Learning about substratum with the artist Kathryn Trumbell Fimreite:
Substratum: Latin: substrātum underlying layer, background; an underlying principle on which something is based; a basis, a foundation, a bedrock; a permanent underlying thing or essence in which properties inhere, more commonly called substance (philosophy); an underlying stratum, esp. that beneath the soil or other surface feature (geology); a lower class or stratum of society, esp. one which is characterized by poverty or criminality (sociology); a language spoken in a particular area at the time of the arrival of a new language, and which has had within that area a detectable influence on the elements or features of the new language (linguistics)
— OED