When you use "Hope of Cain" recipe to upgrade a rare item to a legendary, the game creates a random item of type and level that the rare item was. Hence, to get The Furnace, you need to be upgrading a Rare Two-handed Mace (you can either find one, or craft a Sovereign Persuader at the Blacksmith).
When the game chooses which legendary to drop, it uses weights that determine how rare or common each legendary is. For example, here are all the Legendary two-handed maces and their weights:
- Arthef's Spark of Life - 100
- Crushbane - 100
- Schaefer's Hammer - 100
- Skywarden - 100 - does not drop for Wizard
- Sledge of Athskeleng - 100
- The Furnace - 25
- Wrath of the Bone King - 50
Hence, every time you roll for a legendary 2H Mace as a Wizard, you have 25/475 = 5.26% chance to get The Furnace, 50/475 = 10.53% chance to get a Wrath of the Bone King, and 100/475 = 21.05% chance to get each of four other maces.
You can check here for a table of all weights and drop chances for 2.4.2. In further versions, searching for "diablo 3 legendary drop rates " should get you there.