This month Wayne Strattman, president of Strattman Design, Boston, MA, discusses the use of a polariscope to check for stresses and strains in neon glass work. Strattman teaches scientific glassblowing for both the Boston Center of Adult Education and the Cambridge Center for Adult Education. A former research engineer, Strattman is involved in glass, vacuum and electronic technology as a scientist and a neon artist.
I’ve observed through teaching both scientific and neon glass-blowing that the nature of internal stresses and strains is a difficult subject for students to truly comprehend and get a working feeling for. Most neon glassblowing schools teach the proper techniques of annealing, but rarely mention the subject except in passing. Indeed, watching a fellow glassblower teach a student to “anneal”, or to relieve the strain in a T-seal by passing the torch over it several times, sparked me to write this article.
Briefly put, internal stresses and the strain they introduce into the glass develop whenever glass is heated and cooled unevenly. Stress will often develop because the outside surface that is exposed to ambient air will cool quickly and set, while the internal surface is still hot. As the internal surface cools and shrinks, it is resisted by the hardened exterior. This push/pull of glass molecules on each other produces stress, sometimes on the order of thousands of pounds per square inch, which can lead to fracture if the glass piece is shocked mechanically or thermally.
Compared to other glass families, lead glass used in neon sign work is particularly susceptible to stress, because it has a high coefficient of expansion, meaning that it expands or contracts more than other glasses at a given temperature change. Laboratory ware, which is made of borosilicate glass—a “hard” glass—has only one-third the expansion rate that lead glass has and is therefore more forgiving than lead glass.
A simple instrument used in our scientific glassblowing classes has found its way into our neon classes to help students better understand glass strain. A polariscope, which is an easily constructed device, utilizes polarized light to detect stresses. (An excellent account of how to build a simple polariscope is in Creative Glassblowing, by James E. Hammesfar and Clair Strong, W.H. Freeman and Co., San Francisco.)
Simply put, the theory behind a polariscope is that glass has the ability to alter the plane of polarized light passing through it depending on the magnitude of its internal stresses.
Looking at a piece of worked glass in a polariscope shows bands of light and dark regions, wheras the unworked potions appear uniformly gray. While there is a whole field of study called photoelasticity, suffice it to say that the more bands one sees, the more stress there is within the glass and the more prone it is to breakage.
Observing bends, splices, T-seals, etc., through a polariscope will show many things that can improve one’s glassblowing technique. For example, a hot piece of glass laid on a table will show the area that contacted the table and cooled rapidly, introducing a strain. A simple splice may show strain lines a full diameter’s length or more away from the splice.
One may also learn something about stopcocks sold to the neon trade. The seal between the stopcock and the shop manifold may show strain lines, which form because some stopcocks supplied today are made of slightly different coefficients of expansion that does lead glass. This means that no matter how well one makes the seal, a strain will exist.
T-seals, or side tubulations such as the ones picture here, are areas of high stress due to the geometry of a T-seal. The T-seal pictured on the left shows the strain lines of an unannealed seal. The middle shows the same seal after it has been “flamed” several times according to the technique commonly taught, with little effect on the strain. The right hand seal is a truly annealed T-seal, in which the stress was relieved by passing the torch over it enough for the molecules to realign themselves.
These are but a few examples of how a simple polariscope can be a handy addition to any glass shop, both as an instrument for new glassblowers to learn proper techniques and for professionals to check difficult joints and bends for strains that may require later repairs.