Trials, Tribulations, and Solutions for Flameworking Green Borosilicate

Green was a finicky color when I first started flameworking with borosilicate glass 25 years ago. As the flameworking world grows, so too do borosilicate color manufacturers and the palettes they present. Many new options are available in the range of green glasses. I reached out to a few contemporary borosilicate flameworkers to get their take on experiences with different greens in the past, options that are available these days, and some tips on how to make the most of these glasses.

Suellen Fowler has been a crucial pioneer in the development of colored borosilicate. She has a great deal of insight to share from her early attempts to create green.

“When I began learning about flamework glass in 1969, there was no commercial boro color available, for the most part. At the Glass Workshop at Pepperdine College, we learned to hand mix basic oxides into clear glass, to make colored rods. We had two methods of producing the color green, the first one being chromium oxide, which makes a very rich opaque jade green, but has serious COE [coefficient of expansion] issues. We worked around that by adding in other oxides, such as tin oxide, which lightened the shade of green but was pretty effective in producing a decent opaque green. John Burton often used cobalt and silver oxide as additional elements, which gave more of a lighter forest green. And you could mix a larger proportion of clear in, which would give you a somewhat grainy looking translucent green. There are a number of opaque manufactured greens available today that include chromium oxide in their chemical mix.

The second combination was mixing cobalt and silver oxide in various proportions, which would produce shades from an ultramarine blue to a pretty decent semi opaque leaf green, etc. Achieving the shade of green one wanted was very much like mixing paints together to produce the desired color. The one technical problem with that was that the silver oxide in a certain saturation would leave little silver balls in your cane, that liked to rise to the surface of the bubble and often explode, leaving a spatter of silver on the side.

I began experimenting with other oxides in combination. I discovered formulas for making an opaque, but heat reactive dark green that produced surface tones of blues and purples. These formulas would produce various translucent and transparent greens when mixed with more clear boro. I found that I could strike nearly clear looking rods of the combined oxides and produce a lovely pale leaf green. And because Germanium oxide binds excess silver in the glass, the problem of the exploding silver beads was almost completely eliminated.” As Suellen mentioned, chromium-based greens can present some compatibility issues. These issues are compounded when one applies these greens in an encased design, whether it be for murrine work or in marbles and paperweights.

Jason Lee is an innovator in the use of borosilicate glass for pipes, murrine and the application of murrine into hollow forms. He provided helpful suggestions for handling these often troublesome greens.

“Chromium greens build crystal content over time which eventually causes checking. The keys to succeeding are to bench cool or kiln crash all prep stages til final completion, run the kiln at lowest set point which is 980ºF to prevent popping during work. I always imagine a timer counting up to detonation with chromium green, and just try to finish with the most efficient moves possible to duck that peak of time.” It is helpful to realize the chemical composition of these glasses continue to change dependent on how long they are kept above their strain point. Deep encasement of greens can be troublesome, but Jason suggests Timber and Roswell for such applications in his murrine work.

John Kobuki has been a pioneer with his explorations on the possibilities of compression marbles. This technique requires colored glasses that are as stable as possible for deep encasement within clear glass.

“Since I started with Borosilicate glass in the mid 90s, green has always been a troublesome color, whether it was getting the right combo of gold and silver in fume work or finding a true opaque green that didn't crack when encased. Since mostly all my work involves green being encased, it became quite frustrating. When I taught classes, I would say to not trust any green except the original Corning Green Dwarf and the India Glasstronics green. Both of these were cintered glass and no major cracking issues; however, they are no longer made. There are friendly shades of green in the traditional pulled rod form mostly made with a cobalt and cadmium yellow combo, you can also achieve these shades with a veil of this combo.”

Mike Mason from Torch Talk and Glass Central Station also notes that “Timber and Roswell by Northstar Glass have filled this gap in the market but lack the density of Glasstronics, leaving their discontinued greens highly desirable.”

Pipe maestro Eusheen Goines achieves great results from Plantphibian green from Molten Aura. In the recycler pictured below, the blueish/green areas are tubing he created from layering Glass Alchemy Steel Wool with clear on the inside. Then, he applies a layer of Plantphibian, and a thick layer of clear goes on top of that. He finds it best to work Plantphibian in a neutral flame and create tubing from the “blowout” technique.

This technique involves gathering up a large amount of solid rod into a thick mass; pushing a tungsten rod into the middle of that mass to begin an air cavity, then adding a glass blow tube to the mass to further inflate it into finished tubing. Layering with lots of clear thins out the Plantphibian to create the wonderful range of greens in the recycler (pictured).

Muranese maestro Cesare Toffolo used a transparent green tubing from China for his GAS Conference demonstration of the classic Veronese vase form in Murano in 2018. He used green for that demo “because the Veronese vase that appears in the painting of the Annunciation is green like… many of the glasses of the time.” He had the following insight to offer:

“Chinese glass tubes are not perfect, they have problems of purity and uniformity in thickness, but currently, they are the best we find on the market. Often, to overcome the drawback of imperfect thickness, I twist the glass as is done for a twisted rigadin [optic twist] or a filigree. This mitigates the problem but, sometimes, it can have a negative effect on refraction.”

Working with Chinese tubing led me to treat the glass almost as though it is soft glass. It is helpful to warm into it slowly with an annealing flame before cranking up the heat for shaping. Either putting it directly intoa hot annealer, or flame annealing it back down below the strain point, are crucial as well. Hopefully these insights and suggestions provide a better understanding of how these new options continue to evolve within the growth of using borosilicate glass for artistic work. Enjoy experimenting with these techniques to get better, more consistent results moving forward.