since my metallurgical background is somewhat limited although I was
trying to point out some of the things you covered. Most of the time
we loose folks when we get too technical but some of these
metallurgical questions need your details.

Regards,
Fred