They carefully avoided discussing it in their meetings. They do not think it their province to enter into the political arena. Finally in 1891, Pope Leo XIII spoke. He did not speak of slavery by name. However, he took a position that should have made it clear.
Slavery is incompatible with universal and fundamental human rights.
He wrote that human labor is personal, since the active force inherent in the person cannot be the property of anyone other than the person who exerts it, and it was given to him in the first place by nature for his own benefit. The erroneous doctrine so firmly held and promulgated by the Roman magisterium for so many centuries was implicitly corrected by the Roman magisterium in 1891. They still taught the morality of slavery down to the middle of the present century. Zalba wrote justifying slavery as late as 1958. The common Catholic teaching on slavery was not officially corrected until Vatican II in 1965.