“Firescale: The Chameleon Effect on Sterling Silver”

July 31, 2012

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This article constitutes an entirely new philosophy concerning an unrelenting foe of the metalsmith; firescale. The materials presented directly contradict long-established concepts concerning the cause and treatment of the formation of oxides on sterling silver. While aspects of the article raise more questions than provide answers, it is in the interest of academic freedom and the open sharing of research that we publish this content.
Jamie Obermeier, SNAG Board Liaison
James Thurman, Technical Articles Editor

By Martin Ebbers

Martin Ebbers, alias Martinus, began his formal apprenticeship to become a goldsmith at the age of 15. He achieved his Master Goldsmith Degree and Teaching Certification through studies at the Institute of Goldsmith Arts in Hanau, Germany in 1980. To date Martinus has spent 38 years at the work bench. For decades he has hauled around the “Theory and Practice of Goldsmithing,” the definitive book on jewelry making by Professor Erhard Brepohl. Martinus always carried some unresolved questions on metallurgy. His curiosity has recently led to fascinating new discoveries.

“What to do and what not to do are the two main things that I was told in the early days of my career as a jeweler. As an apprentice in Germany during the 70s, I found the instructors were generally not appreciative or receptive to challenging questions of their accepted beliefs. Thankfully, in my own way, I continued to be inquisitive of what apprentices were supposed to believe.

The standard Oxide Theory, as the cause of discoloration on sterling silver, has never made sense to me; if oxides were the cause of discoloration, then why could my boss electroplate fine silver onto “oxidized” silver surfaces? The entire silver production line was successfully fine silver-plated. This created contradictions and challenging questions for me. I was taught that electroplating only works for metals onto metals. (Electroplating is an option used for industrial production to conceal and correct the discoloration especially on sterling silver). With respect, my boss Elsie was revered for her extensive knowledge, however, when it came to surface discoloration and metallurgy, she rigidly referred to ‘The Brepohl.’

As an apprentice how could I know then, that it would take over 30 years to answer my own questions and solve a mystery.” – Martin Ebbers

As jewelers and metalsmiths we must come to terms with how to prevent, correct or conceal the dreaded phenomena referred to as ‘firescale.’ The objective of this article is to revisit our fears and question the myths that drive standard thinking on ‘firescale,’ by carefully examining contemporary studio practices. Let’s not be afraid to raise important technical questions and generate dialogue amongst peers who wish to understand this phenomenon and may like to apply new knowledge to their jewelry designs.

To delve into the scientific origin of ‘firescale’; in 2011 Martinus traveled to former East Germany to meet with Professor Erhardt Brepohl. He encountered a brilliant scholar and a generous man with a good sense of humor. Brepohl seemed perplexed that surface discoloration during the polishing phase would cause distress for North American jewelers. He emphasized that his research regarding deep layer oxidation is the result of extreme laboratory conditions and does not apply to ordinary studio procedures. Brepohl recognized there may be varying degrees of confusion defining the actual occurrence of ‘firescale’ as his book does not explicitly differentiate between deep layer oxidation and polishing difficulties with sterling silver surfaces. As laboratory conditions were unavailable during the meeting of 2011, Brehpohl and Martinus could not reach a scientific conclusion on the cause of discoloration in the polishing phase of silver.

Prof. Brepohl & Martinus – Bad Doberan, Germany, March 2011

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

Specifics of Sterling
Due to the inflated price of gold, many studio jewelers are turning to sterling silver; a standardized alloy of 925/1000 parts silver and 75/1000 parts copper. The common belief is that ‘firescale’ develops as sterling silver is exposed to oxygen at higher temperatures causing the copper at the surface to convert into oxides. These oxides are believed to remain below the surface as colored blotchy patches, destroying the clear reflectivity of our finished silver pieces. This is incorrect as new knowledge will show! Even with the greatest care, surface discoloration can occur.  However, this discoloration can be managed, if understood, as we will see further on! It is ONLY extreme heat that really gets us into trouble, (i.e.: as intended for Reticulation), causing oxides to recede below the surface resulting in UNSOLVABLE polishing problems and compromised brittle silver.

The Real Firescale
Many thanks to Mrs. Orlich of the Allgemeine Refinery in Germany for her kind assistance providing rare images of sterling silver under various treatments, similar to the research by Professor Brepohl.

