Technical Article: “Metal Millefiori” by James Thurman

May 28, 2015

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It has been my pleasure to serve as Editor for SNAG’s Technical Articles for the past five years, as well as have the opportunity to be an occasional author. At the end of 2015, SNAG will be discontinuing the regular Technical Articles and redirecting its funding and resources towards new programming. As the final Technical Article I will be authoring, I thought it would be appropriate to share some experiments I’ve been doing for the past several years, inspired by the truly unique and innovative work of Ken Cory. I would like to give a very special thanks to Nancy Worden, who not only granted permission for the reproduction of the image of the artwork that inspired this entire project, but who was also shared such inspiring statements throughout the process. — James Thurman, Technical Article Editor

“Metal Millefiori (Ken Cory Revisited)” by James Thurman

More than ten years ago, I came across a truly inspiring book, The Jewelry of Ken Cory: Play Disguised.  Although the designs and aesthetics of his work were very different from my own, I had instant and profound respect for his passionate creativity.  One piece in particular has always stood out in my mind, “(Untitled) Nancy’s Buckle” (1978). (Image 1)

I had never seen anything like it in metal and was intrigued about how it was made.  In the book’s Glossary under “Millefiori Metal,” it says, “Cory probably invented this process, using the same concept that Venetian glassblowers developed to make detailed patterns for glass (in Italian, ‘millefiore’ means ‘thousand flowers’)…The finished effect resembles inlay, and no other metalsmith is known to have used this process.”  Who could resist retracing his steps to explore the process?

nancy's buckle
Image 1
“(Untitled) Nancy’s Buckle” (1978)
Silver, copper, brass, cloisonné enamel, and mirror
2.25” x 3” x .5”
Collection of and image courtesy of Nancy Worden

My initial experimentations focused on variations of soldering different basic configurations together.  As with any soldering operation, it is critical that all metal is very clean and fits tightly together.  To test the importance of an outer tube, I made two different billets: one concentric arrangement of a bronze rod inside of a copper tube inside of a brass tube (Image 2) and one that was a checkerboard of brass and copper square wires (Image 3).

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Image 3

I correctly anticipated that the solder seams of the concentric configuration would hold together better than just the checkerboard.  Ken Cory obviously knew this as well since all of the metal millefiori segments in his piece are contained within a copper tube that has been drawn hexagonal in order to solder best into a sheet.  Combining the approach of the concentric tubes with the square wire, I soldered rectangular bars together and then milled them to fit inside square brass tube. (Image 4)

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Image 4

Once the small billets were created, I explored the potential of the millefiori slices.  The slices were made both lengthwise and widthwise. (Image 5) A perfect impetus for these explorations was the upcoming Pin Swap at the 2013 Yuma Symposium.  Using a rolling mill, I deliberately stressed the millefiori slices to their breaking points to test the relative strengths of the composite metals versus their solder joints.  Somewhat surprisingly, the innermost bronze core was often the part most likely fail or fracture.  Despite this, I still found the overstressed bronze an interesting part of the pins and the related experiment. (Image 6)

Given the range of results of the soldered millefiori slices, I felt confident that almost any configuration of parts successfully soldered together would work, as long as there were no gaps between the component parts and that the outermost component was a seamless tube of any shape (round, square, hexagonal, etc.).  I had stretched and stressed the slices far past what I believe Ken Cory had done in “(Untitled) Nancy’s Buckle (1978)” to prove their viability beyond any doubt.  There is still great potential for further exploration here, both in efficiency of the creation of numerous similar parts (by slicing up billets), as well as the creation of very small extruded patterns.  Similar to the original inspiration from glassworking, images could be created out of numerous larger wires soldered together and then drawn down in size.  I have seen this done successfully as part of a larger pattern-welded steel billet but not in non-ferrous materials except in the case of Ken Cory.  In my experiments, I found that metal millefiori compresses much better than it stretches (less annealing necessary, less stress-fractures/delaminations, etc.).  As an example of this, I was able to successfully roll the concentric billet of brass, copper, and bronze into square stock and then sandwich a set of four inside of another square brass tube.  This could be repeated until the pattern of its components becomes visually indistinguishable.

Inspired by amazing work done with mokume-gane and pattern-welded steel, I wanted to continue to push these explorations further by attempting to create billets through diffusion bonding rather than soldering.  I used the same concentric brass/copper/bronze materials to minimize variables.  I placed the billet inside of a stainless steel envelope (typically used for heat treating of small parts) packed with charcoal.  I was doubtful but thought there was a possibility that the differences in expansion of the different metals might fuse them together.  After firing the package in an electric kiln at 1500F for nine hours, there was some slight adhesion but not enough to hold up to any kind of force from rolling or forging. (Image 7)

At this point, I consulted with several mokume-gane experts to get their input on this project.  I would like to publicly thank James Binnion, Tedd McDonah, and Stephen Walker for their input and advice.  The overall consensus is that significant force needs to be applied to the billet while the billet is at the correct temperature in order for diffusion bonding to occur, just like in traditional mokume-gane billets.  The main difference here is that the layers of mokume-gane are parallel so it is relatively easy to apply the compressing force to the billet through hammering, clamping, or rolling.  In the case of metal millefiori, the compressing force needs to fully surround the billet to be sure that all interior edges bond equally.  On a commercial/industrial scale, this is done through extrusion.  This is how various filled wires and some mokume-gane types are manufactured.

In an attempt to replicate industrial extrusion in a small studio, I machined my own drawplate to attempt to hot extrude the same concentric brass/copper/bronze billet.  The original outside diameter of the billet was .375”.  The drawplate I made had three openings: .370”, .365”, and .360”.  Based on typical drawplate sizes, I hoped that .015” would be enough of a reduction in overall diameter to help the diffusion bonding to occur.  I heated the billets to a bright red (hopefully diffusion temperature but below melting temperature) and forged the billets through the drawplate openings. (Image 8)

8_hot drawing
Image 8

Due to the experimental nature of this process, there were significant variations in billet temperatures during the drawing process.  Despite this, the brass, copper, and bronze did successfully bond through diffusion for the most part.  To test the strength of the bond, I cut the billet into sections, both lengthwise and widthwise. (Image 9)  Just through visual inspection, it can be seen that some sections bonded better than others.  To further test the bond, I rolled the sections in the rolling mill.  (Image 10).

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Image 9
10_Diffusion bonding rolled
Image 10

As expected, some sections delaminated while others held together quite well.  I believe this is enough proof of the potential of diffusion bonded metal millefiori.  However, due to the significant expense in both time and equipment to further explore this approach, it is my hope that this article inspires others to take the next steps.  Just as there is a myriad of commercially available glass millefiori pieces, I can imagine the possibilities of metal millefiori too!

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SNAG would like to take this opportunity to recognize our Corporate Members for their support: Aaron Faber Gallery, Halstead, and Jewelers Mutual Insurance Company.

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