The refining-heat, like most other properties, varies with the carbon; the medium orange given is the proper heat for normal tool-steel of from about 90 to 110 carbon. Steel of 150 carbon will refine at about a dark orange, and steel of 50 to 60 carbon will require about a bright orange to refine it.
This range is small, but it must be observed and worked to if the best results are desired.
A color-blind person can never learn to harden steel properly.
In studying this phenomenon of refining, the conclusion was reached that it occurred at or immediately above the temperature that broke up the crystalline condition of cold steel and brought it fairly into the second, the plastic condition. Farther observation led to the conclusion that the coarser grain and greater hardness caused by higher heats were due to the gradual change from plastic toward granular condition that takes place as the heat increases. Later investigations have given no reason for changing these conclusions.
When the phenomenon of recalescence was observed and investigated by Osmond and others, different theories were advanced in explanation.
Langley concluded that if recalescence occurred at the change from a plastic to a crystalline condition, then the heat absorbed and again set free during such changes would account for the visible phenomenon of recalescence.
Again, if it should prove that recalescence occurred at the refining point, the conjunction of these phenomena would indicate strongly, first, that refining does occur at the point where this change of structure is complete in the reverse order, from crystalline to plastic; and second, the first being true, recalescence would be explained as stated, as indicating the inevitable absorption and emission of heat due to such a change.
Langley fitted up an electric apparatus for heating steel, in a box so placed that the light was practically uniform, that is, so that bright sunlight, or a cloudy sky, or passing clouds would not affect seriously the observation of heat-colors.
Pieces of steel were heated far above recalescence, up to bright lemon, and then allowed to cool slowly; in this way recalescence was shown clearly.
It was found to occur at the refining heat in every case, shifting for different carbons just as the refining heat shifts.