The Crucible of Flame

The Crucible of Flame is an Azerite essence obtained through the quests to unlock the Heart Forge.

Source

 * Rank 1 is learned from Petrified Ebony Scale, a reward from Harnessing the Power (quest line begins with Essential Empowerment).
 * Rank 2 is learned from Dreamglow Dragonscale, a reward from The Stuff Dreams Are Made Of (quest line begins with In Darkness, I Dream, available after reaching level 54 on your Heart of Azeroth).
 * Rank 3 is learned from Tempered Scale of the Scarlet Broodmother, a reward from In the Shadow of Crimson Wings (quest line begins with Investigating the Highlands, available after reaching level 60 on your Heart of Azeroth).
 * Rank 4 is learned from Charged Scale of the Blue Aspect, a reward from A Bolt from the Blue, which is available after reaching level 70 on your Heart of Azeroth.