What can I do if my kiln does not have a plug?
To understand thoroughly what is happening to your glass while firing, observation is key. This means that an observation port is an ideal feature of any kiln.
However, many kilns are made without ventilation or observation ports. This means that several possibilities need to be considered.
The easiest is simply to open the door or lid a small amount to make a brief observation. This means that you have to set up the piece to be fired in such a place it can be seen from a small opening of the door/lid. This brief opening of a small space will not normally cause any problem to the glass or kiln. At the higher temperatures, you need to take personal safety precautions against the heat and light from the kiln.
It is possible to be more radical and drill an observation port through the metal casing and brick or fibre lining of the kiln. This is then filled with a piece of fire brick or roll of fibre blanket. This is sufficient to insulate the heat from the external part of the kiln. This port should be about 50mm diameter to give a decent field of view.
A further refinement is to place a quartz viewing window in the hole you have drilled. This viewing piece will become very hot, but not visibly red. So, you must provide some insulating cover over the window.
But best of all, is to purchase a kiln with a viewing port in the first place.