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Firescale prevention

Hello All,
I have a question about firescale prevention. At a class I took several years ago on silverwork, the instructor claimed that firescale started at 1400*F, so he used medium and easy solder rather than hard. Other instructors (Don Norris) claim everything should be soldered with hard solder, as it makes the solder joint invisible and hard solder won’t polish out of the joint. I was unable to find any info on when firescale starts forming.

So my question is, is it appropriate to use lower temperature solder to avoid firescale. Further, as regards annealing and firescale, silver annealing temp is quoted at 1100-1200*F, but some annealing would take place at lower temperatures. So would it make sense to anneal at a lower temperature to avoid firescale?
Thanks,
-royjohn

I’m going to preface this by saying I am no kind of authority and I look forward to reading answers from those who are so I can learn too, but. My understanding is that firescale comes of having silver excessively hot for excessively long, and that flux or lack thereof allowing oxidation also contributed. I was told that if I was heating pieces just hot enough for hard solder to flow and removing the flame as soon as it had, I shouldn’t have a problem with firescale. That has indeed been my experience in the vast majority of cases.

I tend to do things with multiple soldering stages, so I don’t like the idea of limiting my solder grade options. I feel like there are ways to control firescale without giving up the strength you get from hard solder.

This is mistaken. Heating to even easy solder or annealing temps will give you fire stain/,fire scale (the two are different) fire scale is the surface black layer sterling silver gets as copper oxide forms on the surface. It is removed easily by pickling. Preventing oxygen from reaching the surface and reacting with the metal prevents its formation. Fire stain is the reddish/cream colored faint discoloration that sterling silver forms at the same time as fire scale. It is copper oxide that forms below and within the surface of the silver as oxygen penetrates the surface. Both fire scale and fire stain get worse as temperature and time increase. If temps are kept more moderate and time is controlled, the fire scale is easily pickled off, and the fire stain is shallow enough that ordinary polishing will cut through it. With hotter temps and times, the fire stain is thick enough to be a major problem. Solutions include careful prepolish before soldering so after pickling only light polishing is needed which does not cut through the fire stain layer, so the silver is slightly different color, but is acceptably uniform. The famed Danish silversmith George Jensen and co. Was known for this fire stain finish.

Most current silversmiths, however, prefer to prevent the formation of fire stain and fire scale. A number of fluxes can be applied, though not all soldering fluxes will work since the most active soldering fluxes break down fast enough they let oxygen through before heating is done. That can show up as obvious fire scale/stain right adjacent to solder joints. Solutions to the problem are fluxes that will coat the metal and withstand the heat. Typically these include either boric acid or borax, or both. Classical silversmiths would, through repeated applications, build up a borax layer before soldering. This takes some skill and patience, since initially the coating tends to pull away, thus needing repeated applications until a uniform coating is achieved. Goldsmiths have it easier, since a dip in a simple slurry of alcohol and biric acid powder, burned off, will protect gold alloys (and diamonds), but is not sufficient for silver. A flux called Prips flux (named after Jack Prip) consists of a saturated solution of 2 parts each of boric acid and TSP, and 3 parts boric acid in water (use 60 grams each of the first 2, 90 grams borax, a liter of water) to use, spray the Prips flux onto the silver, when the silver is preheated enough that the flux dries on contact to an even white crust. You will usually want additional soldering flux sparingly applied only to the joints. Done right, the work will emerge from the pickle without that white matte color, but with any prepolish shine mostly intact. The time spent with applying the Prips is more than gained back with less polishing required.

There are several commercially produced fluxes which may offer easier (no preheat to spray) application, such as Cupronil, or Firescoff. And for smaller situations requiring less heat and time, the less aggressive soldering fluxes like Batterns may work well enough.

A final solution, often used in industry is furnace soldering, where the furnaces provide an atmosphere controlled environment which allows no oxygen to reach the metal, thus no firescale or fire stain.

