As the picture above hints, substances like human fat or powdered mummy were once so common that hundreds or perhaps even thousands of antique ceramic jars purpose-built to contain them still exist in antique shops, museums and private collections. This is no secret, but it remains more or less the domain of specialists in early modern history and (judging by the reactions of friends and dinner guests I have broached the subject with!) appears to not be widely known to the general public.
One good popular resource on the subject is this May 2012 Smithsonian article by Maria Dolan, which quotes the authors of two recent academic works on the subject: Louise Noble’s Medicinal Cannibalism in Early Modern Literature and Culture and Richard Sugg’s Mummies, Cannibals and Vampires. As the Smithsonian Magazine article notes, it was a relatively common sight in early modern France and Germany to witness relatives of sick people collecting blood from recently executed criminals to use in medical preparations.
Read more at Res Obscura.