We used to Heat Treat some of their Fasteners at Hammond Heat Treatment, Darlaston.
Their furnaces were set up to continually Harden and Temper Fasteners to 8.8 Tensile Grade. After Tempering they were quenched into a Suds solution which gave them the characteristic Blue/Black finish. Sample were Tensile Tested as whole fasteners along with their hardness and metallurgical examination tests.
Hammond Heat Treatment used to treat the higher tensile grades ie 10.9 and 12.9 as these were less common and did not warrant Atlas to interrupt their continuous Grade 8.8 Heat Treatment
Most of these fasteners were used for Automotive Applications ie Rover, Aston Martin etc