Double Boiler is historically related to Oil.
henley-formulas seats orphan double-boiler beside oil as a first craft peer — object speech, not a text-container or tradition dump.
from Gardner Dexter Hiscox (ed.) · Henley's Twentieth Century Formulas, Recipes and Processes, 1914 (Gutenberg #53143; PD in the USA)
historically related to · sourced · documented