Finely divided or powdered charcoal or bone dust has been mostly used.
Huntsman Crucible Furnace—Original Type
One Type of Oil-Fired Crucible Furnace
Sheffield, England, steel makers, have been very successful in the manufacture of cementation steel. Their usual method is to pack flat strips of best Swedish Walloon iron in charcoal in rectangular stone boxes about four feet wide, three feet high and fourteen feet long. Alternate layers of small-sized charcoal and thin iron bars are piled in these boxes until they are filled, the bars not being allowed to touch one another. When full, top slabs are luted on to the boxes to make them airtight.
Fire is kindled in the firebox below and the heat gradually raised until furnace and boxes are cherry-red in color. This heat is maintained for seven to eleven or more days, depending upon the hardness desired, i.e., the amount of carbon they desire absorbed. The furnace is closed and allowed to cool slowly, which requires another seven or more days.
Upon unpacking the furnace the bars are found to be brittle and of a steely fracture instead of the soft malleable material which was put in. They have become high carbon steel.
Expert workmen are able to judge very closely the hardness of the steel by looking at the fracture and they sort the bars in this way, piling bars of similar hardness together.
