I have a suspition, that you have BaCl2 in your BaSO4.
Reprecipitation of BaCl2 crystals may hapen when BaCl2
solution is saturated and water has a low pH value !!!!!!!
Try to avoid any chloride in your BaSO4 sample.
Test washing water by treatment with AgNO3 solution.
Professor Dr. Stanislaw Halas
Mass Spectrometry Laboratory
Maria Curie - Sklodowska University
20-031 Lublin, POLAND
tel: (48 81) 537 6275 fax: (48 81) 533 3669