Extreme ‘firescale’ magnified: On the bottom, we see crusty oxidation after hours of overheating in a laboratory. Oxygen enters the alloy deeply and internal oxidation occurs. This forces the grains apart.
Actual ‘firescale’ in the workshop: Repeated annealing and pickling has caused deep oxidation in this vase. Serious cracks have opened in the forging of this hollowware piece.

 

New Knowledge and the Chameleon Effect
As a fine jeweler and teacher in Western Canada, Martinus frequently responds to fears and apprehension from his intermediate students concerning unwanted ‘firescale’’ on their work. With respect for his student’s predicament, he endeavored to reach a scientifically-based conclusion on the mysterious occurrence of surface discoloration while polishing silver work. In demonstrations with students, the “Chameleon Effect” a reversed coloring on sterling became evident. Exactly what was happening while polishing the surface layer created more questions than answers. Some of Martinus’ early hypotheses were; polishing compounds smudged into the fine silver layer, oxidation trapped under multiple fine silver layers, or unusual light refraction on spongy fine silver – and strangely enough, if we don’t polish, why is it not there? All of the above lead to the following test.

The “Chameleon Test” by Martinus
Martinus set up a series of steps representing standard studio practices. In this way they can be duplicated by studio jewelers. As the combined behavior of fluxes and acids with heat has not been thoroughly examined to date, this series of images explores the discoloration precisely.

(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.
(3) Flux (early stage): The Flux protection is spotty, red cuprous oxides appear.
(4) Soldering: Higher temperatures cause the flux to spread out absorbing trapped oxidation. The unprotected areas are developing dark cupric oxides.
(5) Acid Bath or Pickling: Acid has dissolved the flux and copper oxides, which are now leaving an irregular pattern of pure silver on the surface. – The Chameleon Effect, stage one.
(6) Polish: Heat energy is generated by the polishing wheel, compelling copper atoms to rise from the sterling base to re-bond with the pure silver surface.
(7) Discoloration- Chameleon Effect, stage two: After polishing, we see that the former white, pure silver areas, (# 5), have become reddish-grey copper surface areas.
(8) Reheating – Chameleon Effect, stage three: With heat applied the effect reverses, the copper layer recedes into the sterling silver leaving a white, frosty, pure silver “bloom.”
(9) Conclusion: clearly oxides are not the cause of polishing difficulties; instead we have copper atoms transitioning between the surface and lower layers.

 

Scientific Support
In February of 2012 similar test plates like the one above were shipped down to the Technology Department at the ARGEN Corporation, a highly sophisticated, precious metals Company in San Diego. With a series of x-ray tests they determined the precise surface structure of our sterling silver test plate and helped with understanding the metals’ behavior.

The previously held belief of oxides being the cause of silver discoloration is clearly IRRELEVANT for studio jewelers. Our expanded knowledge is as complex as it is surprising! Simply explained, the surface of copper-depleted sterling silver executes a “Chameleon Effect.” This happens as the metal is trying to re-establish a balance between copper and fine silver at its surface layer. According to Argens’ Scientists, heat in the form of friction from the polishing wheel, acts as a catalyst compelling the copper atoms to rise and re-bond with fine silver atoms on the surface. This re-bonding is evident in the micro-thin discolored surface areas on the polished silver. A second application of heat by flame will completely reverse this process, as the copper descends downward and a pure silver surface remains. (see images: 5,7, and 9).

In short, if we could polish our pieces without generating heat, the word ‘firescale’ would not exist for jewellers.  Martinus tested this theory by “cold polishing” a silver test plate connected to a cooling steel block with 1000 grid emery paper. Afterward polishing compounds, (Tripoli & Rouge), were applied gently by hand – the conclusion is: discoloration does not appear during a cold polish. A second test was conducted by applying a buffing wheel to the previously “cold polished” surface. And there it was: The Argen conclusion is correct; when heat is applied, a copper-leached, pure silver surface wants to revert back to its original structure and causes copper atoms to rise from the Sterling base below. We discovered that it is the fine silver layer next to the sterling base that affects discoloration problems. This happens at the worst possible time, as we are trying to complete our pieces!

Unfortunately, it is not easy to illustrate cold polishing. Alternatively, the following two images can help to understand how and where the copper rises. We are using a polishing wheel to penetrate three, spongy, FINE silver layers which each time were established after annealing and pickling. The lowest Fine silver layer always attracts copper atoms to rise via friction from the sterling underneath. It’s a very thin but very resistant layer, causing trouble when polishing, before reaching a break through to the sterling base.

