The Nature of the Flame-Flame Fruit
In his previous life, whenever Sephiroth thought of the Flame-Flame Fruit, he would instinctively associate it with the saying, "Not even dogs would eat the Flame-Flame Fruit"...
After all, Akainu had emphasized that the flames of the Flame-Flame Fruit were merely ordinary fire, while his magma could burn even flames themselves.
The Flame-Flame Fruit was an inferior fruit to the Magma Fruit...?
As a science student, when Sephiroth first heard those words, he felt as though his brain had been violently slammed in a door, leaving him completely dumbfounded!
Thus, even after Sephiroth fused the Flame-Flame Fruit into the Nether Flame Blade, he still refused to accept it and wanted to prove the Flame-Flame Fruit's worth.
So, in the laboratory, he used high-precision instruments to test the lower and upper temperature limits of the flames generated by the Nether Flame Blade under normal conditions. In the end, he obtained a fluctuating range of 84.5°C to 1000°C.
In the world of One Piece, naturally occurring magma ranged from 900°C to 1400°C...
Judging from the data alone, the Magma Fruit really did seem to be superior to the Flame-Flame Fruit. Its minimum temperature was practically the latter's maximum temperature... There was simply no comparison!
Looking at it that way, Ace's death in the original story did not seem quite so unjustified.
However, when Sephiroth mixed Armament Haki into the flames produced by the Flame-Flame Fruit, he was delighted to discover that the flames increased to varying degrees depending on how much Armament Haki the user added.
In fact, when Sephiroth ignored his own Haki consumption, the temperature of his flames had briefly broken through ten thousand degrees!
This test result greatly comforted Sephiroth. If the fruit was lacking, Haki could make up for it.
Teacher Kaido