What is "brown stock"

Brown stock ( French: Fond brun or Estouffade) is one of the basic stocks (fonds) in French cuisine. Auguste Escoffier gives a recipe in Le Guide culinaire which contains marrow bones, beef, poultry carcasses, carrots, turnips, leeks, celery, parsnips and onion and is simmered and skimmed for several hours producing a dark brown liquid which is the basis for many other sauces, soups and stews. It is the basis of espagnole sauce and demi-glace.