I talked with a local jeweler and she recommended dipping the
piece in a alcohol and borate solution first then burn off the
alcohol. Flux and then solder. Would this help? Does anyone have
pictures to show the difference between fire scale and oxidation?
That’s the standard commercial jewelers solution to preventing
oxidation and fire scale on GOLD, as well as preventing heat damage
to diamonds. However, simple boric acid is not effective at
preventing fire stain on sterling silver. It may reduce the obvious
black oxide called fire scale, but fire stain is different, and
somewhat more important to prevent. it’s the faint cloudy/milky/rosy
toned shadow you find showing up in your highly polished silver when
you go from tripoli to rouge. The clean silver looks darker, clearer,
more highly reflecting, and the fire stained areas are a bit lightler
and cloudy looking areas. It’s a subtle difference, not obvious
unless both colors show, but typically, when you fabricate something
and get fire stain, your polishing and buffing will be just uneven
enough so it cuts through the fire stain in some areas, and not in
others, leaving an unattractive blotchiness to the surface. Gold, it
should be noted, does not do this. The oxides formed on gold alloys
when you solder it are confined to the surface, so when you pickle
the metal, you’ve removed the oxides and only need to refinish the
metal you see. The problem is that with silver, oxygen penetrates
INTO the surface, oxidizing the copper in the sterling to a certain
depth within the metal, so your silver, instead of being a mix of
silver and copper metal, is a mix of silver and reddish copper
oxides. They’re dilute enough in the silver so the reddish color may
not be all that obvious, but the damage to the finish remains a
problem.
As I and others have posted many times throughout the history of
Orchid, fire stain and fire scale are only a problem with sterling
silver if you don’t bother to prevent it. The solution has been
around and taught in many art schools, published in a number of
books, and otherwise not hidden, since the 60s, or earlier, when John
Prip first devised his flux recipe.
In short, prips flux, used correctly, solves the problem. it IS a
seperate step, and takes a little more time, but what the heck. Do
you expect your jewelery to make itself? it’s just another part of
the process, and once you get used to using it, it’s second nature
and works like a charm. Prips flux is easy to mix up and use, though
if used in too small a soldering area, the overspray can make a
little bit of a mess, but this too is easily solved. It’s cheap to
buy the chemicals, cheaper at least than commercial products, and for
a few dollars in chemicals, you can mix gallons of the stuff, enough
for years of working.
The recipe is simple. 3 parts boric acid, 2 parts each borax and
Sodium phosphate. The latter can be monosodium phosphate, disodium
phosphate, or trisodium phosphate, the last of which is the most
commonly used, since it’s usually the cheapest and easiest to find.
Normally, about 96 grams boric acid and 64 grams each of the other
two will form an almost saturated solution in a quart of water. You
can mix it more dilute if you wish (schools often do, to avoid
wasting flux with students who tend to not know when to stop
applying it) All three chemicals can be simple technical/industrial
grade, and tap water is fine. For borax, you can use some laundry
products, like Borateem, which is mostly borax (the littlle blue
crystals are whiteners, which have no effect on the flux.) Boric acid
can be purchased sometimes as a cockroach powder in hardware stores,
or through chemical supply houses. Trisodium phosphate, also known as
TSP, is widely known as a fairly strong alkaline cleaning agent,
used for things like cleaning walls prior to painting. It’s become
harder to find over the last decade, because, like all phosphate
detergent products, it presents some water pollution issues (It’s a
fertilizer, promoting algae and other water plant growth) So you may
not be able to find it in the paint section of your hardware store.
Some stores carry it still, perthaps on a bottom shelf not so
visible. Be careful if buying it like this that you actually have
TSP, since there are a wide number of products confusingly labeled
TSP, or similar, which actually have different chemicals in them, so
read the label. it should be a white granular dry powder, and the box
needs to say Trisodium Phosphate as the ingredient, not just “TSP”.
If you can’t find it in a hardware store, it’s easily sourced through
any chemical supply house.
However you get the chemicals, dissolve them in hot water, and allow
it to cool. If needed, add more water to keep it all in solution.
