Glass Stress and Annealing
By Joe Walas, East Carolina University
Department of Chemistry Glassblowing Facility

Scientific Glassblowers fabricate simple to complex glass apparatus to be used under laboratory conditions - which can mean harsh chemical exposure, high and/or low pressures, and a host of other environments hostile to people and facilities. An awareness of product design is essential as well as the integrity of the glass structure itself. An element in product safety is the fabrication of stress or strain-free glass apparatus and systems. The following is a very basic introduction to glass stress and annealing.

Blowing glass in the fabrication of scientific glassware involves the use of a torch or burner. The flame is adjusted to varying degrees of sharpness, ranging from a pinpoint for precision work, to a large bushy flame used for heating and forming broad areas.

This process of heating, forming and cooling will introduce strain (often referred to as stress) into the glassware. Invisible to the naked eye, the strain never- the-less is present and is a potential point of failure in the glass apparatus unless relieved. The amount of strain present will be determined by a number of factors including the intensity and size of the torch flame, glass wall thickness and the complexity of the seal itself. The severity of the stresses may be enough to cause glass failure.....sometimes while the glass piece is under construction! Many glassblower's hand anneal the work during the fabrication process, with full furnace annealing prior to customer receipt. At no time should un-annealed glass apparatus be put into laboratory service.

An instrument used by glassblowers as an aid in detecting the presence of glass stress is known as a polariscope. Briefly, the use of two polarized filters held in varying orientation produces a very visible stress pattern when viewed through the instrument. An excellent and detailed explanation of glass stress, strain and polarisers is found in:

Manual of Scientific Glassblowing
i.S.B.N. 0 9518216 0 1
Chapter 11 - Stress and Strain in Glass
Published by
British Society of Scientific Glassblowers

The polariscope contains a light source in the base that houses one of the polarizing filters. The second filter is housed in an adjustable holder (smaller diameter window) on this particular unit. Polariscopes are available in many forms, including battery operated as shown directly above.

The two close-up photographs on the left are samples of a typical stress pattern produced when making a "T" seal. This seal is normally made by holding the torch flame stationary, blowing out a hole in the main glass body to match a second tube (hose connection in this example).

The stress pattern mimics this heat zone. Color and line intensity can be changed by rotating one of the polarized filters, so caution is advised when "reading" the viewer. Best to assume the view indicates only stress - not necessarily the degree or intensity of the stress.

Relieving stresses in the glass can be performed by hand annealing, which is a process of using a torch flame of diminishing intensity and size over time, slowly and evenly allowing the glass to return to room temperature. An alternative is to use a glass annealing furnace. The bell-type furnace shown is programmable to allow a pre- determined ramp to annealing temperatures and cool down rate.