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Blowing - The Trident Seal Test

The trident seal test is a simple glass-to-glass seal used to measure the expansion differential between different glasses. Henry Hagy invented the trident seal test in the mid 1970’s when he was a senior Corning engineering glass scientist. He showed that any residual stress remaining after annealing can be very accurately assessed using a trident seal and a polarimeter. This methodology has become Corning’s standard in-house method for measuring a thermal contraction mismatch between glasses. The trident seal test has now superseded the old butt seals and/or sandwich seals, which are much more labour intensive and not as accurate.

Gaffer Glass also employs the trident seal test on all its glass melts, to ensure compatibility to within ±0.5 x 10-7 with all its blowing colour range and its master clear glasses. The clear master glasses for glass blowers are manufactured by Philips and are offered as pelletised Gaffer Batch – Code #3300 or #4433.

The clear master for our casting range is manufactured in-house at Gaffer Glass Code # 210.

All our casting colours are contraction compatible to less than a coefficient of less than 0.5 x 10-7 tolerance.

The trident seal test is much more sensitive than comparing dilatometric data, because it takes care of different annealing viscosities, set point estimations, surface tensions, and Young’s moduli. Poisson strain corrections are not required. 

By looking through the central cane with a polarimeter after being carefully annealed, an accurate estimation can be made of the degree of tension or compression being brought to bear on the centre cane by the two outer canes.

Gaffer Glass UK Ltd offers a free trident seal service, measuring your individual clear recipe with compatibility with our colour range. Send us your test samples to our distribution facility in Portsmouth. We require a 2-3 mm diameter cane or rod, not out of round or tapered, and about 150mm (6”) in length to perform this test accurately. Outside of the limits of the trident seal test, (which is around 4 LEC points), a very rough approximation can be made with thread and ring tests. It is helpful to send us your recipe, so we can theoretically compute properties prior to testing. These in-house computer program results are also offered as a free service. Properties theoretically computed give viscosity and contraction outcomes.

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