I don't print carbon so I don't know what the heat will do to it and most folks I know that do print carbon do it on fixed out paper. I have flattened plenty of silver gelatin prints though. Try this with a scrap print or two: Warm up two sheets of mat board in the press. Put the print on a piece of mat board and cover it with a release sheet or parchment paper (bakeries use it or the grocery store sells it). cover that with the other sheet of mat board. heat the whole thing with light pressure for 2-3 minutes at medium heat. remove from the press, place on flat surface, cover with a heavy flat weight for 5-10 minutes. This is how I flatten silver prints and it may work for your process too.