Heating mantles are a relatively safe way to heat flammable organic liquids in a round bottomed flask (Figure 1.52). The mantles are cup-shaped and designed for different sizes of round bottomed flask (Figure 1.51a). If a mantle does not fit a round bottomed flask perfectly, sand can be added to ensure good thermal contact (Figure 1.51c).
The mantles should never be connected directly to the outlet, but first to a "Variac" (blue piece of equipment in Figure 1.51b) which then connects to the outlet and delivers variable voltage to the mantle. A Variac set to "100" would be equivalent to plugging the mantle directly into the wall \(\left( 100\% \right)\), while a setting of "50" means the delivered voltage is halved \(\left( 50\% \right)\). By controlling the delivered voltage, Variacs are used to regulate the temperature of a heating mantle. There is variation between devices, and settings must be experimented with to determine appropriate heating rates.
Heating mantles take some time to warm up (so may be pre-heated during setup of an apparatus), and also take some time to cool down. The mantle will remain warm even after turning off the Variac, and therefore flasks have to be removed from the mantle in order to cool (Figure 1.52c).
Safety note: the main hazard with heating mantles is that flammable organic liquids spilled on the surface of a hot mantle do have the possibility of ignition.