While in this country the Reaumur process of annealing is not followed, a “packing” of ore or scale is generally used. Some use an inert packing such as sand, and as first mentioned, some use no packing at all. Really, one of the main purposes of the “packing” as now used is the prevention of warping of the castings in the pots while annealing. The annealing temperature is not so high as in Europe nor is the annealing continued so long, but when packing is used for the shorter time only, some surface carbon is removed and the carbon throughout the center portions of the castings is precipitated in the coke-like form which is known as “temper carbon” to distinguish it from graphite which is shown in photomicrograph No. 35. To the eye, then, fractures of such castings show black centers and white rims. They are known as “black heart” castings and these form the bulk of the malleable cast iron made in this country. [Illustration: FRACTURE OF BLACK HEART IRON Note the white rims and black coke-like interiors. The majority of American malleable iron is of this “black heart” variety. ] We may say, then, that there are in general three varieties of malleable cast iron: the “all black” which is annealed without “packing,” the “black heart,” annealed in “packing” and the most common kind in this country, and in Europe, but very rarely here, the “whiteheart” from which practically all of the carbon has been burned during the “anneal.” [Illustration: MALLEABLE CAST IRON SWIVELS OF WHICH PARTS NO. 2 ARE CAST TIGHTLY AROUND NO. 1 AND LOOSENED ONLY UPON ANNEALING. ] Comparison of photomicrographs No. 35 and No. 30 given on page 181, will show at once one of the reasons for the much greater malleability of malleable cast iron. While the total carbon present is very nearly the same in the two irons, the difference in physical form causes great difference in the malleability of the two. In the gray cast iron, No. 30, the carbon is crystalline and in the form of long brittle flakes which cut through and separate the grains of iron. Thus “planes of cleavage” are formed which make the alloy unable to resist severe shock and cause it to be anything but malleable. It is not so with annealed malleable cast iron. Here the carbon is in the form of small pellets which are imbedded among the grains of pure iron, the malleability of which is not seriously impaired largely because of the continuity of the “pure iron” structure. A second reason for the ability of malleable cast iron to withstand shock is that in the burning out of the carbon of the outer portions of the casting very small cavities are left. These allow the surface to become considerably deformed and battered under successive shocks without great strain on the casting itself.