FIRESCALE... KNOW MORE!
Tips and Answers - by Martin Ebbers
Overcoming the Fears, Myths and Misconceptions
- Definitions - What we do & what we need to know
- The Martinus Firescale Test will help you carefully observe the effects of the silver surface behavior (Thin Layer Oxidation pattern) through common studio procedures.
- Firescale – for the brave examines the scientific aspects of the issue.
- Tips and Solutions Will provide you with valuable, practical knowledge to work fearlessly and successfully
1. What we do & what we need to know
Dear Students and Colleagues: As studio jewelers working with silver – the term "firescale" has become ingrained in us. I would like to start with a simple overview of what is involved when we talk about "firescale". The McGraw-Hill Dictionary of Scientific & Technical Terms defines 'firescale' ('fīr 'skāl) as: "Copper oxide remaining below the surface of silver-copper alloys after annealing and pickling". Partly correct, this definition doesn't differentiate between fine silver layers and the underlying alloy which causes tremendous confusion amongst practicing jewellers.
Depending on temperatures applied, I would state, that "Firescale" can be viewed as a continuum, ranging from mild polishing problems to extreme conditions that result in the breakdown of a silver alloy’s cohesiveness. Let’s keep it simple while looking at physics and chemistry and start with basic facts and processes used in our studios.
- Many shape changes; rolling, drawing, bending, forging harden the inner crystal structure of our alloy.
- Annealing is essential to soften the inner consistency of our metal in order to continue working.
- As a side effect to annealing and soldering, the surface layer of our metal transforms, creating the conditions for discoloration to occur. We will explore this in the next section.
Sterling and Oxides
While heating our work, first reddish, then grayish and later black oxides appear on the surface as a result of 7.5% copper content, (added for strength in Sterling silver). Unfortunately, these unattractive oxides are very hard and can seriously damage drawplates and make solder connections impossible. In addition this unsightly surface causes difficulties to estimate the proportions of our piece accurately. As a consequence of the above, we need to acid treat (or "pickle") our silver work. While "pickling", copper oxides are dissolved and leached out from the silver alloy’s surface, leaving our piece with a matte white, spongy, pure silver layer. Multiple annealing or soldering will increase the layer of pure silver - it is important to understand that this layer cannot oxidize! We can prove this during repeated heating, because our piece doesn’t turn black anymore. In fact, this spongy, pure silver layer causes our polishing difficulties. It is important to know that the Sterling metal below remains unaffected by oxides under regular studio conditions! Lets refer to this as a Thin Layer Oxidation versus the extreme, Deep Layer Oxidation as a consequence of severe overheating. In Deep Layer Oxidation the oxides proceed into the alloy’s interior and polishing is futile. "Firescale" seems to have become synonymous with troubles and the mysterious culprit for all of the above. As nobody could easily question authorities about their extensive research and elaborate explanations, we must rely on our own observations.
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2. The Martinus Firescale Test
This practical studio procedure is an overview that reveals how discoloration occurs. You can easily repeat this process and find proof for yourself! In our images the letter A will refer to areas where Acid will change the surface and F will stand for oxide removal through Flux reactions.
(1) Clean Shapes: A sterling silver plate with an evenly polished surface and no discoloration. (Left: A leaf shape to be soldered onto the plate).
(2) Fluxing (start): A borax-based flux is applied to the leaf shape. While heating the flux solution begins to foam. (Underneath trapped air may contribute to spotty oxidation).
(3) Flux (early stage): At lower temperatures the flux is barely covering and early stage spotty oxides are detected. Surrounding areas are taking on red cuprous oxides.
(4) Soldering: Higher temperatures cause the flux to spread out absorbing and leaching out trapped oxidation from below, red coloring from cuprous oxides & teal coloring, a reaction to cupric oxides). The unprotected areas are developing fully dark cupric oxides.
(5) Acid Bath or Pickling: Acid has dissolved the flux. Also formerly dark areas top right and spotty areas around the leaf shape have been leached of copper oxides by flux and show spongy, lighter whitish, matte surfaces. The visibly darker areas in this image are the fully flux protected and unaffected and still reflective sterling silver surfaces. (Please note that flux not only covers against, but can also dissolve oxidation, creating fine silver layers!)
