I have decided to write this series of articles in response to the reader's request for information on lampworking soft glass. Although I am probably not the most qualified lampworker to do the job, and in spite of the fact there are several articles on the subject already written, I don't see anyone else stepping forward anytime soon. So I guess for now, I'm the guy.
I have about three years experience with soft glass (along with about 15 years as a hard glass lampworker) and have developed a line of bottles, goblets, ornaments, and beads that have marketable quality. Being a hard glass lampworker who has adapted to soft glass, I can pass on things I have learned that should make it easier for others like me to make the transition.
I should say at this point that everything in these articles, indeed, everything I know about soft glass, I have learned from Shane Fero and Fred Birkhill. They have been my soft glass mentors and it is really their knowledge that I pass on to you here.
So, what is "soft" glass? Soft glass is glass that melts at a relatively low temperature and attains a very low viscosity when heated, i.e. soda glass and lead glass. Hard glass is glass that melts at a much higher temperature and maintains a relatively high viscosity when heated, i.e. Borosilicate glass such as Pyrex, or Duran.
Let's start by dispensing with a few myths and misconceptions many hard glass lampworkers (including myself) have had about soft glass.
The first subject we must touch upon is compatibility. Hard glass lampworkers rarely have to worry about compatibility because all brands of clear hard glass and most types of colored hard glass have about the same linear coefficient of expansion (COE): 33(X10 -7). But the many varieties of soft glass all have differentCOE's ranging from around 85 to 104 (X10 -7). If the difference in COE between two glass is more than 4, they should be considered incompatible. Clear lead glass tubing is more tolerant to COE deviation than soda-lime glass. It also tolerates thermal shock much better, so I recommend the beginning soft glass lampworker use lead glass tubing.
Differences in COE are nowhere near as critical for lampworkers as they are for furnace glassblowers, but are still important. It is obviously helpful to know the manufacturer's specifications about a particular type of glass so you can be sure of compatibility, but when you don't know, a simple compatibility test will help. Seal the unknown glass to one you do know and draw them into a thin thread. If compatible, the tread will remain straight. If incompatible, the thread will bend, or even curl, as it cools.
Most of the soft glass work I do is done with 20 - 25mm clear lead glass tubing, or 6mm clear lead glass cane, which has an COE of 91 (X10 -7). With these I use any of three brands of German colored soft glass: Kugler, Zimmerman, and Wiesenthalhuette, all of which have an COE range of 85 -90 (X10 -7). (See supplier list at the end of this article).
Next, we need to talk about the importance of atmosphere in working soft glass. Atmosphere is the second major variable in a flame (the first is volume) and is described as being either "oxidizing" or "reducing." Most soft glass, particularly those which contain lead, must be worked in an oxidizing flame. If the flame is too reducing, the metallic element, especially lead, will condense on the surface, blackening the piece. There are several ways to increase the oxidizing properties of a given flame. One is by changing the mix. You can either add oxygen, or remove gas. A more oxidizing flame can be produced without changing the flame (if the problem is a minor one)by moving the work away from the torch face. Surface-mixers are more efficient in the consumption of gas than pre-mixers so there is less gas in the flame. While it is possible to work soft glass in a pre-mix burner, it is much more difficult to obtain a completely oxidizing flame, and there is a potential for backfiring.
Now for some notes on the material itself. Soft glass can be purchased in several forms. Clear lead glass comes in tubing and rod in varying diameters, much like hard glass. The colored glass (Kugler, Zimmerman, Wiesenthalhuette) comes in frit, powder and short thick rods in hundreds of different colors.
It takes time to learn which colors are best for each application and trial and error is the only teacher. Some useful information can usually be obtained from the dealer, as well as other lampworkers, or furnace glassblowers. C&R Loo puts out a nifty "chip" catalogue with shards of each color in plastic sleeves which makes it a bit easier to select colors. Making matters even more confusing, the colors will vary sometimes from batch to batch. It can be frustrating, but the vast selection of colors and resulting endless possibilities make it well worth the effort.
