Sorry to be dragging my feet on this subject. I have to confess to finding it a little tedious. For me, technical information is like lifeboat maintenance on the Titanic. No one really cares about it (myself included) until something goes wrong.
Then you’d like to know that the equipment doesn’t have holes in it.
Last week I went over basic compatibility testing as it was established at Bullseye and has been practiced in our field, with little or no variation, for the last 25 years. Whether that test is being performed within the factory or the artist’s studio, it is not – and never has been – a measurement of the so-called COE. What is measured is the strain that exists at the interface between the chip and the base glass.
WHAT STRESS?! By Bullseye’s factory standards the four chips on the far right of this bar are considered incompatible. Chips 1570 and 1572 are low relative to the base clear. Chips 1573 and 1580 are high relative to the base. Compare this bar with one in which many of the samples are very obviously out-of-compatible range
The bar above shows test chips fired against a standardized base clear glass and viewed between polarizing lenses.
Whether the stress is high or low and its magnitude can be determined by using a quarter wave plate and by knowing the stress optical coefficient (birefringence constant) of the glass, which allows one to then calculate the amount of the stress.
Rotating the analyzer until the halo of light is extinguished allows the operator to measure the amount of strain. This “angle of extinction” is directly related to the magnitude of the strain in the test glass due to the compatibility mismatch. (The optical retardation is what is actually measured.)
Bullseye accepts and labels as Tested Compatible glasses that show less than 2 degrees LOW and 4 degrees HIGH. It is important to note that these are degrees of retardation, NOT expansion points.
ARE THESE “INDUSTRY STANDARDS”?
What industry? When Bullseye established its in-house standards no other manufacturer was making a line of compatible glasses designed for kilnforming. The factory set its own standards based on its experience in working with artists and in-house studio practice.
Fat-headed as they were, the Bullseye guys were never so arrogant as to claim to be an industry “standard.”
Within the wider community of glass artists – referred to within the US as the “contemporary studio glass movement” – glassblowers had certainly set no standard. The standard was whether the work broke or not. Torchworkers were primarily using Italian rods but Moretti made no claims of being the industry “standard.”
For standards to provide value they need to be administered by an independent testing body. No such organization exists in the studio glass world at this time. Any standards are those that have been proven by time in the field.
It is the responsibility of manufacturers to make available to the user the standards to which they hold their own products. And to publish the testing method used to support those standards.
NEXT: Why the COE is not a true standard.