This section is from the book "Soyer's Standard Cookery", by Nicolas Soyer. Also available from Amazon: Soyer's Standard Cookery.
To make Four Quarts. - Take one pound of brown roux dissolved in a tall, thick saucepan with six quarts of brown stock. Put the saucepan on the fire, and stir the sauce with a spatula or a whisk until it begins to boil. Then remove the spatula, and put the saucepan on a corner of the fire, letting it lean slightly to one side with the help of a wedge, so that boiling may only take place at one point, and the scum be easily removed as it collects.
During despumation change saucepans twice or even three times, straining every time, and adding a quart of brown stock to replace what has evaporated. At length, when the sauce begins to get lighter, and about two hours before finally straining it, add two pounds of fresh tomatoes, roughly cut up, or an equivalent quantity of preserved tomatoes, and about one pound of Mirepoix. The sauce is then reduced so as to measure four quarts when strained, after which it is poured into a wide tureen, and must be kept in motion until quite cool lest a skin should form on its surface. Remember: -
1. Only to use strong, clear stock with a decided taste, and
2. Be very careful of the roux, however it may be made. By following these two rules, a clear, brilliant, and consistent Espagnole will always be obtained in a fairly short time.