By this process, wrought-iron bars are converted into steel. Alternate layers of sifted wood charcoal and iron bars are placed in fire-brick basins or "pots."' the tops of which are made air tight with a layer of clay. This prevents burning the charcoal. The pots are then heated in a large furnace. Maximum heat is reached in about 48 hours, and this is continued for 8 to 12 days. The fire is then allowed to die out and the bars are removed when cold.

The high temperature has caused the iron to absorb the carbon in sufficient quantity to change it into steel.

The product thus obtained is called blister steel because the bars are covered with blisters as a result of the process. This steel is not yet ready for use, as the carbon is very unequally distributed in it, making some spots very hard while the interior remains soft.

To correct this lack of homogeneity the blister steel is further treated by one of the two following-named processes:

(1) Cut or broken into small pieces, melted in covered crucibles, and poured into ingots suitable for rolling or forging to the shape desired. This equalizes the distribution of carbon and any other elements in the steel, gets rid of the slag or cinder which formed a part of the wrought iron, and makes a very superior grade of steel. This is known as crucible steel.

(2) Cut into suitable lengths, piled, welded and rolled as in the case of muck bar. This process helps to average up the distribution of carbon in the steel, particularly when several times repeated, but it does not make the steel homogeneous, as when melted in the crucible, and each repetition of heating and welding adds to the expense of the product. This product is known as shear steel.

The cementation process has practically passed into history, except that its use continues in some place in Europe, particularly in Sheffield, England, where it has been long in vogue, and much skill has been acquired in using it. Steel is there made by this process from the purest wrought iron for a superior grade of cutlery.

The crucible part of the process, however, is still retained, as will be outlined later on, and is an important branch of steel making.