We have not found information on "sel blanc". In bot instances (52v and 159v), it looks much more like "sol blanc" to me.
Two sources identify sol blanc as a white, chalky earth from Saxony (see glossary doc). I feel this makes better sense to me in the two entries.
Question: change to "sol blanc" in TC and TCN and leave in Fr. in TL, with an ed note "a white, calcareous earth mined particularly in Saxony"?
@ps2270?
TT has written to MHS