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Over heated silver, additional questions


#1

Hi all, I’d like to add some additional questions related to Doug’s.

I’ve been told to heat my mold forming the ingot prior to its pour.
How hot should the mold be, and how can I heat it and the crucible at
the same time?

I made an ingot (well, sort of) from sterling scraps and was able
thin it out, slowly bearing down about 2/1000" at a time and carefully
annealing to a dull red. When I got the metal to about 24 guage and
tried to fold it I found that it was brittle so that it fractured
during the folding (I did anneal). I could see a granular texture at
the broken surface. Why should this occur?

How much borax should be added to the scraps? Also, how hot is "hot"
when heating the crucible. Do you pour immediately after all the
scraps have melted, or do you continually heat say for another couple
of minutes to raise the temperature of the melted sterling. (I use MAP
gas).

Looking forward to your responses.

David
(Be careful what you wish for because you might get it; we’ve been
wishing for sunny days and getting lots of them, and now we just might
be in a drought)


#2

Your annealing process has been too hot for too long. Assuming
sterling, annealing starts about 230 C and will be complete in a few
minutes at 600 C. When you anneal you want to get above the point
where recrystalization occurs - about 600 C but not much higher.
Practice is to about 650- —700 C . There are differences in
opinion as to soak time. Some pull immediately and quench, some soak
at temperature a short while. It is better to be on the short side
rather than the long side particularly on thin material. On
recrystalization the metal crystals are reformed at a minimum size and
then will begin to grow to large size and imbrittle as the
temperature and or time increases. The cure is to remelt and start
over. Another related problem occurs in heating when the heated
material is heated without proper protection from atmospheric oxygen.
In this case copper is oxidized deeper into the metal between the
silver crystals causing brittle grain boundaries.

Jesse


#3
    Your annealing process has been too hot for too long. Assuming
sterling, annealing starts about 230 C and will be complete in a
few minutes at 600 C. When you anneal you want to get above the
point where recrystalization occurs  - about 600  C but not much
higher. 

I agree to 600 C - 650 C for annealing stg. Turn the lights off and
see the colour better. And by the way if you’re annealing 999 silver
that only needs 350 C to anneal. Not much hotter than a scone oven
(250 C)! - I do quick scones.

Temperature of the steel mold? Smoking hot. Heat it so that when you
dab it with beeswax it melts and smokes. How more precise can I be?
:wink:

Brian
B r i a n � A d a m
www.adam.co.nz/workshops/bush/


#4
    I've been told to heat my mold forming the ingot prior to its
pour. How hot should the mold be, and how can I heat it and the
crucible at the same time? 

This is a bit of a problem, I admit. Here’s what I do. I have a
couple of heat sources. One is a big fluffy short-flame propane burner
on a stand under a tripod arrangement (actually, under a stainless
mesh pad supported by two insulation bricks). This I use to pre-heat
the crucible and mold. The when things get hot enough I put the hot
mold on the nearby tray (catches any spills), the metal in the
crucible, and get the oxy-propane torch out to melt the metal really
quickly. when it’s molten I touch the mold with wax to check it’s
still hot (and to provide a reduced atmosphere inside) and pour soon
after it’s reached the molten state.

Without the stationary gas burner I guess you put the mold near the
crucible and pre-heat the mold with the oxy-assisted torch (get it all
hot, not just parts of it), then heat the metal. The mold being close
by can benefit from some heat from the torch as well.

    I made an ingot (well, sort of) from sterling scraps and was
able thin it out, slowly bearing down about 2/1000" at a time 

You might be over working the surface. Roll with more effect, I’d
say. Take bigger bites with the rollers. Remember that the inside of
the metal os also annealed, so rolling deep will get it all squished
ad work-hardened together.

    How much borax should be added to the scraps? 

Always include a good amount of clean scrap, or new metal. If the
’scrap’ is merely offcuts from clean new stg, no solder, shiny metal,
etc, then I’d consider his ‘new’ metal. But if you include stg that
you’ve been practising reticulation on (for example) then I’d say that
was probably full of oxides and not good to include.

Bri
B r i a n � A d a m
N E W Z E A L A N D
www.adam.co.nz