There is a question that I have been trying to answer. I have not
taken metallurgy classes but it is my guess, that the answer lies
somewhere therein. When soldering silver, you have to heat the
entire piece. But gold, you can heat the specific place to be
soldered (like sizing a ring). I was wondering if someone could
explain why the difference. Also, does platinum fall into the silver
or gold ranks. Thank you for your help.
Rick
Rick,
The answer is twofold.
In any metal it is not necessary to heat the entire piece… if you
can apply the heat in a fast enough pulse… and that is where Lasers
come in. The direct answer is that it is a function of the metal’s
thermal conductivity. When one solders or welds metal with a torch
there are 5 numbers that come into play
The temperature of your torch
The number of BTU per unit of time that your torch tip generates
Thermal conductivity of the object
Cooling rate of the object (how fast does it loose heat by
radiation or convection / direct contact
Time
If 1 and 2 cannot overcome 3 and 4 then your piece will never get to
the need temperature If 5 is excessive then you have too much chemical
mad structural changes that happen
Also, does platinum fall into the silver or gold ranks. Neither, it is even less thermally conductive than gold
Hope this helps. Kay
PS here is an example of a chart of Thermal conductivity (and some
other properties) Thermal Properties of Metals, Conductivity, Thermal Expansion, Specific Heat
When soldering silver, you have to heat the entire piece. But gold, you can heat the specific place to be soldered (like sizing a ring). I was wondering if someone could explain why the difference. Also, does platinum fall into the silver or gold ranks.
The difference is due to thermal conductivity. Metals differ in the
rate at which they conduct or transfer heat. With some, heat one end
of a piece of metal and the other end very quickly gets very hot too.
Heat other metals at one end, and you can get it glowing red while
still holding the other end comfortably.
Silver is the first. It has the highest thermal conductivity of any
of the metals we use in jewelry. The reason you have to heat the
whole piece is simply that if you attempt to heat just the joint, the
rest of the piece will be heat sinking away your heat at the joint,
and you’ll have a hard time getting it hot enough. With a hot enough
torch, like one using oxygen and propane or natural gas, you can
still heat just the joint. But it can simply seem like it takes more
heat that you expect. Sometimes, MUCH more. You can, for example,
size a silver and turqoise ring by putting the stone half of the ring
under water to keep it cool. But then getting the solder to flow at
the joint may take an amazingly hot flame, and when it does flow, if
you’re using a smaller container of water, you’ll notice that it’s
merrily boiling while you’re doing this. Note that the stone will
still be safe, but it illustrates how much of the torch flame’s heat
is being transferred away from the joint to the water. Heating the
whole piece also works very well with silver because that thermal
conductivity tends to make sure, especially when you too are trying
to heat the whole thing evenly, that all parts of the assembly are
evenly heated to the same temp. This is great for a good uniform
solder flow.
Aluminum and copper are two other metals with very high thermal
conductivity.
Gold alloys, on the other hand, have a lower thermal conductivity.
Heating the whole thing can be done, but isn’t as needed, since
heating just the area of the joint does not result in so much of the
heat being pulled away by the rest of the piece. This means that to
get proper even heating on both sides of a joint, you have to be more
careful to actually control the torch to do just that. It’s not so
automatic. But then you also have more control over what gets hot
and what doesn’t, allowing you more ability to pull the solder in
desired direction with the heat (solder flows towards the hotter
areas of the metal)
Platinum, since you asked, is an even poorer thermal conductor than
is gold, so you can use very pinpoint heating to precisely control
what gets hot and what does not. This is good, because the higher
soldering temperatures of platinum make it difficult to solder with
stones near the solder joints. But the low thermal conductivity
allows you greater ability to then keep stones cool, closer to such a
solder joint area, than you could do with other metals.
Hope that helps.
Peter Rowe
Hi Rick,
Silver can also be heated in a localized place i.e. for sizing a
ring. You place the un-heatable part in water, upside down if a stone
is on the top. The only problem is if the ring is thick, one simply
cannot get enough heat there for the solder to flush. I have used
heat paste too with some success. It all depends on the piece.
Some stones take heat. Diamonds, sapphires and garnets even
amethysts. But one has to watch for heat treated, they can change
colour. Inclusions in diamonds, gas and liquid which can expand and
then broken stone, not pleasant at all and of course for wind passing
over while you solder which will burn the outer skin diamond. As long
as you let it cool down naturally and don’t quench in water all
should be fine. There are other tips for soldering next to or on
stones too if you are interested.
The reason is that silver transfers heat a lot quicker due to it
having a lower specific gravity than gold.
Platinum takes more heat to solder as the solder melting point is
much higher and that can lead to burnt stones.
Platinum, gold and silver are all their own element if you look at a
periodic table you’ll see them individually. So they don’t fall
under each other at all.
The platinum group metals however are a few of which some are used
in the industry. Palladium and lately ruthenium for alloying and
rhodium for electro-plating.
There are some excellent articles to be found through Wikipedia and
such.
Hope this helps.
Leza
Heating for soldering is predominantly a matter of heat conduction.
Gold and silver are both excellent conductors of heat so a lot of it
depends on how big the piece is and how hot your torch can get so as
to heat the area of interest. A very hot but small flame such as you
get with water fuelled hydrogen torches will make a small area very
hot very quickly enabling soldering of new claws on rings without
overly heating the rest of the ring. Now, with high carat gold less
fluxing is needed as there is less oxidation of the job under the
flame so there is a lesser need to play the flame to bring it all up
to temp. Also, generally speaking, the objects we work on made of
gold are smaller than many that are made of silver so the conduction
of heat away from the area of interest is less.
