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Firescale Protection on Underside of Silver


#1

In an earlier note about granulation, the person commented that the
back of a silver piece should be coated with yellow ochre; I assume
this is needed to protect the silver from firescale. When preparing
to torch solder, I only flux and/or use solder-resist where needed
on the top of the piece or any part exposed to direct torch heat,
not the back if it is flat on the soldering board. The back changes
to a darker color (the same as it does if coated with yellow ochre)
but the stain pickles off. I have not noticed any firescale on the
uncoated backside. Is there something about the granulation process
that makes fluxing or coating the back necessary or am I doing
something wrong by not providing any protection when soldering?
Thanks for your help. Nancy www.psi-design.com


#2

The purpose of coating the back of the Fine Silver piece with ochre
in granulation is not because of firescale, but to keep it from
melting down before the granules are set. When using back heat as
with the small trinket (beehive) kiln, the silver backsheet runs the
risk of overheating. I have not had a problem with firescale when
using the Fine Silver. Sandra


#3

I have had trouble with the gold granulation and have coated it with
ocre as taught. The granules are hard to fuse without melting down.
Without coating it, will the fusing process be easier? Thanks, Catherine


#4
I have not had a problem with firescale when using the Fine Silver. 

You wouldn’t. Fire scale and fire stain are problems caused by the
copper content in sterling silver. Silver alloys without the copper,
or pure silver, can’t have problems with copper oxides when there
isn’t any copper.

Peter


#5

At Revere School in SF we were taught to put the yellow ochre on the
back of a flat silver piece that you are placing on the kiln burner
for granulation. The point of this is that gold and silver have a
eutectic reaction. So if you use the same kiln for gold and silver
granulation, and you get silver on the burner and hence on your gold
piece, it will flood. An eutectic reaction is one that the
combination of metals causes the whole piece to melt at the lowest
of the melting temps. To my (admittedly limited) knowledge copper
and silver, and gold and silver have this relationship, but not
copper and gold.

I have not gotten into the molecular structure enough to be able to
say why some metals behave like this- but I have seen it in action
when messing up with some enamelling on copper with silver
cloissons. And we did hear stories in our class at Revere about
hours and hours spent on gold granulation only to have the thing
flood because of a speck of silver.

HTH!
Mary Barker


#6
 An eutectic reaction is one that the combination of metals causes
the whole piece to melt at the lowest of the melting temps. To my 
(admittedly limited) knowledge copper and silver, and gold and
silver have this relationship, but not copper and gold. 

Hi Mary. Copper is the metal that was/is traditionally used to lower
the melting point of golds in gold granulation.

“Eutectic” isn’t really a reaction. It’s simply that specified
ratio of two or more metals that melts at the lowest temperature of
two such metals, and the word indictates both the alloy made with
that ratio, and the temperature such an alloy melts at. There
isn’t any reaction that would melt a whole piece just from contact
with other metals, but if two metals are in contact, their contact
point will melt at the eutectic temperature for that combination of
metals, which is usually well below the melting point of either one
individually. This is what allows granules to fuse to a base piece
without themselves actually melting. The surfaces, "contaminated"
with copper, slightly melt due to the copper contacting the gold or
silver underneath. While the surfaces are thus molten, the copper
can then diffuse into the larger mass of gold or silver, removing it
from the molten area until there is no longer enough there to form the
eutectic alloy, and then that surface area resoldifies. If the
molten surface is also in contact with either the base upon which the
grain has been placed, or another grain, then the molten surface
layer is able to bond to that other surface, and granulation bonding
takes place. If two dissimilar metals are in contact, and heated,
the contact between them will melt at the temperature determined by
the eutectic point of that combination. If instead of it being just
a thin surface film of another metal, as in granulation, it’s an
actual larger mass of metal, then it’s possible that as the
componants of the molten area dissipate into each other, more is
simply supplied by the larger masses in contact, and in this
situation, it’s somtimes possible for two pieces of metal to just melt
into each other at surprisingly low temps. The classic example of
this phenominon, which is sometimes a not very welcome surprise to
the jeweler who may have been trying to just solder those two pieces
together, is the case of silver and brass, where it’s very easy to
find one’s brass overlay just slumping into the silver one was trying
to decorate. When you consider that brass, added to silver, forms he
alloy for silver solders, it becomes more understandable…

cheers
Peter Rowe


#7

Hi Peter and Mary,

A eutectic alloy is distinguished by the fact that it transitions
from solid to liquid without an intermediate mushy stage not that it
is the lowest melting point in the system. Silver and copper do have
a eutectic alloy it is 28.1% silver to 77.9% copper and melts at
1434.2 F or 779 C. Neither the gold and copper or gold and silver
systems have a eutectic alloy. As Peter pointed out the copper is an
agent used to locally lower the melting point of the gold in the
granulation process but it is not a eutectic alloying. BTW it is
possible for a binary alloy system to have more than one eutectic
alloy each with a different melting point. As for the eutectic
reaction well if you really want to know the ASM Handbook defines it
as:

The three-phase reaction that takes place at the invariant point E,
where a liquid phases, freezes into a mixture of two solid phases,
is called a eutectic reaction (from the Greek word for “easily
melted”).

Jim