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Alloying bronze for casting


#1

Would this be a good method to alloy bronze for centrifugal casting?
To alloy 90-10 bronze (90%cu–10%sn), melt 360 grams copper, with
borax, in the crucible. When molten,add 40 grams tin, stir and cast.
Would very much of the tin be lost, as a gas, if cast as quickly as
possible, once alloyed, or is there a better method ?

thanks,
Andy


#2
When molten, add 40 grams tin, stir and cast. Would very much of
the tin be lost, as a gas, if cast as quickly as possible, once
alloyed, or is there a better method ? 

If you melt the copper first, that charge will be higher in
temperature than the melt point of the tin, some of your tin will
vaporise.

I’ve done this before, if you melt the copper first in an open
crucible, then add your tin, your bronze will be noticeably different
in colour to a bronze where the tin melts first.

If you use a lidded crucible in a furnace the vaporisation will be
lessened, but vaporisation will still happen.

Regards Charles A.


#3

Wouldn’t it be better to mix the alloy and make an ingot, then remelt
the required amount for your casting? The remelt temp will be lower.

All the tin should remain in the mix, as I don’t see how the tin
could vapourise at 1060C, the liquidus of copper, when its boiling
point is 2,270C (4,118F). You’d just get super-heated bronze. Let it
cool a little before pouring.

Brian
Auckland
New Zealand
www.adam.co.nz


#4
All the tin should remain in the mix, as I don't see how the tin
could vapourise at 1060C, the liquidus of copper, when its boiling
point is 2,270C (4,118F). You'd just get super-heated bronze. Let
it cool a little before pouring. 

You are absolutely correct about the boiling point of tin. In theory
if it is in the liquid phase liquid you will get a minuscule amount
of vapor from it but the vast majority of that will fall right back
into the melt and the amount lost will be virtually unmeasurable. So
until you hit the boiling point there will not be any tin lost to
vapor.

Jim

James Binnion
James Binnion Metal Arts


#5
If you melt the copper first, that charge will be higher in
temperature than the melt point of the tin, some of your tin will
vaporise. 

Would the melting and flow temperature of both be lower if the tin
was melted first before adding the tin?

D. Ralph


#6

Hi Brian,

All the tin should remain in the mix, as I don't see how the tin
could vapourise at 1060C, the liquidus of copper, when its boiling
point is 2,270C (4,118F). You'd just get super-heated bronze. Let
it cool a little before pouring. 

Remelting 90/10 bronze once is okay, but you start to lose tin the
more you re-melt it, especially if you boil the bronze, but it should
be okay if he’s careful.

How could tin vaporise(?)… well it does, the following is why :-

The melt point of copper is 1083 C (1981 F), and the boiling point
of copper is 2595 C (4703 F). The melt point of tin is is 232 C (449
F), and the boiling point of tin is 2270 C (4118 F).

As you know the melt point is not the flow point.

Regards Charles A.


#7

Hi,

Would the melting and flow temperature of both be lower if the tin
was melted first before adding the tin? 

That’s correct, the tin lowers the melt point of the copper.

What this means is that your melts are more efficient, you use less
energy, and time than if you melt the other way around.

I know this to be true with gas and radiant coil electric furnaces.
Not sure if it would make much a difference with induction with
regard to time and energy. Induction is something I haven’t played
with (I’ll get around to building an induction furnace one day).

Regards Charles A.


#8

Mr. Binnion,

Thank you for this detailed

https://orchid.ganoksin.com/t/bronze-confusion

I have purchased the bronze alloy (510) from McMaster Carr in sheet
form. I have fabricated an anticlastic bangle bracelet in 22 gauge.
It has formed nicely using my anticlastic stakes. The 510 alloy
contains 93.4-95.3% copper, 0.3% zinc, 0.05% lead, 4.2%-5.8% tin,
0.10% iorn, and 0.03-0.35% phosphorous. Should I be wearing gloves
as I fabricate this metal because of the lead content? Also, should I
use different files for this alloy so as not to contaminate other
metals? Thank you for sharing your extensive knowledge.

Elaine Shah


#9

Charles,

If by melt point and flow point you mean liquidus and solidus then
yes with alloys that’s true. But with pure metals such as copper,
tin, silver, gold, the liquidus and the solidus are the same
temperature.

Brian


#10

Hi Brian,

If by melt point and flow point you mean liquidus and solidus then
yes with alloys that's true. But with pure metals such as copper,
tin, silver, gold, the liquidus and the solidus are the same
temperature. 

You know this, but I’ll spell it out for others that may not.

Solidus would be equivalent to the “melt point”, which means just
below this point the substance is completely solid.

The Liquidus is equivalent to the “flow point”, where the metal is a
free flowing fluid.

What you say is true, I’ll adopt the liquidus and solidus as terms
for further discussion.

I do use pure elements in jewellery manufacture, but mostly it’s
alloys, and I think most people are in the same situation (if not
totally using alloys).

The problem with vaporisation is that it does happen in 90/10
bronze, but it’s a problem that really is only really significant if
you re-melt a lot of times. The alloying of pure elements to make
bronze is dependent on when and how you add those elements.

When I make some new bronze, (which is just a mixture, not a new
element) I find that it is better to make your charge with the tin
at the bottom of the crucible, and cover with granulated fine copper.
I am using gas to do my melts, so I have to monitor the melt so that
I don’t boil the resultant alloy. This produces a nice warm bronze
alloy. I weigh my elements as precisely as I can. The smallest melt I
do is 250 grams, and currently the largest is 1.5 kg (soon to be
larger when I get my Mario working).

