In a tall narrow stockpot, bring the bones and water to a boil. Skim the fat. Lower the heat and simmer gently, so that bubbles are barely breaking on the surface, for 12 hours, skimming the fat as necessary. Partially covering the pot with a lid or using an otoshi-buta will mean less evaporation.
Strain through a fine-mesh sieve. Skim the fat. Cool to roo