Vitreous enamel adventures

Firing glass powder onto metal is an ancient technique commonly known as enameling or, more precisely, vitreous enameling. It is highly durable, beautiful in appearance and pleasing to the touch. I was recently tasked with creating enameled medallions for inlay into stainless steel bar pulls for a new building project. They would be 1 inch squares with a readily identifiable red and white graphic image. A group of 4 pairs of 6 foot tall, 1 1/4 inch diameter, stainless steel door handles providing access through tempered glass doors to the high-spec, inner sanctum of this temple to consumer products.

An acquaintance who had worked in my building, and who now works for an international design firm based in Portland, was the source of this new project. He had been given the job of overseeing the construction of the project and, when given the challenge of getting this particular  task underway, he thought to contact me. I could only claim childhood experience and an ongoing fascination with enameling, but the machining work to inlay the medallions was right up my alley. So I chose to accept the challenge and dive into preparations.

A suitable kiln was required, so I made a search of Craigslist and found an Evenheat Copper – halfway there! A simple modification would make the kiln a bit more suitable to the particular requirements of enamel work. I cut a wide slot into the lower half of the hinged door and made a removable fire brick sub-door to provide quick access for inserting the enameled parts.

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Kiln with modification #1

But before the test firings could begin, another unexpected challenge soon emerged. As presented to me, the task seemed to imply the handles would be solid material, since the inlays would be inset perhaps 1/4 inch deep into the face. But when the pulls arrived, it was quickly realized that they were fabricated from tubing – a bit of a “duh” moment for all concerned. Nonetheless, it meant something would have to be done to create the proper cavity for the inlays. I worked out a few possible scenarios and settled on a stainless steel insert consisting of two matching parts that would be inserted through a precut clearance hole in the handle.

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Test fitting the stainless steel inserts.

Each part would have two threaded holes with a set screw in one used to press against the back side of the tubing, forcing the inserts against the front wall, while the second hole would contain a screw projecting upwards to facilitate moving the inserts through the hole. Epoxy would be used to adhere the inserts in place augmented by the mechanical force of the set screws left in place. Once cured, the assembly would be machined as if the material was solid.

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The completed inlay recess

While that issue was being resolved, I continued launching into the enamel process. Initial samples of copper substrate and enamel supplies from the major supplier, Thompson Enamel, were obtained along with a few additional tools for handling the enamel parts. I knew I was in for some trial and error and I wasn’t disappointed. While I don’t think I discovered every possible way to go wrong, I made very good progress toward achieving that goal. Lot’s of interesting results, but consistency was the real goal. Try, fire, record, repeat – how many times? Pretty soon I was beginning to contemplate using stainless rather than copper for the substrate. I didn’t find much rigorous data to go beyond Thompson’s recommendations pointing to 4oo series stainless. But my other parts and the pulls themselves were 304 and it is cheaper and easier to obtain through my channels, so I decided to take a chance and make the switch. The oxidation issue with copper and the red enamels was beginning to grate on me. Stainless at least had fewer oxidation issues. And I had begun to worry a bit about galvanic issues though the data didn’t fully support that concern.

Rather than using my Tormach CNC mill to cut out the parts, I ordered what would become only the first of several batches of 304 square blanks from my local laser fab shop. There were plenty of expected failures, presumably due to expansion rate issues, cleaning procedures or black magic. But eventually a solid candidate for the base white coat materialized from among Thompson’s offerings.

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White base coat on 304 stainless

The next step was to find a red that might work on top of the white. But the trick was now to find a means of applying the material so that the clean stripe motif could be created. The white base was applied by the traditional sifting method which works just fine for that purpose. But the detail in the thin stripe needed to be sharp and consistent across all the final parts. So my initial concept was to use a silk screen technique to lay down the stripe. I built an indexing screen fixture using the CNC mill and obtained some photo stencil screen material. Everything but the result was according to plan. The screen produced a pixelated result that was not sufficient to get the result I needed. I was considering weather a finer screen might work when it occurred to me I could convert this setup into an indexed knife guide for cutting airbrush frisket. First I tried a few hand cut masks to see if the procedure was viable and then I made up some brass cutting guides to fit in place of the screen material.

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Silk screen fixture without the screen in place

That did the trick and still allowed for re-masking and re-coating the red enamel on the same part. Initially I used a brushable version of the enamel. But there were problems with brush marks and and poor color match. I wanted to look at spraying the material, but that meant that the enamel must be prepared in a different way. Rather than the 80 or 150 mesh granular form I used for the white base, the sprayable form must be finer than 320 mesh. And usually a special medium is used to mix the final sprayable material. Thompson had a very limited supply of red applicable to this process and I tried it for a mask-and-spray version of the procedure. The process was fine, if a little finicky. The color and/or flow to proper sheen was not.

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Trial, and mostly, error

By this time I had become aware of a variety of suppliers of enamel and still hadn’t turned up a color candidate that might work. And then I stumbled across a company that deals with a closely associated area of hot glass work (no metal involved). They had a couple of reds that looked right and, on close reading, the material looked to be suitable for glass-on-metal as well. The supplier was actually close by and I went to see them and discuss the project. I came home with some red powdered enamel and airbrush medium and went back to work.

By this time I had become aware of a variety of suppliers of enamel and Next up: putting it all together . . .

 

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