A classic meat stock should be as clear as possible, so that it can be made into soup broths and aspics that will be attractive to the eye. Many of the details of stock making have to do with removing impurities, especially the soluble cell proteins that coagulate into unsightly gray particles.
The bones and often meat as well (and skin, if any) are first washed thoroughly. To make a light stock, they are then put in a pot of cold water that is brought to the boil; they’re then removed from the pot and rinsed. This blanching step removes surface impurities and coagulates surface proteins on the bones and meat so that they won’t cloud the cooking liquid. To make a dark stock for brown sauces, the bones and meat are first roasted in a hot oven to produce color and a more intense roasted-meat flavor with the Maillard reactions between proteins and carbohydrates. This process also coagulates the surface proteins and makes blanching unnecessary.