WHEATON Technical Data - page 8

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Technical Data, Glass
Glass Manufacturing Terminology
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.
The mixed raw materials used in manufacturing glass that have been blended and
proportionally mixed for delivery to the glass furnace.
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.
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.
Waste or broken glass. Clean cullet is always used in the batch.
Mass per unit volume measured in grams per cubic centimeter.
The wall thickness or the evenness of the glass distribution throughout the container.
To attack the glass surface with a strong chemical agent, usually hydro-fluoric acid.
Usually used in decorating glass.
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.
The shaping of hot glass.
The shaping of glass using air pressure.
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
Flame re-working of a blank or tubing cane, typically on a lathe.
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).
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.
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.
The degree of residual stress in annealed glass as measured using polarized light
The attack on glass surface by atmospheric elements.
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