A friend of mine alloys his own bronze also. However, he melts in an
electric kiln (it’s a burnout oven, but it does the job), and melts
the copper first, then adds the tin.

There is a noticeable difference between our alloys.

Both methods don’t boil the copper, yet the colour variation is
there.

Now back to helping our friend, and casting aside the colour
variation, as an issue.

Why should our friend alloy bronze, tin first?

If you melt the tin first, you will use less fuel/energy and time to
alloy your bronze. The other way around you will use more
fuel/energy and time. It does not matter whether you use gas or
radiant coil (as stated in another post I’m not sure about induction,
as I haven’t played with it).

Regards Charles A.


#11
You are absolutely correct about the boiling point of tin. In
theory if it is in the liquid phase liquid you will get a minuscule
amount of vapor from it but the vast majority of that will fall
right back into the melt and the amount lost will be virtually
unmeasurable. So until you hit the boiling point there will not be
any tin lost to vapor. 

The Odd thing here is, as I remember it, my mom would hang wet
laundry on the line, at MUCH below the boiling point of water, and it
would all dry! There is a criteria called “vapor pressure” that causes
what we call evaporation, well below boiling point.

Tom Parish


#12
If by melt point and flow point you mean liquidus and solidus then
yes with alloys that's true. But with pure metals such as copper,
tin, silver, gold, the liquidus and the solidus are the same
temperature. 

And for those alloys where the ratio of the metals happens to be a
eutectic point, that is true (both liquidus and solidus the same)
even for alloys. Not all alloys have eutectic points, but some do.
Silver and copper, for example, can be made as a eutectic alloy…

Peter


#13
How could tin vaporise(?)... well it does, the following is why :-
The melt point of copper is 1083 C (1981 F), and the boiling point
of copper is 2595 C (4703 F). The melt point of tin is is 232 C
(449 F), and the boiling point of tin is 2270 C (4118 F). As you
know the melt point is not the flow point. 

To get any metal to vaporize in amounts of significance to us as
metal workers you need to heat it to the boiling point. You may be
loosing tin to oxidation as dross or slag but you are definitely not
losing tin to vaporization.

James Binnion
James Binnion Metal Arts


#14

I need to correct this, it should read

You may be loosing tin to oxidation as dross or slag but you are
definitely not losing a significant amount of tin to vaporization. 

Somehow “a significant amount of” got left out before the send
button was pressed, sorry about that.

James Binnion
James Binnion Metal Arts


#15
The Odd thing here is, as I remember it, my mom would hang wet
laundry on the line, at MUCH below the boiling point of water, and
it would all dry! There is a criteria called "vapor pressure" that
causes what we call evaporation, well below boiling point. 

Yes there is a property called vapor pressure, boiling temperature is
typically listed at atmospheric pressure but boiling temperature
changes with pressure. So vapor pressure changes with temperature.
Even solids lose some atoms to vaporization, but most atoms that
break free of the surface actually end up returning to the liquid or
solid rather than escaping to the atmosphere because they strike a
gas atom and are reflected back to the surface of the liquid or
solid. As the temperature increases or the pressure decreases more
atoms attempt to escape until you reach the boiling
temperature/pressure at that point the atoms escaping the surface
exceed the number being reflected back. So part of why your mothers
clothes dried so fast on the line and the water on the clothes is a
lot nearer to the boiling point of water than the molten tin in the
bronze is to its boiling point so more vapor is being generated from
the clothes. But the biggest reason for why they dry out on the line
is the huge surface area. Surface area plays a major role in the
evaporation rate. If you took the volume of water in those clothes
and put it into a drinking cup and waited for it to evaporate you
would be there for many many days.

So I stand by my statement that there is virtually no loss of tin by
evaporation at the melting temperature of the bronze.

James Binnion
James Binnion Metal Arts


#16
Should I be wearing gloves as I fabricate this metal because of the
lead content? Also, should I use different files for this alloy so
as not to contaminate other metals? Thank you for sharing your
extensive knowledge. 

A large number of copper alloys (brass, bronze, nickel silver) have
trace amounts of lead in them. This is a good reason to have
adequate ventilation when soldering, grinding or polishing them. But
normal studio hygiene like washing your hands before eating or
drinking should be enough to keep you from ingesting the metals
including lead that you are handling.

James Binnion
James Binnion Metal Arts


#17

Which alloys have no eutectic?

Richard Furrer
Sturgeon Bay, WI


#18
Which alloys have no eutectic? 

More than do have them. Copper-Zinc, Copper-Tin, Copper-Gold,
Gold-Silver for example.

James Binnion
James Binnion Metal Arts


#19
Which alloys have no eutectic? 

In general, binary alloys where each metal is completely soluable in
the other won’t have a eutectic point. For us, the most common
example is silver and gold. Each of them is completely intersoluable
in the other over the whole temperature range, and the phase diagram
looks like a simple lens shape, without that single meeting point of
liquidus and solidus temperatures in the middle somewhere that
signifies a eutectic point. Silver and copper, and gold and copper,
however, both have eutectic points.

Peter


#20
A large number of copper alloys (brass, bronze, nickel silver)
have trace amounts of lead in them. This is a good reason to have
adequate ventilation when soldering, grinding or polishing
them..... 

Or just don’t use alloys with lead in them. :wink: CIA