This matter would be considerably simplified if, instead of the colour, the degrees of temperature were specified; thus tempered to 460° F. (238° C.) would mean the same degree of hardness as a straw colour, and all the degrees of hardness above that would be specified in less degrees of temperature, and all degrees of softness down to a blue with a green tinge would be included in ranges of temperature up to 630° F. (332° C). The degrees of softness below that, being indistinguishable by either a colour test or any other known indication save the ease with which it can be cut, would still remain unknown quantities or degrees. This plan would remove another technical objection to the colour test, which is, that the presence of a colour obtained on a piece of steel by subjecting it to heat is no evidence that the steel possesses any above its normal degree of hardness; for steel, wrought - iron, or even cast - iron, that has been softened to the lowest degree, will assume, on a polished surface, all the colours, providing that they are heated to the necessary temperature; hence the presence of colour is simply proof that the steel has been heated to a certain temperature, but by no means proof that it possesses temper, or indeed that the process of heating has in any way modified the degree of hardness or softness.

Tempering, when performed by a second operation, reducing the hardness obtained by a previous one, is dependent for its uniformity upon the uniformity of the first one; hence if a number of pieces of steel of the same grade be heated to an equal temperature and plunged in water until cooled, and subsequently tempered to the same shade of colour, they will all possess an equal degree of hardness; but if other pieces of steel of a different quality or grade (this may be further specified by saying" containing a different percentage of carbon") be subjected to precisely the same processes, leaving upon them the same temper colour, while this latter batch will be uniform in hardness, it will not possess the same degree of hardness as the pieces of the first batch: hence temper colour may be used as proof of equality in the degree of temper in pieces of the same steel, but is not indicative of any determinate and uniform degree of hardness. In tool hardening, this fact assumes but little practical difference, because for tools a special quality of steel termed tool steel is supplied, which will harden sufficiently to give accuracy to the colour test tempering, when heated to any degree of heat from blood - red to yellow - red, the difference of hardness in steel quenched, from either of these degrees of heat, being too small to be of practical moment in all tools comparatively inexpensive to make.

In tools that are expensive, it is desirable to give the exact degree of temper which experiment has determined as the best. It will be noted that in the colour test the shades of yellow alone extend over a range of 70° F. of temperature, and tool users know that within these 70° F. lies a wide range of hardness. It is better, then, to adopt a tempering process that will determine with approximate accuracy the first heating temperature, such, for example, - as by heating the article in some flux such as melted lead, melted salt, or melted glass, plunging it into a cooling mixture or liquid whose temperature can be maintained, by suitable means, somewhat equable, and drawing the temper in a bath maintained at the required thermometrical temperature. By this means, the steel used being a brand known for its uniformity, both the hardening and the tempering will have the greatest practically obtainable degree of temper, and the tools will be hardened more answerably to the requirements of the duty than is obtainable under the colour test.

This plan is indeed largely resorted to when large numbers of pieces require tempering; but if the articles arc large, or tempering requires to be done piecemeal and at odd times, it will not pay, as a rule, to keep heating and tempering mixtures constantly ready upon the fire, and the open fire and colour gauge must be resorted to. It is under these latter conditions that the whole of the skill of the hardener and temperer is called into play, since, from the moment the steel is placed in the fire until it is cooled, all through to the temperature of the water, judgment and expertness are qualities absolutely essential to entire success.