(6) Polish: An abrasive final polish, using various brushes and wheels, Tripelie and Rouge for a clear, reflective, finished surface. At this point discoloration begins to appear!
(7) Discoloration: Through polishing we see that the formerly spongy, white areas (image #5) transitioned into a reddish-grey discoloration where even cleaning has no affect. Is this the Firescale most people fear ???
(8) Testing for Oxides: After a quick soap cleaning in the Ultra Sonic, we lightly reheat our plate. Without any chemicals involved the effect reverses from reddish to white, in the flame! Where do we observe oxides? (Note: on a sterling surface the 7.5% copper oxidizes, however fine silver cannot!)
(9) Observation: As a result from heating, we see oxidation on the exposed sterling surface, - (done for contrast and demonstration only in this sample!) In comparison with image #7, we too discover that the areas expected to oxidize even more, (which appeared reddish/gray previously), are behaving like fine silver surfaces without any traces of oxides.
Conclusion:
We can conclude that the spongy, copper-leached silver layer is the vehicle for discoloration during polishing. Through decades of experience working with these copper leached, fine silver surfaces as a design element, I have found that the physical characteristics of fine silver makes it challenging to polish on the wheel.
Its malleability and the receptivity that spongy fine silver has to dirt and polishing compounds is most likely the source of discoloration and reason for our polishing problems.
To date I couldn’t confirm what scientifically causes the discoloration and why it burns off so easily however the good news is: This Thin Layer Oxidation Pattern can be controlled and fears around overheating can be a thing of the past, see (Part 2 and 3)
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3. Firescale – for the brave
Throughout work related literature, I have found only vague descriptions and insufficient translations, mostly stressing the rare and worst case scenarios! As mentioned before, depending on the temperatures applied the effects of "firescale" and oxidation can be seen as a continuum, ranging from simple surface discoloration issues to the destruction of the physical integrity of a piece, (#11).
We begin with the extreme, the Deep Layer Oxidation (see #10 microscopic cross section and #11, both Allgemeine Refinery Germany). Here in hollow ware work extreme heat and multiple picklings result in the loss of malleability and ultimately the breakdown of the metal’s cohesiveness. Through temperature related expansion and silver’s high receptivity and storage capacity to oxygen when heated, the copper and silver crystals run into a deepening loss of connectivity towards the interior of an alloy. Sometimes we may find these unwanted deep effects in reticulation as well. This is definitely an extreme example of "Firescale" and no one would consider polishing this surface!
Usually we deal with mild forms of difficulties which I am reluctant to refer to as Firescale! For clarity I would prefer to speak of Thin Layer Oxidation patterns, see image # 12 (Allgemeine Refinery Germany) and from earlier on, Martinus #1 to #7. This second type of oxidation doesn’t affect the integrity of our metal, and be assured; this has nothing to do with overheating. No one considered examining the interactions and effects between soldering and annealing Sterling with treatments involving: fluxes, brief heating and acids. As described in the Martinus Firescale Test, discoloration problems stem from the development of spongy fine silver layers that build up as a result of repeated treatments and cause challenges in the polishing phase.
To date we do not have qualified, scientific information which takes into consideration our daily studio procedures and so, I decided to contact the highest authority on the matter.
Prof. Brepohl & Martinus
Bad Doberan, Germany
March 2011
The Master Goldsmith, scientist and author, Professor PhD. Erhard Brepohl invited me to discuss "Firescale" and discoloration in Sterling silver at his house in Germany. In the 1950s Professor Brepohl lead extensive research involving specialized laboratory conditions while masterminding the theoretical foundation for all the later jewelry-making books. We agreed that "Firescale", according to his own research and as defined in the McGraw-Hill Dictionary, is a rare case scenario and usually does not relate to our work as studio jewelers. Also admittedly these definitions are not stating such differences clearly enough and unfortunately, misinterpretations and unquestioned translations have created unnecessary fears amongst jewelers.
While in agreement, that Fine Silver is the true vehicle of the issue, Professor Brepohl offers two unconventional suggestions to avoid difficulties in the polishing phase:
1) We can choose to minimize the fine silver layer by acid treating our work at the very end, only once. Oxidation would become our surface protection during assembly. Please keep in mind we would continually scrape clean the solder connections throughout assembly and polishing around the scraped surfaces can become tricky.
