The book of trades; or, Familiar descriptions of the most useful trades, manufactures, and arts practised in England : and the manner in which the workmen perform their various employments.
(undated, inscribed 1829)
The cutler., pp. 40-41
41 THE CUTLER. Tiix Cutler makes all those articles denominated edge tools, but more parti- cularly knives, forks, scissors, penknives, razors, and swords. The chief art in this business consists in softening hard- ened steel by the application of a heat not greater than that which was employed in hardening it. For this purpose it is gradually heated according to the tem- per required, and cooled again either gradually or rapidly, this making no dif- ference; after which the steel is found to be softened or tempered exactly inl proportion to the beat which it has un- dergone. While the steel is tempering, its surface displays a variety of colours, in proportion as it becomes more and more heated, which the workmen inge- niously take advantage of to ascertain the degree of temper. The first is a light - straw colour, which indicates the highest or hardest temper; after this a full yellow; then a brown, then a red- dish blue, a light blue, and lastly a full deep blue, passing into black; which being the other extremity of the series, denotes the lowest degree of temper in this metal.
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