Lehr (glassmaking)
Encyclopedia
A lehr is a temperature-controlled kiln
for annealing
objects made of glass
. The name derives from the German verb lehren meaning to teach and is cognate with the English lere also meaning to learn or acquire knowledge of (something).
Rapid cooling of molten glass generates an uneven temperature distribution in the body of the glass which results in mechanical stress
sufficient to cause cracking before the object has reached ambient temperature, or to result in susceptibility to cracking in later use, often resulting from thermal shock
. To prevent this, objects manufactured from molten glass are annealed by cooling gradually in a lehr from a temperature just below the solidification point of the glass. Anneal cooling rate depends on the thickness of the glass, and can range from several tens of degrees Celsius per hour for thin sections to a fraction of a degree Celsius per hour for thick slabs or castings.
In glass manufacture, a lehr is typically a long kiln with a temperature gradient
from end to end, through which newly-made glass objects such as glasses or vases are transported on a conveyor belt
. However, the same effect can be obtained in a small kiln by controlling the cooling rate with an electronic temperature controller.
Kiln
A kiln is a thermally insulated chamber, or oven, in which a controlled temperature regime is produced. Uses include the hardening, burning or drying of materials...
for annealing
Annealing (glass)
Annealing is a process of slowly cooling glass to relieve internal stresses after it was formed. The process may be carried out in a temperature-controlled kiln known as a Lehr. Glass which has not been annealed is liable to crack or shatter when subjected to a relatively small temperature change...
objects made of glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
. The name derives from the German verb lehren meaning to teach and is cognate with the English lere also meaning to learn or acquire knowledge of (something).
Rapid cooling of molten glass generates an uneven temperature distribution in the body of the glass which results in mechanical stress
Stress (physics)
In continuum mechanics, stress is a measure of the internal forces acting within a deformable body. Quantitatively, it is a measure of the average force per unit area of a surface within the body on which internal forces act. These internal forces are a reaction to external forces applied on the body...
sufficient to cause cracking before the object has reached ambient temperature, or to result in susceptibility to cracking in later use, often resulting from thermal shock
Thermal shock
Thermal shock is the name given to cracking as a result of rapid temperature change. Glass and ceramic objects are particularly vulnerable to this form of failure, due to their low toughness, low thermal conductivity, and high thermal expansion coefficients...
. To prevent this, objects manufactured from molten glass are annealed by cooling gradually in a lehr from a temperature just below the solidification point of the glass. Anneal cooling rate depends on the thickness of the glass, and can range from several tens of degrees Celsius per hour for thin sections to a fraction of a degree Celsius per hour for thick slabs or castings.
In glass manufacture, a lehr is typically a long kiln with a temperature gradient
Gradient
In vector calculus, the gradient of a scalar field is a vector field that points in the direction of the greatest rate of increase of the scalar field, and whose magnitude is the greatest rate of change....
from end to end, through which newly-made glass objects such as glasses or vases are transported on a conveyor belt
Conveyor belt
A conveyor belt consists of two or more pulleys, with a continuous loop of material - the conveyor belt - that rotates about them. One or both of the pulleys are powered, moving the belt and the material on the belt forward. The powered pulley is called the drive pulley while the unpowered pulley...
. However, the same effect can be obtained in a small kiln by controlling the cooling rate with an electronic temperature controller.