Ancient Furnace

Ancient Furnace is an item in The Legend of Zelda: Breath of the Wild. They are high-tech furnaces created by the Sheikah 10,000 years before the events of Breath of the Wild. The furnaces produce Blue Flame which is used by Purah and Robbie to power their labs respective Guidance Stones (in Robbie's case he had modified his Guidance Stone into the Ancient Oven Cherry). One Ancient Furnace is located in Hateno Village and is used by Purah to power the Hateno Ancient Tech Lab. Another is located in Deep Akkala and is used by Robbie to power the Akkala Ancient Tech Lab and Cherry. Both furnaces play a key role in the quest Locked Mementos and side quest Robbie's Research.

According to a loading screen tip, they have been burning for 10,000 years.