We report here the results of a study of the substrate quality of boron nitride powder samples subjected to different cleaning treatments. Argon vapor pressure isotherms were used to determine the resulting quality. We found that washing the BN powder in methanol prior to heating it under vacuum to 900 degrees C is an effective treatment for producing high-quality substrates. On the other hand, we found that vacuum heat treatment to 900 degrees C, alone, is not always sufficient to eliminate all the impurities present in the BN powder. We have also used cycles of adsorption followed by desorption measurements to investigate the occurrence of capillary condensation for Ar on BN. We found hysteresis loops characteristic for this phenomenon.