Cut the washed bones into pieces approximately 3-4 inches (8-10 cm) long.
Place the bones in a stockpot and cover them with cold water. If blanching, bring the water to a boil, skimming off the scum that rises to the surface. Drain off the water and the impurities. Then add the 3 gallons (11 liters) of cold water and bring to a boil. Reduce to a simmer.
If not blanching the bones, bring the cold water to a boil. Reduce to a simmer and skim the scum that forms.
Add the mirepoix and sachet to the simmering stock.
Continue simmering and skimming the stock for 6 to 8 hours. (If only chicken bones are used, simmer for 5 to 6 hours.)