Crucible steel is made by rolling a pure grade of wrought iron into flat bars. These bars are cut up and placed in piles in a crucible with layers of pounded charcoal between them. The piles are then subjected to a high temperature for several days. When withdrawn from the furnace the steel is found to have absorbed some of the carbon and to be hard and fusible. As its surface is covered with small bubbles, it is called blister steel. When the bars are heated with a flux, it is called shear steel, because this grade of steel is most suitable for making shears, scissors, etc.

When melted and run into ingots it is termed cast steel, which is the strongest form of steel. These ingots are broken up and melted in crucibles to be cast into any desired shape.

Crucible steel is used to a large extent for making machine-shop tools and is often spoken of as tool steel. The best grade usually contains fron .9 to 1.1% of carbon and has a wide range of uses. Other grades contain:

.1

to

.5%

carbon, used for

lathe tools and small drills.

.5

to

•75%

" " "

battering tools, hot work.

.75

to

1%

" " "

dies, axes, large-sized drills.

The properties of tool steel are due to the carbon they contain.