Soldering - Heat protection

Does anyone have experience in heat flow of an item being soldered?

If we have a sterling item (which conducts heat really well) to
solder, we need to heat up the entire piece so that the opposite end
from the joint doesn’t act as a heat sink. Ok. Now, what if the other
end has a stone and we need to protect it with say a wet tissue or a
commercial product that cools it off? Why doesn’t this heat
protection work as a heat sink? Or it does? Any suggestions please?

Thanks a million.

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Now, what if the other end has a stone and we need to protect it
with say a wet tissue or a commercial product that cools it off?
Why doesn't this heat protection work as a heat sink? 

It does work as a heat sink. The heat you put in near the joint gets
conducted throughout the item; the temperature near the heat source
(flame?) is the highest, and it falls the further you move away. The
temperature at the heat source rises until you remove the heat
source, or it reaches the point where all the heat you can apply
gets conducted away before it can raise the temperature any more. At
this point the whole item is at its maximum temperature (but its not
uniform): all the heat put in by the flame is being being lost by
radiation and convection in the air. If the solder flows at or
before this point, then you have made the joint.

If you place part of the item in something that can absorb or
dissipate the heat faster than it can be supplied, then its
temperature will not rise any more; this is what happens if you
immerse it in wet sand, for example. The heat that gets into the wet
sand causes the water to boil rather than raising the temperature of
the item. The stone reaches the temperature of the boiling water,
but no higher. The wet sand acts as a rather better heat sink than
the air. This does, of course, imply the use of a rather large heat
source - a good large, hot, flame that can deliver plenty of heat so
as the create a rather steep temperature gradient in the item. I
wouldn’t use wet tissue; the water boils away so quickly that you end
up with dry tissue before the solder flows. At this point two bad
things are likely to happen: the stone cracks and the tissue catches

Heat shields don’t work in this instance: they are effective in
protecting a stone from the direct heat of a flame, but are the exact
opposite of what you want for protection from conducted heat; the
temperature inside the heat shield would build up to destroy the
stone even faster than if you didn’t use it.

I use a small ceramic pot, filled with sand, which I carefully fill
with water until it overflows. Fill it slowly because it takes time
to filter through the sand. I then immerse the stone and as much of
the item as possible, in the wet sand, flux the joint and apply the
solder. I then carefully apply the heat until the flux melts, then
apply full heat - the water starts to boil (its a bit worrying the
first time you do it), the temperature builds up, the water
continues to boil until the solder flows. The flame is then removed
immediately. If the solder doesn’t flow within a few seconds, then
the flame is not supplying enough heat.

Regards, Gary Wooding

To try and answer your question we need to consider how an item is
heated. there are 3 sorts of heating for an object, conduction,
convection and radiation. With your silver ring your torch provides
all 3 to a piece, the proportion of each dependant upon the size of
the object. Now, heating a ring shank at 1 side will result in the
direct heat being radiant (from the flame), covection of the heated
air around it (can be used to your advantage and this will be evident
if you use a heat resistant mat) and indirct heat as both radiant and
conductive. Try soldering a large piece with too small a torch and
then try again by boxing in the work with fire bricks and you will
get a much better chance of soldering your piece. The additional
radiation from the heated brick plus the convected hot air gives you
better conservation of the energy of the flame.

Now, back to the ring. the direct heat on the shank will give little
in the way of convected or secondary radiant heat (flame size
depending)but the metal conducts very well so the stone will get
heated. If you coat the stone with Quimijoy or a similar thermal
barrier this protects the stone from the radiant and convected heat
but not the conducted heat. So the aim of the barrier coating can be
undermined by heating your shank for too long. You can get the solder
to run without heating the whole job, you need to get more superheat
into your joint. This means a very hot flame on a very small area.
these water cracker (hydrogen-oxygen) microflame torches are perfect
for this. Downside? you can get bubbles in the solder joint from
uneven heating, outgassing and flux deposits. the thermal barrier
material is a very poor conductor of heat so doesnt act as a heat
sink itself. The wet tissue removes energy by evaporating the water
and that does therefore act as a heat sink and is thus not as useful
as a proper thermal barrier. The best superheat source available for
us is a laser, so if I need an already set ring resizing I send it
off for laser welding. You can blow inert gas over the job and you
dont get any oxidation either. I know that this last bit doesnt
answer your question but it does solve a problem.

Nick Royall