2) Or, once assembly is complete, we have the option of dipping our piece briefly, for a few seconds in Nitric Acid to dissolve the fine silver layer prior to polishing. Be aware of hand and eye protection, fumes are also hazardous!
Introductory quote from the highly esteemed resource book – "The Theory and Practice of Goldsmithing":
"The mutual exchange of experience is an important means of further education. Those who allow their colleagues to participate willingly in their own experiences will often get useful information in return."
Professor PhD. Erhard Brepohl
Three Questions for Scientists
1) If the discolorations in our sample (image #7) are cuprous oxides, have they been trapped in the fine silver layers disconnected from the sterling and burned off somehow in the heat? (image #8).
2) Could (image #7) possibly show discoloration related to light refraction?
3) Is polishing compound smudged into the spongy fine silver (image #6) causing discoloration and possibly its own resistance toward abrasion?
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4. Tips and Solutions
As a practicing jeweler, I found, that embracing and demystifying metallurgic processes can be a catalyst for artistic insights and a source of new design options. Let’s delve into the Essential Techniques:
1) Protection
If you need a good polish, cover the entire piece with flux or protective liquids at all times, it is as important while annealing the materials. Doing so, you avoid oxidation to enter the surface, this way copper won’t be diluted and your surface stays Sterling consequently polishing will be easy. To cover your surface well against oxidation, use Borax type flux and cover fluids, the green fluoride types are troubles with silver, since they will liquefy and spread out at temperatures too high and therefore too late! (This is also tremendously important to soldering!)
A self mix option for a wide temperature range: Borax and Boric Acid 50/50 dissolved in Methylhydrate until saturated for finer distribution over the piece. (Use an unbreakable container and a lid that can shut down an accidentally caught-on flame). Water based mixes are harder to apply and pastier so that solder tends to loose its running capacity while floating liquid on or inside the flux without sinking through to the metal surface underneath.
2) Removal
Pure silver is hard to detect on the slightly yellower Sterling, but if you make sure when filing or sanding, that all fine silver sponges are gone, polishing will be easy! Here you can also consider Prof. Brepohl’s Nitric Acid dip.
3) Acceptance
If your intention is a whiter fine silver surface, and a lesser polish is acceptable, you can also reheat and acid treat two to four times after completing your work piece. Be gentle with heat so that solder doesn’t flow again and fine silver appears all over your object. A fine brass wire brush, wetted with water and soap can massage the spongy surface to a semi-polish. The advantage here is that sulphites in the air won’t tarnish your work nearly as much as polished Sterling surfaces do over time.
4) Concealing
Silver plating will cover the piece evenly, but work marks or even sanding lines cannot be covered. Electrolytic plating is used mostly in industrial production. It is very detailed in the process, so that it can only find mentioning here. All good jewelry books though take care of the specifics if you need to explore your options.
5) Soldering Fine Silver Surfaces
Be aware that spongy, fine silver is a receptive material that fills up its openings with solder, while inhibiting the solder to connect through to the alloy level. For assembly work with hinges and other mechanisms this generally causes the soldered spots to break off under pressure! If you need solid connections make sure you are connecting properly with the alloy level by sanding or scraping the surface of your components!
6) Rare and Nasty
As a little addition to oxides and polishing troubles, you may come across "Comma Silver" real cupric oxides, tiny, black, pointy crystals sticking out of the surface, causing lines behind them in the polishing direction. They are harder than our files and only emery paper or diamond tools can remove them before you can continue polishing. These oxides seem to form in the silver’s melting or casting phase.
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(#10) Firescale, the extreme under the microscope: On the lower side, we see crusty oxidation after hours of over heating in a laboratory. Oxygen enters the alloy deeply. Internal oxidation occurs while copper & silver crystals become isolated, loosing connectivity.
(#11) Firescale, frequent annealing and pickling have caused deep oxidation, deep cracks open in the forging of this hollow ware piece.
(#12). Correctly annealed sheet silver shows no interior oxidation, the top and bottom layer of a 925 silver alloy displays the typical Thin Layer Oxidation. Throughout all treatments and studio operations the changes in this thin layer need to be examined very carefully.
Here’s where all of our common polishing difficulties originate!
(#7) Sudden discolorations in the polishing phase, related to Thin Layer Oxidation patterns. Cleaning can’t remove this surface, heat can!