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..
"Mill-track" spawn originated from the spawn found in covered roadways at mills or along tram-car tracks where horses were used. The accumulation of manure trodden down in these places and sometimes mixed with sawdust or earth, provided a congenial place for the growth of the mycelium. The spawn was likely introduced here through spores taken in with the food of the horse, or brought there from highways, if they were not already in the soil from mushrooms grown there. It would be then multiplied by the growth of the spawn, and from spores of mushrooms which might appear and ripen. The well tramped material in which the mycelium grew here, when broken up, formed convenient blocks of spawn for storage and transportation, and probably led to the manufacture of brick spawn.
The manufactured spawn, on the other hand, is that which is propagated artificially by the special preparation of the substratum or material in which the mycelium is to grow. This material is inoculated either with a piece of natural spawn, or with pieces of previously manufactured spawn. It is put upon the market in two different forms; the brick spawn, and the flake spawn. The latter is sometimes known as the French spawn, while the former, being largely manufactured in England, is sometimes spoken of as the English spawn.
The brick spawn is so called because the material in which the mycelium is present is in the form of bricks. These bricks are about 5 by 8 inches by 1 1/2 inches in thickness, and weigh about 1 1/4 pounds each when dried. The proportions of different kinds of material used in the manufacture of brick spawn probably vary with different manufacturers, since there is a difference in the size and texture of bricks from different sources. One method of making the brick spawn is as follows : Equal parts of horse dung, and cow dung, and loam soil are thoroughly mixed together to a consistency of mortar. This is pressed into the form of bricks and stood on edge to dry. When partly dry, a piece of spawn about an inch in diameter is pressed into one side of each brick. The bricks are then stood up again until thoroughly dried. They are then piled upon a layer of fresh horse manure about 8 inches deep, the pile of bricks being about 3 feet high. This pile is then covered over loosely with fresh horse manure, a sufficient amount to produce, when heating, a temperature of about 100° F. They are left in this condition until the mycelium or "fiber" has thoroughly permeated the bricks. The spawn is now completed, and the bricks are allowed to dry. In this condition they are put upon the market. The bricks made with a very high percentage of soil often have the appearance of dried soil, with a slight admixture of vegetable matter.
Brick spawn. Three " bricks," one marked to show into how many pieces one brick may be broken.
Brick spawn from other sources presents a very different texture and contains probably a much larger percentage of horse manure, or, at least, a much smaller percentage of soil. The appearance of the brick is not that of soil with a slight admixture of vegetable materials, but has much the appearance of a dried and compressed mixture of horse dung and cow dung, with an abundance of the "fiber" or mycelium, "the greyish moldy, or thready matter," which constitutes the vital part of the spawn. In the selection of spawn this is an important item, that is, the presence of an abundance of "fiber" or mycelium. It can be seen on the surface, usually showing an abundance of these whitish threads or sheets, or a distinct moldy appearance is presented. On breaking the brick the great abundance of the "fiber" or whitish mycelium is seen all through it. This indicates that the brick possesses a high percentage of the "fiber," an important part of the spawn.
One not accustomed to the quality of spawn can therefore judge to a certain extent by the appearance of the bricks as to the quality, at least they can judge as to the presence of an abundance or a scanty quantity of the "fiber." Since the spawn remains in good condition for several years, there is usually no danger in the use of spawn which may be one or two years old. But it does deteriorate to some extent with age, and young spawn is therefore to be preferred to old spawn, provided the other desirable qualities are equal. Those who attempt to cultivate mushrooms, and depend on commercial or manufactured spawn, should see to it that the spawn purchased possesses these desirable qualities of texture, and the presence of an abundance of the mycelium. That which appears devoid of an abundance of mycelium should be rejected, and good spawn should be called for. There is no more reason why a grower should accept a worthless spawn from his seedsman than that he should accept "addled" eggs from his grocer. In this business, that is, the manufacture and sale of spawn, poor material is apt to be thrown on the market just as in the case of seeds, poor material may find its way upon the market. Sometimes this occurs through unscrupulous dealers, at other times through their ignorance, or through their failure to know the quality of the product they are handling.
There are some brands of spawn, that is, those manufactured by certain houses, which rank very high among those who know the qualities and the value of good spawn. Some large growers send direct to the manufacturer for their spawn, and where it is to be obtained in large quantities this is a desirable thing to do, since the cost is much less. Where obtained from seedsmen in large quantities, the prices are much lower than where small quantities are purchased. One of these brands of spawn, the Barter spawn, is for sale by several different dealers, by Mr. H. E. Hicks, Kennett Square, Pa., by Henry F. Michell, 1018 Market street, Philadelphia, and by Henry Dreer, 724 Chestnut street, Philadelphia. Another brick spawn, known as "Watson Prolific," is for sale by George C. Watson, Juniper and Walnut streets, Philadelphia. James Vicks Sons, Rochester, N. Y., and Peter Henderson & Co., New York City, have their spawn manufactured expressly for their trade.
The Barter spawn is said to be made fresh every year, or every other year. Instead of the "continued culture" of spawn, that is, inoculating the bricks each succeeding year from the same line of spawn, which is, as it were, used over and over again, a return is made each year, or in the alternate years, to the natural or virgin spawn, which is obtained from old manure heaps. In this way, the Barter spawn* is within two to three, or four, generations of the natural spawn. The number of generations distant the brick is from the natural spawn, depends upon the number of times it may have been multiplied before it is inoculated into the bricks. That is, the natural spawn is probably first grown in large beds in order to multiply, to produce a sufficiently large quantity for the inoculation of the immense number of bricks to be manufactured. For it is likely that a sufficient amount of natural spawn could not be obtained to inoculate all the bricks manufactured in one year. If a sufficient amount of the natural or virgin spawn could be obtained to inoculate all the bricks of one year's manufacture, this would produce a spawn removed only one generation from that of natural spawn.
If the natural spawn were first grown in beds, and from here inoculated into bricks, this particular brick spawn would be removed two generations from the natural spawn. So the number of times that successive inoculations are made to multiply the spawn, the manufactured products are removed that many generations from the natural spawn. Where recourse is had to the natural, or virgin spawn only once in two years, the second year's product would then be further removed from the natural spawn than the first year's product. Where we know that it is removed but one or a few generations from the natural spawn, it is a more desirable kind. For the nearer it is to the natural spawn, other things being equal, the more vigorous the mycelium, and the finer will be the mushrooms produced.
* I have not learned the history of the other kinds of spawn referred to above.
The brick spawn is sometimes manufactured in this country by growers for their own use, but at present it is manufactured on such a large scale in England that little or no saving is effected by an attempt to manufacture one's own brick spawn in this country.