When you buy a colored soft glass rod, it will be about 1"-1 1/2" thick and is obviously not lampworkable in that form. To make a lampworkable cane, take a hammer and a sharp chisel and knock off a 1" -2" chunk. Place it in your oven and bring it up to 900 degrees, or so. Use a 12" piece of 8mm pyrex as a punti rod, heat the tip, and stick it on the chunk. Heat the chunk for a few seconds in a vigorous pure gas flame (no oxygen). Then dial in a medium oxidizing flame and continue to heat the chunk. Stick on another pyrex punti and rotate the chunk in the flame until it melts into a ball, then draw it slowly out into a rod (see the accompanying illustration). The trick here is how fast you draw the molten ball. Too fast and it is on the floor. Too slow and the rod will be too thick. Just right and you should be able to pull clean cane from 4mm to 8mm thick (a hard glass lampworker, accustomed to the quick cooling properties of borosilicate glass, will definitely tend to pull too soon, too fast). If you have a big enough chunk, you might have to enlist the help of a partner to complete the pull, which can sometimes be up to ten feet long. Striped cane can be made by rolling the molten ball into a cylinder and drawing on contrasting colored stripes before pulling. Once the pull is complete, lay the entire rod on the floor to cool. If your floor is not concrete like mine is, you may need to lay down some wooden slats to protect it from burn marks. As the rod cools, the pyrex puntis will pop off. Cut the cooled rod into 18" -20" sections. If you are left with thick ends, save them for future use, but remember that the tips of these pieces are contaminated with pyrex, which must be pinched off before using. Sound like a pain? It's not. In fact, pulling cane is great fun and I personally look forward to the task and often enlist the help of my nine year old son who eagerly helps out.
To conclude this introductory article, I would like to discuss some of the major differences you will encounter between hard glass and soft glass. First, you will probably discover the hard way that soft glass is more sensitive to thermal shock than hard glass. If you heat soft glass too suddenly, it will shatter. It must be heated gradually. The best way to do this is by pre-heating the glass to be worked in a pure gas flame. The temperature of a pure gas flame is around 800 degrees, very near the annealing temperature of 810- 825 degrees. This is not hot enough to shock the glass so; in most cases, a piece of soft glass can be plunged into a pure gas flame with little risk of shattering. The flame is also not hot enough to cause reduction, despite the obviously high content of gas in the flame. A pure gas flame is also used for re-heating a piece while working. If a piece you are working on gets too cool, a short soaking in a pure gas flame will re-heat the piece with minimal risk of cracking. I have often seen Fred Birkhill pre-heating, or re-heating a piece in a pure gas flame, then he will raise it to his face and feel the radiated heat to check for temperature. A pure gas flame is also used for annealing.
The second major difference you will notice is in viscosity. Soft glass gets much softer than hard glass, even runny like honey. This is both an advantage and a disadvantage. The advantage is that shrinking tubing, blending in decorations, re-shaping, and correcting wall thickness are much easier to accomplish since the glass flows so readily. The disadvantage is that very soft glass can be difficult to control. The softness varies directly according to how much heat is applied, so obviously, the lampworker must learn to adjust the heat application to control viscosity. This takes practice, but is not that difficult to learn.
The third major difference is in the time it takes the glass to "set," that is, change from molten to solid. Soft glass takes much longer to set than hard glass. Again, this is both an advantage and a disadvantage. The disadvantage is that the time between moves is increased slowing down the procedure. The advantage is that there is more time to make adjustments and correct mistakes, particularly when working tubing. It is easier to blow a symmetrical bubble in soft glass, because it can be blown slowly and in gradual steps, rather than all at once as with hard glass.
The fourth major difference is in price. Soft glass cost less than a third of what hard glass costs by weight and that is something I'm sure anyone can adjust to quite easily.
Adapting to these differences requires little more than practice and before long, will become second nature. If I can do it, anyone can. In the next article, I will discuss one of the best ways to learn about soft glass... by making beads!