(10) Sterling Silver Test Plate & Diagram of Cross Section A = fine silver “bloom”, B to D = three layers of copper depleted fine silver, E = copper rising through friction, F = Sterling base

Once Sterling’s behavior is understood, you have to choose your option.

Option 1: Flux protection
To achieve the best polish we should aim to avoid surface oxidation all together, protecting all sterling surfaces with fluxes. This way polishing becomes a beautifully easy final step. Note: Green fluoride-based fluxes are not recommended due to higher work temperatures; oxidation will enter before the fluxes take effect. (For making your own flux mix see Martinus website). If we don’t succeed completely and encounter oxidation, pickling will cause fine silver layers which can mechanically be removed. Importantly, sterling silver beneath any fine silver layer is usually unaffected. You can trust that polishing will be just fine!

Option 2: Embracing Matte Fine Silver Surfaces
Let’s take what we have learned and make the best of it! As a practicing jeweler, demystifying metallurgic processes can be the catalyst for new design options. Let’s use oxidation and acid treatments as a design feature to create color enhancements. That can easily be done, it takes a minimum of two, or better three oxidations and picklings to achieve a beautifully frosted, pristine white “bloom” on your sterling silver surface. For best results, Martinus recommends gently brushing your surfaces in between picklings with a fine, wet, soapy, brass wire brush. Also, take time to lightly brush your piece even after the final pickling.

Contrasts of Matte and Polished
If our jewelry designs involve contrasting matte and polished surfaces; the polishing phase will require special consideration. As expected, once we begin our general polish, some discoloration may occur. This should not concern us anymore – we simply focus on the reflectivity of all surfaces. After cleaning, the next step is to create a light oxidation under a gentle flame, (Image 11), then pickle until all surfaces become white again. A light buffing with rouge finishes the piece to a perfectly reflective, pristine white surface.  It also has the advantage of less future tarnishing, (Image 12). For designs in gold, you can use the same techniques; light oxidation while maintaining a reflective surface as well as careful buffing. For both metals, heating to a full glow results in dull surfaces causing the necessity for a total general polishing all over again.

(11)
(12)

Gold – Color Enhancements
Gold responds in the same way although it doesn’t cause any discoloration issues. Fortunately, due to the higher copper content in ‘Peach Gold’ or ‘Rose Gold,’ we can achieve more intense, vibrant hues in a pristine matte yellow “bloom” which works best when heated to a full glow. Gold alloys of 14 karat are generally easy to work with, 18 karat alloys will display stronger hues but 10 karat alloys simply don’t have enough gold content, (in some of these alloy compositions, zinc can also cause troubles as a reddish discoloration).

Acids and Methods in Use
In general, earlier methods, (prior to alternative acid products), applied Sulphuric Acid to sterling silver with excellent results. Today ‘Sparex’™ works comparably and won’t burn holes in your clothes! Hydrochloric Acid was used historically to create an exceptionally beautiful surface on gold; use extreme CAUTION as it is hazardous. ‘Wilacid’™ from Germany works comparably and it is low risk – like ‘Sparex’™. (Note: ‘Wilacid’™ cannot be used for silver). To conduct your own research on acids and treatments, refer to the goldsmith Oppi Untracht who covers the topic at great length in “Jewelry – Concepts and Technology” pages 416-420 and/or the “Theory and Practice of Goldsmithing” by Brepohl pages 353-354, see: “Bringing up the Fine Silver.”

After decades at the workbench embracing fine silver and fine gold surfaces as design elements, Martinus advises students that clean acids are important for a dependable color enhancement. Used acids from silver projects will make gold turn a pale green. Contaminated acids with nickel from white gold will make silver turn a dull shade of grey.

Martin Ebbers offers personalized training in fine jewelry design, hand fabrication and serial production techniques. He has designed a sophisticated line of fine jewelry and owns the Martinus Studio-Gallery located on spectacular Salt Spring Island in British Columbia, Canada.

If readers are willing to share, Martinus and students will gratefully appreciate their knowledge. Please email to <ask@martinus-gold.com>.
Also check the Training pages at <www.martinus-gold.com>

Copyright Martin Ebbers – May 15, 2012
All rights reserved

 

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