caput mortuum

From the Latin caput mortuum (literally “dead head”).

• IPA^(key): /ˈkæpət ˈmɔː(ɹ)tjʊəm/

caput mortuum (uncountable)

1. (alchemy) Worthless residue of sublimation or distillation.
2. Cardinal purple, a variety of haematite iron oxide pigment.
   • 2018, Kate Atkinson, Transcription, page 227:

     Were they working their way through the colours and now had reached the more abstruse layers of the spectrum? What would be next — caput mortuum, heliotrope? The colours of the day.

Literally, "dead head". Figurative use from alchemical literature.

caput mortuum n (genitive capitis mortuum); third declension

1. a dead man's head, a skull
2. worthless residue, useless substance
   • 1591 "Caput mortuum, ut est apud Paracelsum, sign. materias, quae sunt in fundo vasis chimici" (Antonio Ricciardo, Commentaria symbolica, p. 138)
3. Cardinal purple, a variety of haematite iron oxide pigment

Third-declension noun (neuter, imparisyllabic non-i-stem) with an indeclinable portion.

Unadapted borrowing from Latin caput mortuum.

caput mortuum c

1. (alchemy) caput mortuum; residue
   • 1771 June 15, “Lärda saker och rön. [Learned things and findings]”, in Stockholms vecko-blad‎, page 1:

     Caput mortuum ger och efter afbränningen något fast lut-salt.

     Caput mortuum gives and after burning some solid lye-salt.

   Synonym: nigredo

2. (colour) a brown-purple pigment, mainly of iron oxide

   Synonym: (dated) engelskt rött

3. (colour) mummy brown; a reddish-brown pigment, made from ground mummy and resin

   Synonyms: egyptiskt brun, mumia, mumiebrunt