Is there any way to predict the melting temperatures of complex
alloys? I am investigating some medieval objects that have profound
casting flaws in most of them. They appear to have been cast from a
mixture of scrap silver and bronze. The alloys contain silver,
copper, tin, lead and zinc as well as trace elements, but each one
is different. Typical analyses are between 17 and 82% Ag, 14 to 75%
Cu, 2 to 13 % Sn, 1.1 to 3 % Pb and 0.1 to 1.3 % Zn.
The objects are the St. Ninian’s Isle Treasure, circa 800 AD found
in the Shetland Islands. One of the most interesting pieces from a
“bad casting” point of view is a brooch (Wilson #24) which has severe
pitting and sponge porosity and also some incomplete filling of the
mold. There is an old solder repair of a casting crack near the gate.
The analysis of this piece is Ag 32%, Cu 60%, Sn 5%,Pb 1.9%, Zn 1%.
Is there any way to predict the melting temperatures of complex
alloys?
I don’t know why I do this to myself, was looking for a phase
diagram calculator, but can’t seem to find one, but I did find a page
that will give you pretty good estimates.
ASM wants a lot of money for access to that DB. I believe it is $500
per year. And the most complex phase diagram available is a ternary
(3 element) diagram. Steven was looking at greater numbers of
components. There are some thermodynamic analysis programs that can
calculate phase diagrams but I am not sure if they will handle that
many components either.
There are some very powerful software packages been run at different
Universities for alloy developments which may be of assistance in
calculating precise melting ranges for your alloy compositions. One
thing to consider in your investigations is what elements are present
as deliberate additions and what are present as high levels of
impurities as a consequence of poor refining techniques.
Three is closer than none, but you are right, a ball park may not
be good enough.
If we could find a site that explained the maths, we could write a
program or make a very complicated spreadsheet.
I suspect it way beyond the current state of the art so solve such
equations for more than 4 metals and certainly it would be impossible
to present them graphically in full. I have heard that there are a
few quaternary phase diagrams but, it is not possible to display them
as a whole. You can only plot diagrams where one or more elements are
held at a static percentage of the alloy. The problem is you need
more than 3 dimension to present a complete quaternary PD. Even most
ternary diagrams you see are isothermal sections of the whole not the
complete diagram.
Working out specific gravity and putting it onto a spreadsheet is a
walk in the park compared to this problem.
I’m thinking that all we want is a close enough temperature range.
We know the melt points of elements, and we know the melt points of
standard alloys, maybe there’s a way to get a “close enough for the
workshop” that can help our friend out?
Personally, I monitor my melts closely, I melt with gas, so knowing
the exact temperature isn’t as important for me, as observation.
I’ve been thinking further about your old silver alloys. As they are
copper-rich silver alloys their structure is different to a
conventional sterling silver alloy, or even the 90% coin silver and
80% continental silver. At silver contents below 72% the structure
of silver-copper alloys changes so that you have a copper-rich
primary phase with a silver rich secondary phase. From the
description of your castings this silver phase should be quite
coarse.
As these pieces will have been cast using rudimentary technology then
the porosity is probably an effect of an attempt to fire refine the
metals (similar to tough pitch copper manufacture). In fire refining
air is introduced into the molten metal (in this case copper-rich) to
oxidise the impurities. Any excess oxygen is absorbed by the molten
copper and silver and then the second stage of the refining process
is to introduce a ‘green log’ (freshly cut wood with a high moisture
content) to carry out the ‘poling’ operation. This introduces
hydrogen into the molten metal which co-exists in equilibrium with
the oxygen. When solidified this equilibrium is disturbed and the
oxygen and hydrogen react together to form steam which becomes
entrapped in the casting. This gives the high levels of porosity that
you are seeing. In tough pitch copper production the oxygen content
of the copper is carefully controlled so that the volume of the steam
cavities naturally counteracts the contraction of the metal.
I suspect it way beyond the current state of the art so solve such
equations for more than 4 metals and certainly it would be
impossible to present them graphically in full.
“Ball-park” would be close enough. The actual alloys used in the St.
Ninian’s Isle Treasure are a rather minor point to what I am hoping
to explain, which is much more concerned with the molding technique.
But I find it very interesting that several of the castings were so
poor in quality, but they were gilded and finished anyway. I would
have imagined that in the 8th century the metal would have been so
valuable in relation to the labor that it would make sense to start
over again and hope for better results.