Peter Rowe

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First of all I am a craft style silversmith. The work is quite affordable and for me it is artistically satisfying. I don’t set stones or do much casting in gold. The bulk of my work sells at price points less than $500.00 with $90.00-$100.00 being the average. Until this last year my work was done for craft fairs and fine arts shows.

Fire scale is the evil we all face when we work with sterling silver. You can remove it with chemicals, polishing, and sanding. Early on I decided that the look of sterling as it ages is worth the grief of contending with fire scale. I believe the method I use to deal with scale is from Tim McCreight’s books. If the piece has large open surfaces I will build a bath by boiling water and letting it absorb into suspension as much Borax as the water will take. Then I will slowly warm the metal to a light straw color and dip it into the Borax/water suspension. I let the liquid dry on the metal to coat the silver and create a barrier. Then I will solder through the barrier. Typically I will use a Boric Acid and Alcohol flux on wide open surfaces or combinations of B/A flux and Battern’s or Green Flux. Depending on the piece I will redo these steps after each time I pickle the silver.

Another way of dealing with scale I learned from my Dad. If you can’t plan to eliminate the scale by one method or another try and use it in the design. I do a lot of stamp work and typically I will polish once before stamping. But I will also stamp through the firescale and drive it into the depth of the stamping to make the pattern POP when the surface around the stamping is sanded and polished.

And depending on the piece I am making, bracelets for instance, I have developed the finishing process to the point where I don’t worry about it. Our polishing system is such that scale is largely eliminated to the point where you don’t see it without a glass.

Don Meixner

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As Don Meixner suggest, firestain can be a design element. The great Danish silversmith, Georg Jensen, used firestain as part of many of his raised and planished designs.

I’ve only ever found pripps flux to work, and then only when applied to hot work (so the water instantly flashes off) with an airbrush set up (see http://collarsandcuffs.co.uk/avoiding_firestain.htm for set up instructions using a foot pump and jam jar to make an applicator I wrote up many years ago).
If using this bear in mind that pripps is a terrible soldering flux, so you need to use a propper solder flux on the join

Chris, the applicator I was taught to use in 1972, and which I’ve never found better, is a simple ceramics mouth atomizer. Used by potters to spray glazes, they just dont clog up, and deliver a uniform fine spray. Cost is usually around 5 dollars or less. These consist of a thinner and longer tube, a hinge, and a shorter tapered and wider tube. You adjust it so with the thin tube in the liquid, you blow into the wide end, which then blows across the end of the narrow tube, yielding a spray.

You are correct in that the proper use is preheating the metal so the flux dries instantly to a thin white crust. In a pinch, you can brush it on if the metal is very clean, but you will still be battling surface tension causing the flux to pull away leaving holes in the coverage.

And yeah, its not a soldering flux. The TSP (trisodium phosphate, or other variants) appear to be a wetting agent, but solder doesnt like it)

The biggest plus point fir it though, is its low cost. The TSP can be "Cascade brand dishwasher powder (make sure it is not phosphate free), and the borax can be found in the laundry aisle as “Boraxo” or various other brands where borax is the main ingredient. That leaves only boric acid to buy as an actual chemical… the result can be a half gallon jug of flux, enough for several years perhaps, for just a few dollars…

As described by Peter, following is a picture of my Prips applicator. You can find the mouthpiece on amazon along with real TSP. It is attached to a pill bottle. Be sure to drill a hole in the top so that air can replace the Prips solution as it is removed. Peter, thanks for the very detailed description of firescale and how it can be controlled…Rob

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Following is a link to a source for the atomizer…Rob

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Rather than a mouth atomizer, which I had tried twice and didn’t like (the first time was in a middle school ceramics class), I use a pump hairspray bottle. It puts out a fine spray that is easy to direct.

I cut about 3/8 inch off of the bottom of the sprayer tube to prevent the inevitable precipitation particles from being sucked in and clogging the tube. I also use a polyethylene food container as a backsplash to prevent spray from messing up the bench. Another advantage it has is that after coating a piece, I can pour the solution collected in the backsplash back into the sprayer (being careful not to allow precipitate in). I estimate that doing so allows me to reuse about 60-70% of what I’ve sprayed.

