Glassware Terminology and Definitions

Anneal – the use of controlled cooling from a determined temperature to prevent or remove objectionable stresses in glassware.

Annealing point (AP) – the temperature corresponding to a rate of elongation of 0.0136 cm/min when measured according to ASTM Method C 336, Test for Annealing Point and Strain Point of Glass by Fiber Elongation. As prescribed by this test, a rate of cooling of approximately 4°C/min with a fiber of approximately 0.065 cm in diameter, and a suspended load of 1000 g. Numerically, the annealing point approximates log = 13.0 poises where, within a few minutes, internal stress is substantially relieved.

blister – a relatively large, gaseous bubble or inclusion in glass. Considered an imperfection.

blown glass – glassware formed through the use of air pressure, either by mouth or compressed air.

borosilicate glass – any silicate glass containing at least 5% boric oxide. Borosilicate glass has a low coefficient of thermal expansion and relatively high resistance to chemical attack. KIMAX® PYREX® and SCHOTT® are examples of trademarks for borosilicate glasses.

check – a crack in the surface of glass. Considered an imperfection.

chip – a small, missing fragment of glass from an otherwise regular surface. Considered an imperfection.

etch – to attack a glass surface with a strong chemical agent such as hydrofluoric acid. Commonly used for marking and decorating glass.

expansion coefficient – the average increase in length per unit length per °C change in temperature over the range of 0 to 300°C. The temperatures correspond to annealed glass, since the expansion coefficient is slightly affected by annealing.

finish – the neck design and opening size that accepts a closure on a bottle or vial.

fire-cracks – glassware cracks caused by localized temperature shock.

fire-polish – the application of fire to glassware in order to smooth, round or add gloss.

forming – shaping of hot glass.

fuse – to join glass parts by the application of heat from a flame.

glassblowing – shaping of hot glass using air pressure.

graduated glassware – glassware decorated with one or more lines (graduations) used for volumetric measuring purposes.

hard glass – refers to glasses with a high softening point, commonly borosilicate.

lampworking – the use of a gas flame to shape and form glass rod and tubing. Also known as flameworking and torchworking.

lehr – a long, tunnel-shaped oven through which a conveyor belt transports glass to be annealed.

mold-mark – a seam or mark in glassware that results from the mold in which the glass item was formed.

out-of-round – the asymmetry or lack of roundness in a glass item.

sealing – the joining of glass parts by the application of heat from a flame.

seed – a very small, gaseous bubble or inclusion in glass.

soda-lime glass – refers to glasses with a substantial portion of lime in the formulation, also known as soft glasses.

softening point (SP) – the temperature at which a uniform fiber, 0.55 to 0.75 mm in diameter and 235 mm in length, elongates under its own weight at a rate of 1 mm/min when the upper 100 mm of its length is heated in the manner prescribed in ASTM Method C338. Test for Softening Point of Glass at a rate of approximately 5°C/min. For glass of density near 2.5, this temperature corresponds to a viscosity of 107.6 poises.

stone – a crystalline contamination in glass. Considered an imperfection.

strain point (StP) – the temperature corresponding to a rate of elongation of 0.00043 cm/min as measured by ASTM Method C336. This test prescribes a linear extrapolation of the data obtained in the annealing point determination at the rate designated above. In a few hours, internal stress is substantially relieved.

stress – the existence of tension or compression within glassware. Usually caused by incomplete annealing, inhomogeneity or temperature gradient during manufacture. Can also be introduced by rough handling, bumping, and dropping of glassware.

thermal shock resistance – the relative ability to withstand heating and cooling without causing defect.

weathering – the effects of atmospheric elements on the surface of glass.

working range – the range of surface temperature in which glass is formed into ware in a specific process. The “upper end” refers to the temperature at which the glass is ready for working (generally corresponding to a viscosity of 103 to 104 poises), while the “lower end” refers to the temperature at which it is sufficiently viscous to hold its formed shape (generally corresponding to a viscosity greater than 106 poises). For comparative purposes, when no specific process is considered, the working range of glass is assumed to correspond to a viscosity range from 104 to 107.6 poises.