It appears to be a cast-iron steam train furnace with a bubbling cauldron of Elixir on top of it. It is also covered with various other props, including a ladder on the side of it, and a pile of firewood on the other.
The Furnace can spawn up to 12 Fire Spirits over its lifetime, which would cost 8 Elixir to deploy manually, creating a profit of 4 Elixir.
The Furnace can be used with other spawner buildings to deal high instant splash damage.
Mirroring them can spawn four spirits at a time. Using Golem and Wizard with the two furnaces is very deadly.
If too many Fire Spirits gather because of cards like the Mirror, Zap can give the opponent a vast Elixir profit.
It can act as a useful defense along with being a support building, thanks to its Fire Spirits which deal area damage.
The Furnace ironically is a good counter against spawner decks although it's a spawner card, due to the fact that the Fire Spirits spawned deal splash damage.
The Furnace is good at supporting counter attacks as it takes out many small troops, such as Minions, Spear Goblins, and Goblins, that opponents may play due to being low on Elixir from their attack. Prepared in advance, the player will not have to worry about cards like Minions, as the spawned Fire Spirits take care of them on their own.
If no other options are available, a Furnace can stop a push with low hitpoints troops since it will immediately spawn at least one Fire Spirit, wiping out a lot of the push and acting as a distraction.
Keep in mind that at Tournament Rules levels, one of the spawned Fire Spirits will be able to attack the tower. Due to the Furnace's 50 second lifetime, this will deal valuable chip damage to a tower.
Mini P.E.K.K.A. is a great pair with the Furnace's Fire Spirits, as the Fire Spirits take out any Goblins, or multiple troop card (excluding Skeleton Army), while Mini P.E.K.K.A.'s priority is the tower.
Poison is a powerful counter to the Furnace, as it will not only damage the spawner but will also most likely take out the Fire Spirits.
Since the Fire Spirits spawned from the Furnace explode on impact, Skeletons from the Tombstone can distract Fire Spirits easily and stop almost all the waves of Fire Spirits.
The Furnace is a great support card if placed behind a tank, eliminating nearly any type of cheap card that the enemy will place down to counter it. For example, a great combo is Hog Rider or Giant Skeleton with a Furnace. If the opponent spawns cards such as Minions or Goblins to either distract or destroy, the Fire Spirits will take them out, and if left unchecked behind a tank can decimate the tower with their powerful splash.
On 18/5/16, a Balance Update decreased the Furnace's elixir cost to 4 (from 5), its hitpoints by 14%, and its lifetime to 40 seconds (from 50 seconds).
On 21/6/16, a Balance Update increased the Furnace's lifetime to 50 sec (from 40 sec).
It is the only building unlocked at Arena 5 (Spell Valley).
It is the second spawner building to not take the appearance of a 'Hut', the first being Tombstone.
It seems that it has Elixir in the pot.
The pot on top of the Furnace resembles the Cauldron decoration from Clash of Clans. The pot on top of the Furnace also jumps into the air when spawning Fire Spirits, although an Ice Wizard or stunning spells can cause the pot to desynchronise with the Fire Spirits spawning.
The ladder and the stack of firewood's places on the sides on the Furnace are swapped around in game compared to the card picture, implying that the card picture is from the enemy view.
If 3 Fire Spirits cost 2 Elixir, the Furnace needs to spawn the Fire Spirits 4 times to be a profit over the actual Fire Spirits card.
It will periodically have a red shimmer. It can be used to predict when the next pair of Fire Spirits will spawn.
It is currently the only building that was not present during Clash Royale's soft launch.