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
Aluminum and copper are two other metals with very high thermal
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.