What it shows:
Black body radiators in thermal equilibrium should emit the same spectrum of radiation, so inside a kiln at high temperature objects should appear the same color whatever their material.
How it works:
Place a piece of brick and an iron ball into a kiln (ours is a Blue M Electric Co. kiln with 25cm × 12cm × 10cm oven) that has a temperature range up to around 1000°C. Close the door and crank up the temperature to maximum. Depending on the type of kiln, it will take around 20 minutes to reach equilibrium (a good length of time for a lecture). As soon as the door is opened there will of course be differential cooling, so the effect is clear only for a couple of seconds, so have the audience ready and waiting.
Setting it up:
Set the kiln up with the oven door facing the audience. Turn it on to "High" — it takes about 2 hours to reach 1000˚C from room temperature, so plan accordingly. Use a color camera with zoom lens to get a close-up of the interior. Make sure the camera is correctly color-balanced. Before opening the oven, stop down the camera so as not to drown it. You should test this before class to know how far to stop down the lens (it obviously depends on the camera/lens combination you are using). Your don't have time to fiddle with the camera when performing the experiment — the objects cool down quickly once the door is opened and the effect is spoiled if you wait too long.
Scrounge a kiln from your ceramics department and load what you like in it (check melting points first). Make sure the door is suitable for easy access and viewing.