8
Glass Oven
B-585
3 Function
3.1 Working with the glass ovens
The core of the glass oven comprises two borosilicate glass
tubes lying one within the other. The outer tube (1) protects
against damage to the heating tube, and at the same time
prevents contact with parts that are electrically live. A metal
screen (2) around the outer jacket provides additional protec-
tion. The transparent, electrically conductive semi-conductor
coating, does the actual heating. Temperatures up to 300°C
are attained, the oven temperature being monitored via an
integral temperature sensor (4). A lock nut (5) can be used
to tilt the oven and to secure it in a suitable position.
The basic equipment can be used for drying,
distilling and sublimating, if the appropriate ac-
cessories are used.
To avoid thermal damage to, or decomposition of, samples
requires gentle heating techniques. Conventionally, this has
been done by heating the sample via an intermediate medium
(e.g. water or oil) whose temperature is regulated just above
the required setpoint for the sample. In bulb-tube distillations,
a semi-conductor coating is used to provide direct, yet still
gentle, heating. The product may be heated directly because
the heat source provides evenly across the entire surface
of the heater. Because the thermal capacity and the thermal
conductivity of the vapour-coated glass are significantly bet-
ter than those of the gas/atmosphere outside, most of the
heat given off is taken up by the glass and radiated inwards.
There are further advantages in addition to the gentle heating:
Because 75% of the heat is supplied to the product through
radiation and only 25% via heated gas, the oven heats up
very quickly and also cools down relatively quickly when it is
switched off. In addition, because the semi-conductor is fully
transparent, the sample can be observed during the drying
process, so that any decomposition can be identified quickly
and any necessary countermeasures can be taken in time.
3 Function
