The hole is also a good place to stick a thermocouple.
The venting in ceramic work is for all the vapors and gasses formed when the pottery and glaze is fired. It is not needed with knife blades, but there is no real reason to plug it up, either.
For those who often come across a small ceramic kiln and say, "Gee,it's only $30. I wish I could use that for knives, but what can I do with it?".
Well, take the small hole in the top and make it 3" round. Take a piece of 3" schedule 80 stainless pipe and weld a cap on one end. Put this through the hole and close the top. Seal around the pipe with satanite. You now have a small salt pot. It is great for low temp jobs like salt quenching, marquenching, salt bluing,etc. Unless it is deep enough to get al but 3" of the whole salt tube in the kiln, it won't work for austenitizing, but the joy of a 400F quench is something to be experience ( and far more user friendly than a 1500F volcano). You can pull the blade out, straighten it with gloved hands, stick it back in, straighten some more, and when all is well, drop it into the slack bucket.
