Preparing ceramic samples for thermoluminescence dating according to methods used in the earth sciences (Zimmerman, 1971, and Fleming, 1970) allowed the author to approximate results obtained using archaeomagnetism. A sandstone kiln at Savignies (Beauvaisis, northern France) could not be dated directly by thermoluminescence because ground water had contaminated the walls. The magnetic separation method used to solve this problem is explained. Results are presented and discussed. -- AATA