- "An ancient furnace can be simply described as a singular place that gathers massive amounts of energy. Get the blue flame from Tumlea Heights, put it in the furnace, and then Cherry... I mean...the ancient oven will be fixed. However! At my age, it's not so easy to climb mountains and cross rivers. Hm. And my wife is jealous of Cherry, so she refuses to get the blue flame for me... *sigh*"
- — Robbie
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) and Travel Gates. One Ancient Furnace is located in Hateno Village near Zelkoa Pond and is used by Purah to power the Hateno Ancient Tech Lab. Another is located at Tumlea Heights 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.
In the past, both Purah and Robbie used the Blue Flame produced by the Ancient Furnaces to light their respective labs' furnaces, however Robbie eventually became too old and had to rely on his assistant and later wife Jerrin and son Granté, however his son eventually left home to see the world and Jerrin refused to relight the lab's furnace due to her jealousy of the Ancient Oven Cherry which was named after Robbie's first love. Purah however was able to keep lighting her furnace thanks to having regained her youth via the Anti-ageing Rune she developed, though was unable to after her body regressed to a six-year old and her assistant Symin proved unsuited to the task, forcing Purah to try to relight it herself but was spotted by one of the village children (implied to be Karin) and she was forced to remain in her lab as the presence of a Sheikah child in the village could cause the villagers to ask too many questions which could result in her condition being discovered.
Unlike Blue Fire in past games, the Blue Flame produced by Ancient Furnaces is depicted as a source of energy unlike Blue Fire which is a cold flame capable of melting Red Ice. It is unclear if they are the same type of flame or not, as the only similarity is their unique color that differentiates them from normal fire. Additionally Blue Flame is depicted as producing normal flames when used to light Campfires or Cooking Pots, though appears blue when used to light a Torch, wooden weapon, Blue Flame Lantern, or Arrow. It is also hot as opposed to a cold flame of Blue Fire as Link's temperature gets warmer when near Blue Flame in Breath of the Wild. It should be noted that in the real world blue-white flames are often produce by the hottest flames, though blue color flame may also be the result of a flame's chemical composition. As very hot flames can become hot enough to be considered plasma, it is possible that blue flame is the result of the Ancient Furnace converting energy into plasma which would make sense given the Sheikah's ability to create energy blade weapons and shields, which is similar to the blue flame and may be a more refined version of the blue flame energy. Like normal flame, Blue Flames can be extinguished by rainfall, though the Ancient Furnace and any Blue Flame Lanterns will remain lit even if it rains.
According to Robbie, an Ancient Furnace can be described as a singular place that gathers massive amounts of energy, implying that this energy is converted into the Blue Flame it produces which is powerful enough to power Ancient technology such as Guidance Stones and Travel Gates. However unlike the Furnaces, Ancient Furnaces apparently produce Blue Flame rather than being powered by it like the Furnaces that power the Ancient Tech Labs, as those smaller furnaces blue flame eventually burn out, which is unlike the Ancient Furnaces which according to a loading screen tip, have been burning for 10,000 years, which is presumably due to the Ancient Furnaces converting gathered energy into Blue Flame thus explaining why it has not burned out during all that time while the smaller furnaces simply use the flame retrieved from the Ancient Furnaces as a power source.