To keep the sprayer from clogging after use, I spray water through it to eliminate any residual Prips in the tube and nozzle.

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Gary…I tried the spray bottle too, but it just kept getting plugged up, so I went with the atomizer. What do you do with the alcohol lamp in your picture? Thanks…Rob

Hi, Rob. I use the alcohol lamp to heat my piece when applying the Prips. Slower than the torch, but I prefer it. One thing I didn’t mention is that, after the application is finished for a piece, I wipe the nozzle with the paper towel (not pictured) that I have hanging from under the plastic container to keep spray from hitting my pliers and other tools below.

I’ve become quite a fan of Firescoff in recent years. It works quite well at preventing firescale for what I do (primarily gold work, with the occasional silver job), even with a lot of it being torch welding rather than using solder (I will always try to weld to pieces together before resorting to using solder).

If I had one gripe, and this applies to the spray application described above with other solutions, it would be that the spray method is very wasteful. When using Firescoff, I feel that maybe only 10% of what I spray actually gets on the piece, with the rest of it disappearing in to the ether. Granted, it’s a small price to pay for not having to spend extra time removing scale from intricate pieces.

A lot depends on your torch and your flame. As others have said, the formation of scale is a function of available oxygen. You can prevent oxygen from reaching the surface using flux, but you will have more difficulty controlling solder flow if you do so.
A soft, brushy, reducing flame is your friend to prevent scale. Yellow and red at the tips indicate more fuel than oxygen, thus consuming what woul be free oxygen that would cause scale.

A neutral flame is your next best. Single cone or brushy blue flame.

An oxidizing flame is very hot and great for targeted soldering, double blue cones. But if used on a large area you will get gratuitous scale.

Sadly, we don’t always have choices as to our flame or flux setups. If scale is unavoidable, you can always use depletion to build a fine silver coating on the surface. Heat silver until it blackens, pickle, repeat.

Hope this helps.

-Jon

I never thought about using an alcohol lamp. I only have three of them. It always amazes me the number of different ways that we use to get to the same point. It is hard to convince someone just starting out that, with some exceptions, there isn’t just one way to do most of what we do. Thanks…Rob

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Long ago I read that the spray bottle can be put upside down in a cup of water to avoid the clogging. Never tried it, though.

I believe that would do the job, lastleaf, and then the plastic cap (which eventually becomes lost anyway :slight_smile: wouldn’t be needed.

Many thanks to all for the comments on firescale/firestain. I have learned a lot. Peter Rowe, sadly TSP is no longer available in any Calgon/Cascade DW powder as far as I could see in my local Walmart and on line. It is available on ebay and Amazon, cheapest about $7/lb. Boric acid is available cheaply as roach powder. I also ran across the MSDS for Cupronil and apparently it is the same as Prip’s except that disodium phosphate is substituted for the TSP. Of course, the home-made wouldn’t have the pretty blue color. I have not tried this, so you’re on your own, Sundance, if you do. -royjohn

TSP…cheapest about $7/lb

$3.98 at Home Depot

https://www.homedepot.com/p/SAVOGRAN-1-lb-Box-TSP-Heavy-Duty-Cleaner-10621/202935861

Neil A

4lbs. for $8.99 at Ace Hardware according to the website.

$3.96·homedepot.com

SAVOGRAN 1 lb. Box TSP Heavy Duty Cleaner

$4.99·Ace Hardware

Savogran TSP No Scent All Purpose Cleaner Powder 16 oz.

$11.89·Walmart - SIM Supply Inc

Savogran CO TSP 5-lb. Cleaner 10622, Size: 4.5

$10.39·Little Hardware

TSP POWDER A.P. CLEANER 4.5#

There are a number of TSP substitutes as well but I don’t think they will work for our purposes.

Don