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From: (Bruce Hamilton)
Newsgroups: sci.chem
Subject: Re: Pyrex Glass
Date: Thu, 30 Apr 1998 08:44:50 GMT (TheMstrMnd) wrote:

>What is the upper temperature thresh hold that simple lab glassware
>is safe to work at?  What is its melting point?  Thanks

The reason Pyrex is used widely is because it withstands thermal shock
( rapid heating or cooling ) or thermal stress ( constant thermal
gradient ), due to a low coefficient of expansion ( 33 x 10^-7 per
degree C ), compared to previously used laboratory glasses like soda
lime ( 92 x 10^-7 per C ).

So the safe working temperature is defined, in part, by how you get to it,
and what other factors are important. If the glass is containing material,
then thermal gradients and other important parameters ( eg pressure,
surface scratches, thickness ), will limit the temperature.

In general, the maximum safe working temperature for most Pyrex laboratory
ware is taken to be the strain point ( 515C ), provided the object isn't
subjected to conditions or environments that would cause stresses or
surface damage, in which case any use at temperatures above 150C should
be evaluated for safety, and thermal shock considerations often limit
maximum temperature to 200-240C ( such as using on hotplates whilst
adding cold material ). It is usually assumed that Pyrex glass will
start to deform if held at temperatures around 600C.

The named points in Pyrex glass viscosity/temperature profile are:-
1. Strain Point ( 515C ) = viscosity of 10^13.5 Ns/m2 (10^14.5 poise)
2. Annealing Point ( 565C ) = viscosity of 10^12.4 Ns/m2 (10^13.4 poise)
3. Softening Point ( 820C ) = viscosity of 10^6.6 Ns/m2 (10^7.6 poise)
4. Working Point ( 1245C ) = viscosity of 10^3.0 Ns/m2 (10^4.0 poise)

The specified points are defined by the actual viscosity of the glass.
The thermal gradient used for annealing is performed after considering
the glass thickness, and the maximum temperature is usually 5C above
the annealing point. The working temperature is used when moulding
or blowing the glass.

For comparison, typical soda lime glass has a strain point of 495C,
an annealing point of 524C, and a softening point of 707C. The lower
stresses because of the lower expansion explain the improved thermal
performance of Pyrex, not the viscosity/temperature curve.

          Bruce Hamilton

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