How did Georg Brandt identify cobalt as the source of blue color in ores?

In 1742, the Swedish chemist Georg Brandt successfully identified that the blue color found in certain ores—which had been used for centuries to pigment glass and ceramics—was caused by a previously unknown metal, which he named cobalt.12

Prior to his discovery, these materials were often referred to as kobold ores, which were mistakenly thought to contain copper but were actually poisonous arsenic-bearing minerals. By isolating the element, Brandt demonstrated that the characteristic blue hue was a property of the cobalt itself rather than the other elements present in the ore. This achievement marked the first time a metal unknown to the ancient world had been discovered and isolated.213