. Metallographic etching reagents: I, for copper. Fig. 2.—Structure of hot-rolled copper revealed by progressive etching with a 10 percent solution of ammonium persulphate. X500 c and d illustrate good practice for developing etching pits. Etching period: a, i minute; b, i minute,30 seconds; c, 2 minutes, 10 seconds; d, 3 minutes 646 Scientific Papers of the Bureau of Standards [Vol. 16. i;u^?r m:9m>.:%:T-^^

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