Notes on Soldering Silver
Silver is joined by a process that silversmiths call soldering by
convention, but that metalurgists call brazing. True soldering
involves only wetting of the bulk metal by the solder, essentially a
surface process. Brazing is done at higher temperature, and involves
diffusion exchange between the bulk metal and the braze. Sterling silver
is .925 Ag, .075 Cu. A very hard (very high-melting) jeweler's silver
"solder" is .75 Ag, .225 Cu, and .025 Zn. That melts at 1841F, and is
suitable for fine silver, but it's too close to sterling's melting point
for most uses. In use, as the liquidus temperature is maintained, silver
diffuses into the solder, and zinc and copper diffuse out, raising the
melting point and delocalizing the joint. The color is close enough so
that the joint is invisible when polished. The tarnish properties are
different, and a slightly tarnished piece with joints that have cooled
soon after melting and wetting will show a fine dark line at the joint.
When the heat is held for a minute or so, this "line" becomes too
diffuse to be noticed by eye (but a photometer can show that it's still
there).
The common silver solders that jewelers use go by the names "easy",
"medium", and "hard". These names refer to the melting points; Easy has
the lowest. "Medium" is the easiest to use because it flows - wets the
sterling - most readily. "Easy" is the most difficult. The right flux is
important, too. There are liquid fluxes that clean up easily and work
well for experienced users. Paste fluxes don't burn out with extended
heating, and are often better for beginners and for large jobs where it
can be a long time from the first joint to the last. The glassy residue
can be removed by boiling in water. Painting the entire piece with paste
flux can minimize the risk of forming "fire scale".
To help clean the piece after soldering, it is quenched in "pickle", a
moderately strong sulfuric acid bath, or a pretty concentrated solution
of sodium bisulfate. Hydrochloric (muriatic) acid is better for copper
and brass. (Platers call it "bright dip".) Use two containers, one for
the pickle, the other for rinse water. Pickle will eat holes in cotton
and other vegetable fibers, and it can splash when a hot piece is dropped
into it. Wear an apron. Don't pick up a piece in the pickle with iron
tongs. Some dissolved copper will plate onto the silver because of the
galvanic action. Copper tongs are standard; stainless steel might work.
A hook of copper wire might suffice, and I don't hesitate to use my
fingers if I have no broken skin.
A piece of firebrick makes a good work surface. For small pieces, and
especially when soldering with an alcohol lamp and blow pipe, a charcoal
block is dandy.
Silver solder will not fill visible gaps. Its strong wetting action
causes it to flow between surfaces that are in contact. It flows toward
hotter parts, and will flow on the surface away from the intended joint
if the joint is somewhat cooler than the bulk in one side. Ocher or
jewelers rouge can be made into a thin paste with water and applied as
an anti-flux to keep the solder away from where it isn't wanted. India
ink works too, and seems to be the preferred anti-flux for making hinges.
I use cut pieces of solder; these are approximately one-millimeter squares
(or whatever shape results) cut with shears from sheet. Silver solder
also comes as wire, ribbon, and occasionally, pre-cut pieces. (I find wire
or ribbon occasionally, but rarely, useful.) The pieces are placed along
the joint at intervals between 1/4 and 1/2 inch, depending on the thickness
of the material for butt joints, the size of the desired fillet for tee
joints, and so on. A single piece suffices to fasten a post to an ear ring
or butt-join 16-gauge wire. The pieces can be placed by touching one --
but not the cup in which it lies -- with a fine brush moistened with flux.
When the brush is touched to the fluxed joint, it will come off the brush
and stay on the joint.
An iron poker is handy for leading the molten solder when it is reluctant to
flow and to push an unmelted piece back into place if the action of the flux
has moved it. Keep the tip of the poker hot, so that something touched by it
won't be chilled. Some sacrifice a scriber for this. I made mine from 1/8"
square rod, twisted where I hold it, and ground to a point. The wire from a
hanger (a thick one is nicer) folded so that about 6" is double and 3 inches
is single makes a nice poker too. Twist the doubled part to make a
comfortable grip and point the free end. Don't leave a hot poker lying
around; quench it in the rinse water before putting it down.
Silver-soldering fluxes are either pastes or liquid, but in either case,
they contain salts that have water of crystalization which is expelled by
the heat of the soldering torch. Care is needed to expel the water slowly
enough so that the solder pieces are not displaced (or popped across the
room!).
Polishing takes more work than the rest. There are polishing wheels that
can replace grinding wheels, and Dremel tools are good for small work. The
abrasives you will need are Tripoli (rottenstone) for the first go (cut) and
rouge for finishing (color), both in stick form. I have seen these in Sears.
DON'T get Tripoli on your rouge wheels. Keep them separate and wash the work
with soapy water and a toothbrush to remove all traces of a coarse abrasive
before going to a finer. (Just like grinding a mirror.)
INDIAN JEWELERS SUPPLY COMPANY
601 E. Coal Ave. Gallup, NM 87301
(800) 545-6540
Fax (888) 722-4172
E-mail orders: orders@ijsinc.com
http://www.ijsinc.com
RIO GRANDE JEWELERS SUPPLY
800-545-6566
Fax 800-965-2329
Orders from the web site
http://www.riogrande.com/
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