This section is from the book "The Epicurean", by Charles Ranhofer. Also available from Amazon: The Epicurean, a Complete Treatise of Analytical and Practical Studies on the Culinary Art.
The reason sauces are reduced is to give them the delicacy and succulence necessary to their finish, by incorporating into them fumets, stocks and the required condiments. The stocks that are to be added to the sauces in order to reduce them should be as concentrated as possible so as to avoid any superfluous labor. In order to reduce a strained, despumated sauce, pour it into a flat saucepan having a thick bottom, set it on a brisk fire and let the sauce boil while stirring it well with a spatula, and pressing it down in a way that the sauce will fail to adhere to the bottom, and therefore burn. Mix in gradually the stock needed for its improvement, and continue to boil until it has acquired the necessary consistency and succulence, then take it off, and strain it through a tammy.
 
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