To use it, spray it on preheated metal so that it freezes/dries on
contact to a white slightly crusty film which is thick enough to hide
the metallic reflective surface of your silver. It should not go on
wet, or it will not form a stable clear coating. If, upon heating,
some of it balls up or draws back from areas leaving them
unprotected, spray a bit more on till it stays there. That can happen
if you put too much on, put it on wet with the metal too cool, or if
the metal isn’t clean.
Ordinary trigger bottle sprayers tend to quickly clog up. You can
use an external mix air brush, such as this one from harbor freight
Notice that with this, as with other cheap airbrushes, the
paint/liquid mix feed tube comes up and ends IN FRONT of the air
nozzle, not inside of it. That’s what keeps it from clogging. The
liquid never goes through any sort of small orifice to clog.
Eveh simpler, avoiding plastic air tubes on air brushes while you’re
also using a torch, or air cans or compressors, etc, are mouth
atomizer sprayers. They are the type I prefer. They last almost
forever, are too simple for words, and work well. Sold in ceramics
supply houses for application of glazes. here is a link to one source
(though they may have an annoying minimum order). halfway down the
page, you see numbers 126-86 and 126-87
note that they’re shown folded down for storage. In use, the two
tubes are positioned at right angles to each other (and L shape)
A few other notes to be aware of. When TSP started to become hard to
pick up in hardware stores, Fred Fenster, the amazing silver and
pewter smith who taught at the University of Wisconsin for many
years, and who first introduced me to Prips flux in my first
sophomore metals class some 35 years ago, started experimenting. The
same sort of inspired “kitchen chemistry” that had John Prip coming
up with the original formula, had Fred coming up with an altered
version, which he calls “Frips” flux. In short, he figured that since
Cascade dishwashing powdered detergent (that green box) is primarily
composed of TSP as the main cleaning agent, it might work just as
well as pure TSP. Or so goes the story from some other former
students of Freds who first posted that info here on Orchid a number
of years ago. Personally, I’ve not tried this mix, since I’ve
already got enough TSP to last me a long time. But those
aformentioned posts suggest that Fred, at least, felt this mix was
actually an improvement on the original. I’d guess that the Cascade
formula includes other surfactants, things to promote clean rinsing,
lowering surface tension of the surface, etc. etc. And since that
function is the main action of the TSP in the flux (allowing the
melting boric acid and borax to wet the silver and coat it instead of
pulling away), it sounds reasonable to me that the Cascade version
might be better. But I don’t know, since as I said, I’ve not tried
it, and won’t need to till I’ve run out of the gallon or so of prips
flux I already have sitting here. Search through past archive
postings on orchid for mention of Frips flux, and you should get the
needed details on it’s use.
Now, if this whole posting is too long, and you’re scared of taking
the time to source the materials and mix your own in order to save
money, you CAN buy the mix from suppliers, though if bought as a
liquid, most suppliers seem to sell it pretty dilute, which is
annoying to me. Or use Cupronil, which I never liked as well, but it
does sort of work, or the new Firescoff product. It DOES work very
well. Maybe even a little better than Prips, in terms of not burning
off if you overheat the metal, or in terms of being also a decent
soldering flux (Prips is too, but not an especially active flux). The
main trouble with Firescoff is, of course, the cost. Might as well
sell your first born kids for that that stuff costs. Even if it works
very well, (and it’s used the same way as prips flux, sprayed on
preheated metal), I’ll bet that once you mix up some prips, and
compare performance and costs of the two, you’ll agree with me that
prips or frips is just fine…
Nuff rambling. Email me or post on the list if you need more info on
this. But the bottom line is that there’s no reasonable reason you
should be having trouble with fire scale or fire stain on sterling
silver when you fabricate it. It’s completely avoidable, without
needing fancy alloys, fancy acid dips or other removal methods, or
much of anything else other than proper use of this sort of
preventative. This is one situation where ignorance is very much not
blissful. Learn to avoid fire scale and fire stain, and the small
amount of extra time this step takes will be saved back several times
over in not having to polish or abrade it off later.
cheers
Peter Rowe
Seattle