Glass

Annealing Point

The temperature at which internal stresses in glass are significantly reduced. In the annealing operation, glass is gradually cooled from above the annealing point temperature to below the strain point temperature. This slow cooling relieves residual thermal stresses that would develop if the glass were allowed to cool in an uncontrolled manner.Batch
The mixed raw materials used in manufacturing glass that have been blended and proportionally mixed for delivery to the glass furnace.

Blank

Usually refers to a glass parison that is formed during the first step of glass molding. The piece is then transferred to a lamp worker or glass blower for final shape configuration.

Blister

A gaseous inclusion or bubble in the glass.

Blow Mold

Usually a metal mold used to form a piece of glass from a hot gob.

Borosilicate Glass

A high silicate glass that has at least 5% boron oxide.

Contraction Coefficient

The fractional change in length of a piece of glass per degree change in temperature on cooling from the annealing point to ambient temperature.

Cullet

Waste or broken glass. Clean cullet is always used in the batch.

Density

Mass per unit volume measured in grams per cubic centimeter.

Distribution

The wall thickness or the evenness of the glass distribution throughout the container.

Etch

To attack the glass surface with a strong chemical agent, usually hydro-fluoric acid.
Usually used in decorating glass.

Finish

The part of a bottle which holds the stopper or closure. The area that has the threads (generally a shortened term for thread finish). The first part made on an automatic machine, but the last part (or finish) to be made when bottles were hand blown. On labware, may refer to an interchangeable ground joint.

Forming

The shaping of hot glass.

Glassblowing

The shaping of glass using air pressure.

Gob

A portion of hot glass that is delivered from the furnace for forming.

Hard Glass
A glass with a high softening point or high viscosity (usually borosilicate).Hot End
A manufacturing term for the area of a glass manufacturing plant where molten glass is processed.Lampworking

Flame re-working of a blank or tubing cane, typically on a lathe.

Lehr

A long belt-fed, tunnel-shaped oven used to heat glass to the annealing point and then slowly cool it to room temperature to remove any residual thermal stresses in the glass. Can also be a large oven where glass is manually loaded and unloaded (batch lehr).

Linear Coefficient of Expansion

The fractional change in length of a piece of glass per degree change in temperature. The coefficient of expansion generally indicates the thermal endurance of the glass. Glasses with a low linear coefficient of expansion can be subjected to greater rapid temperature changes with less chance of fracture than glasses with a high coefficient of expansion. (Generally, Type I glass has a lower COE than Type III).

Melt

The amount of glass that is melted at one time.

Mold Mark

The mark in the bottom of the container that denotes the manufacturer.

Pressed Glass

Glassware that is formed by pressing a gob between a mold and a plunger.

Soda-Lime (or Soft) Glass

A glass with a substantial portion of lime in the formula.

Softening Point

Temperature at which a thread or rod of glass rapidly deforms under its own weight.

Strain Point

The temperature at which thermal residual stresses become permanent upon cooling. Temperatures above the strain point will introduce permanent stresses that can cause or contribute to fracture. At temperatures below the strain point, the glass can be temporarily heated and cooled without introducing permanent stress. The strain point can be considered the maximum service temperature.

Tank

The furnace that melts the raw materials into molten glass. Temperatures in the tank vary depending on the glass type being melted, but are typically in excess of 1200°C.

Temper

The degree of residual stress in annealed glass as measured using polarized light techniques.

Weathering

The attack on glass surface by atmospheric elements.