After pricing other options, I bought this cast iron Dutch oven for a crucible:
Stansport Non-Seasoned Cast Iron Dutch Oven, Flat Bottom The description was accurate except one detail: it was not only seasoned, there was a layer of grease in the bottom nearly 1/8 inch thick. It came with instructions for seasoning, which gave me some clues about unreasoning.
I put a little water in the bottom and heated it on the stove until the water was boiling, then turned off the heat. When it stopped boiling and the grease floated to the water's surface, I poured off the grease and water into an empty can, so as not to clog the drain. I heated more water in an electric coffee maker to avoid thermal stress, poured it into the crucible, and repeated the process until I stopped seeing grease.
Then I ran the crucible in the dishwasher on the pots and pans cycle, heated it on the stove to 220 degrees F to dry it, and put it in the oven at 400 degrees F for 2 hours.