It is a device for extracting fats from bones in a high-pressure steam environment, which also renders them brittle enough to be easily ground into bone meal.
Research in the 1980s found that many bone meal preparations were contaminated with lead and other toxic metals, and is no longer recommended as a calcium source.
Stock-free growers do not keep domesticated animals and do not use animal products such as farmed animal manures or animal parts (bone meal, blood meal, fish meal).
This included leather, fats for soap, bone for bone meal and manufactured articles, fertilizers, glycerine, hair for brushes, and even an array of pharmaceuticals.
Blood meal, bone meal, and other animal by-products are permitted in certified organic production as soil amendments, though they can not be fed to organic livestock.