Is the cleansing fire of purgatory a real physical fire?

There are only a few propositions that the Church has dogmatically defined concerning purgatory:

• There is purification after death.
• This purification involves some kind of pain.
• Those being purified can be assisted by prayers, devotions, and Masses.

Other questions concerning purgatory (e.g., its duration, whether it is a place, whether it involves actively inflicted punishments, whether it involves material fire) are speculative.

According to Ludwig Ott:

The Latin Fathers, the schoolmen, and many theologians of modern times, in view of 1 Corinthians 3:15, assume a physical fire. However, the biblical foundation for this is inadequate. Out of consideration for the separated Greeks, who reject the notion of a purifying fire, the official declarations of the councils speak only of purifying punishments (poena purgatoriae), not of purifying fire. (Fundamentals of Catholic Dogma, TAN, 485)