Are Catholics to believe that the cleansing fire of purgatory is real physical fire?
There are only a few propositions that the Church has dogmatically defined concerning purgatory:
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)