Service Temperatures
Borosilicate glass, which makes up the majority of lab glass, may fracture if rapidly heated or cooled through a 150 °C (302 °F) temperature gradient. This is particularly true of large volume flasks, that can take hours to safely warm up. Gentle thermal cycling should be used when working with volumes more than hundreds of mLs to two liters. Whenever working with borosilicate glass, it is advisable to avoid sharp transitions between temperatures when the heating and cooling elements have a high thermal inertia. Glassware can be wrapped with tinfoil or insulated with wool to smooth out temperature gradients.
500 °C (932 °F) is the maximum service temperature for borosilicate glass as, at 510 °C (950 °F), thermal strain begins to appear in the structures. Operation at this temperature should be avoided and only intermittent. Bear in mind that glassware under vacuum will also have around one atmosphere of pressure on its surface before heating and so will be more likely to fracture as temperature transitions increase. Vacuum operation should be used if the atmospheric temperatures required are above a few hundred degrees Celsius, as this often has a dramatic effect on boiling points; significantly lowering them.
Borosilicate anneals at 560 °C (1,040 °F), this removes built in strain in the glass.
At 820 °C (1,510 °F), borosilicate glass softens and is likely to deform. And at 1,215 °C (2,219 °F) it becomes workable.
Quartz glass is far more resilient to thermal shock and can be operated continuously at 1,000 °C (1,830 °F). Thermal strain appears at 1,120 °C (2,050 °F), annealing occurs at 1,215 °C (2,219 °F) and it becomes workable at 1,685 °C (3,065 °F).
It is common for students and those new to working with glassware to set hotplates to a high value initially to rapidly warm a solution or solid. This is not only bad practice, as it can scorch the contents, it will almost universally burst large flasks, and this is one of the reasons why large flasks are often heated in water, oil, sand and steam baths or using a mantle that surrounds most, or all, of the flask.
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