Technical Article
Total Page：16
File Type：pdf, Size：1020Kb
SNAG NEWS Volume 20 Number 4 August 2012 TECHNICAL ARTICLE 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 “Firescale: The Chameleon Effect on Sterling Silver” 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. 18 SNAG NEWS Volume 20 Number 4 August 2012 at left: 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. 19 SNAG NEWS Volume 20 Number 4 August 2012 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 (2) Fluxing (start): A borax- (3) Flux (early stage): The Flux silver plate with an evenly based flux is applied to the leaf protection is spotty, red cuprous polished surface and no shape. oxides appear . discoloration. (Left: A leaf shape to be soldered onto the plate). 20 SNAG NEWS Volume 20 Number 4 August 2012 (4) Soldering: Higher (5) Acid Bath or Pickling: Acid (6) Polish: Heat energy is temperatures cause the flux to has dissolved the flux and copper generated by the polishing spread out absorbing trapped oxides, which are now leaving an wheel, compelling copper atoms oxidation. The unprotected irregular pattern of pure silver on to rise from the sterling base to areas are developing dark the surface. - The Chameleon re-bond with the pure silver cupric oxides. Effect, stage one. surface. (7) Discoloration- Chameleon (8) Reheating - Chameleon (9) Conclusion: clearly oxides Effect, stage two: After Effect, stage three: With heat are not the cause of polishing polishing, we see that the applied the effect reverses, the difficulties; instead we have former white, copper layer recedes into the copper atoms transitioning pure silver areas, (# 5), have sterling silver leaving a white, between the surface and lower become reddish-grey copper frosty, pure silver “bloom”. layers. surface areas. 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.