What I am fishing for by asking about melting temperature is the
possibility that these various alloys have a wide range of casting
temperatures, but that the craftsmen tended to treat them all the
same. If so, some would have been very much overheated, which would
account for the huge pits, cracks and dire porosity. But there are
also cold shuts and incomplete fills at the extremes of some of the
same pieces. These have previously been described as “wear” but I
think any experienced caster would recongnize them as metal that
froze before it flowed all the way into the edges of the cavity.
I would suppose, and please tell me if I am wrong, that undersized
gates or poor gate design and/or molds that were too low a
temperature would account for the incomplete fills and cold shuts,
even if the metal was overheated. Does that make sense?
This introduces hydrogen into the molten metal which co-exists in
equilibrium with the oxygen. When solidified this equilibrium is
disturbed and the oxygen and hydrogen react together to form steam
which becomes entrapped in the casting. This gives the high levels
of porosity that you are seeing. In tough pitch copper production
the oxygen content of the copper is carefully controlled so that
the volume of the steam cavities naturally counteracts the
contraction of the metal.
Reducing gasses like hydrogen(H2), carbon monoxide(CO),
acetylene(C2H2) are highly soluble in molten copper but the presence
of oxygen which is more soluble in molten copper reduces the ability
of the copper to absorb reducing gasses. Molten copper tends to
oxidize unless strong measures are taken to de-oxidize it. The old
school way to do so was/is to use a green stick or log and immersing
it in the molten metal. The green log introduced to the melt when
poling adds CO, H2, and C2H2 to the copper. The steam which is
liberated by the process is not soluble in molten copper so it stirs
the copper as it bubbles to the surface and escapes this helps to
distribute and mix the reducing gasses into the molten copper. The
CO, H2, and C2H2 combine with the Cu2O and reduce the oxide to
either CO2 or H2O both of which are insoluble in molten copper so
they again escape as bubbles. This will continue till all the CO,
H2, and C2H2 or all the Cu2O is gone if it is allowed to remain
molten long enough. If too much of the Cu2O is removed then the
reducing gasses will remain in the molten copper as it has a great
capacity to hold them in its molten state. They then come out of
solution as the copper solidifies creating a raised surface or in
extreme cases worm like protrusions on the surface of the ingot
resulting in porous metal. This is referred to as being Over Polled
or Over Pitch copper. If there is too much Cu2O left in the copper
from not enough polling it will result in an ingot with a sunken
surface and the metal will not be porous but will be hot short and
brittle due to too much Cu2O in the matrix. This is referred to as
Under Poled or Under Pitch copper. If the correct amount of Cu2O is
left in the copper the ingot will have a level surface and be strong
and easily worked with no cracking this is when it is in the Tough
Pitch state.
The way you typically get steam bubbles in the copper is if there is
excess hydrogen or other reducing gasses in the atmosphere as the
copper solidifies or after it is solid as hydrogen easily permeates
hot solid copper. In either case the reducing gas reacts with Cu2O
and reduces it to copper resulting in super heated H2O, this steam
cannot diffuse out of the solidified metal as it will easily do when
the metal is molten and so it will push the soft hot copper matrix
apart and create a cavity.
I would suppose, and please tell me if I am wrong, that undersized
gates or poor gate design and/or molds that were too low a
temperature would account for the incomplete fills and cold shuts,
even if the metal was overheated. Does that make sense?
Funnily enough thermocouples were not available then, so it was
observation that determined if a melt was successful.
It was not unheard of that people lost concentration. Using a side
example Charcoal burners used to stay awake for days tending the
charcoal mounds… a moment of inattention and the charcoal would be
ash. Needle makers used to watch a fire for 24 hours before putting
the needles into this furnace for a week.
Through history there are examples of bad castings and good
castings.
I would suppose, and please tell me if I am wrong, that undersized
gates or poor gate design and/or molds that were too low a
temperature would account for the incomplete fills and cold shuts,
even if the metal was overheated. Does that make sense?
It is likely that any and all the things you mention could be at
issue. From poorly refined metal to the unscrupulous persons adding
base metals to “extend” the volume of the silver. The lack of
understanding of cause and effect in gate design, no temperature
control. What is amazing is that they ever got any good castings. I
think it would be very educational if at least once in their career a
modern craftsperson were required to melt silver in a bellows driven
charcoal fire in a hand made crucible then pour into a stone or clay
mold. It is quite educational
The old school way to do so was/is to use a green stick or log and
immersing it in the molten metal. Nothing wrong with old school, go
the green stick method :-)
Not suggesting there is anything wrong with it, it works fine.
Mostly just offering the correct info about tough pitch and steam
bubbles in copper.
