About 15 years ago, I developed a highly effective technique for successfully working glass on copper tubing. I had been looking for a way to add glass to my bed frame, and wondered if I could somehow put glass on metal. Since the coefficient of expansion of copper is 80% higher than that of glass, it appeared unlikely that it would be possible to “fit” glass and copper tubing. Through beginner’s naïveté, I was able to solve the problem (see Figure 2). I have been using this technique extensively, and to the satisfaction of many clients, since that time.
In retrospect, the reason blowing glass on copper tubing works has little to do with the coefficient of expansion and fitting the copper to the glass and much more to do with the elastic limit, where Young’s Modulus breaks down and the copper deforms. After copper has been brought to a red-hot temperature, it is very deformable and will move to where the neighboring glass wants to move.
The result of using copper pipe to blow glass is that the finished product has a copper tube permanently embedded into the glass. Having a copper tube extending from the glass opens the possibility of using glass in a variety of new applications. The copper tube acts as a useful intermediary for the attachment of glass to a post (Figure 1), an armature (Figures 3-5), or to a larger sculpture(Figure 7). By inserting either steel or wood into the shank, the piece can be made strong enough to support weight, such as on a coat rack, and it can be combined with other hardware for more complex uses such as a doorknob or a lamp base.The durability of this seal has been tested in New England winters for over a decade.
Initially, I was cautious about using blown glass in public settings where the glass is easily accessible to the public. But, having used this technique on several public projects, I can vouch to the durability of the system even in very public places where passersby frequently caress the glass finials admiringly.
Having used this technique extensively, I have learned much about how to successfully carry out the process. To gather glass onto copper tubing, the copper tubing is heated to a dull red glow (about 800°C) which anneals the copper. A layer of oxidized copper is formed on the copper which imbues a deep red color to the copper. The copper can be gathered upon without the preheating, but the result is the shiny pink copper color and it is difficult to do this without gathering an assortment of bubbles on the pipe. This is a matter of taste, but I think that the bubbles and shiny pink look ugly; I heat the tubing to a dull red. Do not over do it! Copper melts at 1084°C (1983°F). A warm glory hole can
easily melt the copper and, what is worse, the hot glass can also melt copper. Gathering on copper that is too hot may result in losing copper into the tank: if the furnace is freestanding, the copper can drill a hole in the bottom of the pot. This is a serious concern!! In practice, I turn down turn down the temperature about 100 degrees from the temperature I blow on normal pipes, to around 1050°C.
The rest is familiar enough. The glass can be worked as if it were supported by stainless. Frit is easy to use, multiple gathers are possible… The heaviest pieces (lamp bases) that I have done are about 10 lbs (5Kg) on ¾ in tube (see Figure 6). When making something this heavy on copper tubing, special care is needed not to overheat the tube and a gentle and graceful technique is always of value.
When the piece is finished, the tubing can be cut with a pipe-cutter (I prefer Rothenberger, but others will do) I usually leave 2-3 in sticking out of the glass, but you can leave on anything that you want as long as it fits into the annealer. Less than 2 inches is sometimes a problem because the copper in this region is often heat softened and does not cut as nicely.
Some smaller details:
I started out using standard nominal ½ inch type M copper tube. This is still the standard although I have had uniform success with nominal diameters between 1/16 to ¾ inch. I sometimes have success with diameters of an inch and above, but it works sporadically (sometimes failing months after being made) and I do not recommend it. I have used type M and L in most gauges and type K in smaller gauges to good effect.
Blowing on copper tubing can involve putting ones lips on the tube. While copper is probably not toxic at this exposure level, the copper does have a flavor, which I do not like. This can be remedied with a piece of tape around the pipe on the blowing end or by use of a rubber insulator of the sort that is sold to insulate electrical alligator clamps.
Since many gauges of copper tubing may be used and since these can be fit into one another, sculptures that involve nesting of 2 or more glass pieces are possible (see Figure 7). Surface tension of the hot glass can wet the inside of the end of the tube. After the fact, this may be cold-worked away. A small amount of kiln shelf wash or the bead resist used by torch workers inhibits creep of the glass to the inside of the tube.
In summary, this technique has been used in my studio to create lushly adorned glass and metal fences and railings, for finials on curtain rods and newel posts, for coat racks, in the creation of multi-piece bird baths, to create lamp bases of glass, and in the assembling of door knobs. The versatility of this technique is remarkable and I think that in the future it will be of value to many glass artists.
Rodman G. Miller