This lag is peculiar in that it grows less the more slowly we heat or cool the steel, and, if the heating or cooling is done slowly enough, the lag disappears almost entirely, i.e., the pause of the pyrometer needle occurs at the same temperature on the upward as on the downward way. Conversely, the disagreement or split grows or widens the faster the temperature is raised or lowered. Here is the vital point. By extremely sudden cooling, such as quenching in water, the lag becomes so great that it never catches up at all and any structure with its consequent properties which was brought about in the steel by the higher temperature is thus frozen or fixed and made to “persist” after the steel has become cold. It is just at this point, the “point of recalescence,” that steel changes from its soft and malleable, to its extremely hard and brittle condition. If it is quenched from temperatures above this point, it is extremely hard, if from temperatures below it, even those only a little below, it is soft and ductile. It is from just a little above this point, then, usually between 1350° F., and 1500° F., that the blacksmith hardens his tools by plunging them into cold water. [Illustration: HEATING AND COOLING CURVES OF STEEL WITH .46 PER CENT OF CARBON ] Steels of Other Composition Now it should be noted particularly that the specimen with which we have been experimenting is a tool steel of .90% carbon or thereabouts. This is important, for, while all of the carbon steels show this same critical temperature, at which occurs the point of recalescence, those containing from .45% to about .85% carbon have another point somewhat higher on the temperature scale, and steels which contain from .10% to .45% of carbon have two others, or three points in all. Further, steels having less than .10% of carbon and iron with no carbon at all have the two upper points but no point at 1290° F. This lower one has disappeared. All of this means that if instead of a piece of .90% carbon steel we had used one having .60% of carbon, say, we would have found two different critical ranges or points at which the pyrometer paused, the one at 1290° F., and another when we got to 1360° F. Had the steel been one containing .30% carbon we would have discovered pauses at three different points, viz., at 1290° F., at 1395° F., and at 1480° F. With very low carbon steel or with wrought iron, the pyrometer would have registered two pauses, one at about 1395° F., and the other at 1650° F. When records are carefully kept of the time which is required for the temperature to rise or lower over each and every twenty-five degree period, say, on the upward and downward way, and these are “plotted,” what are called “heating” and “cooling” curves can be drawn through the stars and dots so set down and these form a record of the behavior of the pyrometer needle at each temperature along the scale. Two illustrations of such curves are shown.[9] Footnote 9: