Since within certain limits and under certain conditions the capacity of a stamp battery depends on the number of blows given per minute and on the momentum of the fall, various contrivances have been suggested with a view to increase both of these.

In pneumatic stamps, such as the Husband and Phoenix stamps, a crank shaft raises the stem, which is attached to an air cylinder. The air in this is compressed, and the stamp head attached to a piston is thus raised. It is forced down to deliver the blow by similar means.

In spring stamps, such as the Elephant stamp, a crank shaft raises the stamps against the action of powerful springs, which increase the power of the blow. The rate of wear and cost of repairs have been found to be high in all these stamps, and they have never been much used.

Morison’s high-speed stamp is lifted like an ordinary pneumatic stamp, except that the compression cylinder contains water instead of air.

The ordinary form of steam stamp consists of a direct-acting vertical engine, having a steam cylinder and slide valve at the top, the piston-rod being rigidly connected with the stamp. Each stamp head works in a separate rectangular cast- iron mortar, with screens both at the front and back, and sometimes all round. The screens are of Russia sheet-iron with punched holes of about 1/6 to 1/10 inch in diameter, the steam stamp being best adapted for coarse crushing. The speed of working is from 90 to 100 blows per minute, and the out-put is from 100 tons to as much as 225 tons of ore per head in twenty- four hours. It is obvious that gold could not be economically saved on plates inside the mortar of one of these stamps, and, as a matter of fact, until recently they were only employed in coarsely crushing the copper ores of the Lake Superior region. Nevertheless, the curious fact seems to be well established that these stamps, with their heavy blow, do not make so much slimes as the ordinary gravitation stamp. They have been tried in crushing silver ores in Montana, and gold ores at the Homestake Mine, in the Black Hills, through a 30-mesh screen. As their capacity is so great their use is limited to cases in which large quantities of ore are available, one Ball steam stamp, such as is used in the Lake Superior district, being equal to at least fifty head of gravitation stamps. They may be useful in preparing ores for re-grinding.

The chief advantages of the steam stamp are economy of space and labour. The advantage of subdividing the work among a number of batteries is that stoppages for repairs and breakages affect only a small part of the crushing capacity at one time.