# Hygroscopic (not hyDroscopic like you may think) A material's ability to absorb moisture from thin air.   When a hygroscopic substance is left out in the open - it will quietly suck moisture right out of the air.   A previously  dry powder may become moist and even wet if left unprotected.   Needless to say, any presence of  hygroscopic materials in our primer compounds must be avoided.  It can be difficult to troubleshoot as a member from Mobile, AL and Tempe, AZ using the exact same compound will report different results.   Unfortunately shinny materials (like lead hypophosphite) can appear wet. Sodium nitrate is a by product of "the lab" and is highly hygroscopic.   Fortunately it is  very soluble (dissolves easily) and this problem "goes away" during the rinse phase of the lab. The term (hygroscopic) in mentioned often when discussing  filaments for 3D printers.