In an effort to clarify some of the confusion surrounding past marketing literature distributed by Spectrum Glass Co., recent litigation, and the article New Knowledge recently posted on their System 96 website, we offer the following basic explanation of working range and its relationship to the terms "long" and "short" glass.
Simply stated, the working range of glass is that range of temperatures that corresponds to the point where glass just begins to soften up to the point where glass is too soft to control. The ASTM and the American Ceramics Society committees on Glass Definitions summarized the definition now widely used in today's glass industries. (see pp. 677-680 in The Handbook of Glass Manufacture by Tooley and pp. 72-74 in Technical Glasses by Volf).
WORKING RANGE: "the range of temperatures in which glass is formed into ware in a specific process. For comparison purposes, when no specific process is considered, the working range of glass is assumed to correspond to a viscosity range from the working point to the softening point. (4 to 7.6 Log10 Poise)"
A LONG glass will have a significantly longer temperature range from the working point to the softening point than a SHORT glass.
Since glass blowers are hand working glasses in this range they are able to readily distinguish a long glass from a short glass. High lead glasses such as Steuben crystal are considered long glasses by experienced glass blowers. Both Spectrum S96 and Bullseye clear glasses would be characterized as short glasses by these same experienced glass blowers.
Fig 1. shows the same data as represented on the System 96 New Knowledge graph. Clearly the temperature/viscosity curves of Bullseye and Spectrum are virtually identical. The Spectrum range is insignificantly wider than the Bullseye range. But if both curves are compared to a glass that is accepted to be a long glass (Steuben), it becomes readily apparent why neither Bullseye nor Spectrum qualify to be called long. Dr Pye of Alfred University concluded in his analysis of the two glasses it was "extremely doubtful that even experienced users of these glasses would be able to distinguish differences".
Fig 2. shows the viscosity/temperature range where most fusing is done: the range between tack fuse and full fuse. Within this range Bullseye is actually the longer of the two glasses —but again, by an insignificant amount— not an amount that would cause us to claim that Bullseye is longer than System 96.
Clear also from the smooth curves on all these graphs is, of course, the fact that none of these glasses is "stepped melt". That would be contradictory to the very essence of the material.
ANNEALING POINT: The temperature corresponding to a viscosity of 13.0 Log10 Poise. It is the temperature at which internal stress in a glass is safely relieved within 15 minutes and is determined by the ASTM C336 test for annealing point.
SOFTENING POINT: The temperature corresponding to the viscosity of 7.6 Log10 Poise. It practically corresponds to the temperature at which tubes can be bent in the flame of a lamp-blowing burner. It would most closely represent the lower temperature of the fusing range.
WORKING POINT: The temperature corresponding to the viscosity of 4 Log10 Poise. At this point the glass is sufficiently soft for the shaping (blowing, pressing) in a glass forming process.
FLOW POINT: The temperature corresponding to the viscosity of 5 Log10 Poise. At this point glass begins to flow freely if unrestrained. It would most closely represent the upper temperature of the fusing range.
VISCOSITY: A fluid's resistance to flow, measured in units called Poises. The higher the Poise number, the stiffer or more resistant to flow is the material.
POISE: The unit of measurement of viscosity of a fluid. It is a unit of force per area per second expressed in units of dynes/centimeter/second.