How can I lower voltage coming in to a kiln?
A crude way to reduce the power would be to put a rectifier in series with the power. It would reduce the RMS voltage to 71%. For example, a 30A 600V such as the
STTH30R06W would be suitable and is very inexpensive. You would need to keep it away from the heat of the kiln and to add a suitable electrically insulated heat sink. 

If you want to do closed-loop control you would need a sensor capable of kiln temperatures (a heavy gauge CA/K type or a platinum-rhodium type such as R/S), and a controller with ramped setpoint capability, and a contactor capable of handling the heater current. You may also need to add a redundant over temperature cutoff for safety reasons, if one is not already present.