Baths & Alternatives
This is a prerequisite for a lot of laboratory work as it protects the work itself and decreases the possibility of thermal strain fracturing the glass; see service temperatures for more information on this.
A common method is to fill a bowl surrounding the flask with water, oil, sand or steam, or to use a wrap around heating mantle.
However, baths can be extremely dangerous if they spill, overheat or ignite, they have a high thermal inertia (and so take a long time to cool down) and mantles can be very expensive and are designed for specific flask volumes. There are two alternative methods that can be used instead, where appropriate.
When a heat source's minimum temperature is high, the glassware can be suspended slightly above the surface of the plate. This will not only reduce the ultimate temperature on the glass, it will slow down the rate of heat exchange and encourage more even heating; as there is no longer direct contact via a few points with the plate. Doing so works well for low boiling point operations.
If the glassware must be run at higher temperatures, a teepee setup can be used; so named as it looks a little like a tipi. This is when the glassware is suspended above the plate, but the flask is surrounded by a skirt of tinfoil. The skirt should start at the neck of the flask and drape down to the surface of the plate, not touching the sides of the flask. Having the base of the skirt cover the majority of the plates surface will effect better heat transfer. The flask will now be warmed indirectly by the hot air collecting under the skirt but, unlike simply suspending the glassware, it can now reach hundreds of degrees Celsius and is better protected from drafts.
Both these methods are useful as they are either cheaper or free, effective, safe and feature low thermal inertia transfer methods, meaning the chemist does not have to wait for a bath to cool down after use.
Baths are most useful when the heat source has little or no control over it. With the advent of variable temperature hotplates and wrap around mantles, their necessity has somewhat declined. The same can be said for many round bottom flask operations, which require the use of a bath.
Read more about this topic: Laboratory Glassware, Gentle & Even Heating
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