This section is from the book "Studies of American Fungi: Mushrooms, Edible, Poisonous, Etc.", by George Francis Atkinson. Also available from Amazon: Studies of American Fungi: Mushrooms, Edible, Poisonous, Etc..
The flake spawn, or "flakes," is commonly known as the French spawn, because it is so extensively manufactured in France. It is made by breaking down beds through which the mycelium has run, and before the crop of mushrooms appears. That is, the bed is spawned in the ordinary way. When the mycelium has thoroughly permeated the bed, it is taken down and broken into irregular pieces, six to eight inches in diameter. Thus, the French spawn, where the beds are made entirely of horse manure, with no admixture of soil, consist merely of the fermented and cured manure, through which the mycelium has run, the material, of course, being thoroughly dried. This spawn may be removed one or several generations from the natural spawn.
French spawn, or " flakes," ready to plant.
The French growers depend on natural spawn much more than American growers do. The natural spawn is collected from old manure heaps. Beds made up in the ordinary way for the cultivation of mushrooms are planted with this. The mycelium is allowed to run until it has thoroughly permeated the manure. These beds are broken down and used to spawn the beds for the crop. In this case the crop would be grown from spawn only one generation removed from the virgin spawn. If a sufficient amount of natural spawn could not be obtained, to provide the amount required one generation old, it might be run through the second generation before being used. From the appearance of any spawn, of course, the purchaser cannot tell how many generations it is removed from the natural spawn. For this quality of the spawn one must depend upon the knowledge which we may have of the methods practiced by the different producers of spawn, if it is possible even to determine this.