If I was soldering a new ball foot on to a toast rack for example, I
would use a small very hot flame on the work area rather than try to
bring the whole thing up to temp. Proper fluxing would be needed
otherwise my solder will tend to ball up rather than flow where I
want it. If I heated the whole piece the solder will run well but
could go to places where I didnt want it if I was too generous with
the amount and I an also more likely to get firestaining.
platinum doesnt oxidise very easily and thus the soldering will be
akin to gold inasmuch as a concentrated flame will normally be used
to even melt the solder.
Nick Royall
Rick,
I have no formal metallurgy training, just burnt fingers 
Silver
conducts heat like crazy. If you try to heat one spot the heat will
flow to all others. Copper and high copper alloys are almost as bad.
Gold is far less conductive (depends on the alloy) Platinum even less
conductive.
You can still get away with murder on silver with a hot enough
torch. There really aren’t any hard and fast rules if you are really
bad. For starting out just follow the rules, they do make sense. Lots
of time yet to break them and make disasters. Have fun.
jeffD
Demand Designs
Analog/Digital Modelling & Goldsmithing
http://www.gmavt.net/~jdemand
The reason is that silver transfers heat a lot quicker due to it having a lower specific gravity than gold
It is the property of thermal conductivity that determines the heat
transfer not specific gravity. If specific gravity was the issue then
copper, iron, aluminum, titanium etc would transfer heat better than
silver as they have a lower specific gravity, and they are all worse
than silver at thermal transfer. Silver is the best conductor of heat
of all the metals.
James Binnion
James Binnion Metal Arts
Some stones take heat. Diamonds, sapphires and garnets even amethysts.
I think it’s a really bad idea to take heat to a Garnet or Amethyst.
I now avoid taking heat to modern Sapphires and Rubies since so many
of them are now filled.
and of course for wind passing over while you solder which will burn the outer skin diamond.
Wind? I ALWAYS put boric acid and alcohol on diamonds to protect
them while heating. However NEVER do this to a corundum stone. It
will fuse with the stone and come off later in the pickle leaving a
pock marked surface on the stone.
Have fun and make lots of jewelry.
Jo haemer
www.timothywgreen.com
Without quoting numbers silver and stirling silver are super
conductors of heat. The heat you apply to the joint will flow away
into the whole piece as quick as you are putting it in. Thus you
need to fill up the whole piece with heat before concentrating on
the joint being soldered.
Gold alloys conduct heat slower than silver, so the heat drains into
the whole piece slower. The whole piece does not need to be filled
up with heat like silver does, but a little filling up will help.
Thick or heavy sections of gold need more time for the heat to
penetrate into the centre to ensure the solder flows through the
whole joint.
Platinum needs a very hot heat source so the comparison is
different. It behaves like gold only you must get accustomed to a
bright orange glow before anything happens.
Regardless of the alloy, if you are soldering a small item onto a
big item then apply proportionately more heat to the big item. Both
items should reach soldering temp together so that the solder will
flow onto both pieces at the same time.
An extreme example would be soldering a vertical stainless wire
(very slow conductor and small size) onto a large silver brooch
(very high conductor and large size). The silver brooch needs to be
filled up with heat while the stainless wire is completely out of
the heat zone, held in tweezers, and poised in readyness. When the
silver is full of heat a pallion of solder on the silver will flow.
This is the moment to plunge the stainless wire into the molten
solder. The stainless wire will heat up during the plunge and will
be at the correct soldering temp as it hits the silver brooch and as
the solder slowly solidifies due to the large body of heat contained
in the silver!
A similar effect can be achieved in this extreme example by having
the parts lined up and in contact, and shielding the small part from
the flame. Only heat the big part. The small part will get all the
heat it needs from it’s contact with the big part. With silver the
heat will be pretty much equal all through the silver at the same
time. With gold the heat will build up or cool down quicker in the
thin places and slower in the thick places.
Alastair
Hi Rick,
The answer is that silver is a far better heat conductor than gold.
Although the solders melt at around the same temperatures, the
superior conductivity of silver means that the heat tends to leak
away from the joint rather than building the temperature up
sufficiently to melt the solder.
Heat always flows from hot to cold (never the other way round), and
the flow rate is dependant on the temperature difference. If you heat
the whole silver piece, the temperature difference is less, so the
heat flow is less, so the joint can get hot enough to melt the
solder. I hope that make sense.
Platinum is a worse conductor than gold.
Regards, Gary Wooding
Cool, thanks for that James. I did not know that, now I have another
closed loop of knowledge. Is silver not a better conductor of
electricity than copper too?
Leza
I’d recommend never heating an amethyst. It loses its color very
easily and I’ve know the stone to loose a lot of its intensity
simply by being left in direct hard sunlight.
If you want to verify this just heat a chip or tumbled stone with a
torch.
It’s only ever done if the customer desperately wants the piece
fixed and unsetting then resetting is more trouble than it’s worth.
And I’ve managed to solder next to all sorts of stones with great
success. It seems to only work with quick, extremely controlled
direction of heat and then allow to cool very slowly. I.E. no
quenching…ever. Customer is always warned of course and the
challenge to get it right is Fun.
As always, approach with caution everyone.
I have experimented with old stones too just to see what sort of
abuse they can take. And surprised myself.
Cool, thanks for that James. I did not know that, now I have another closed loop of knowledge. Is silver not a better conductor of electricity than copper too?
As long as we are not talking about superconductors silver is the
best conductor of both electricity and heat of all metals.
Jim
James Binnion
James Binnion Metal Arts