If your ivory is thick, use a rip fence on your bandsaw and take the bulk off the back side & use it on a small folder, Then take some of the blue masking tape & put it on the bark side of the ivory, squirt some hot glue on your magnetic chuck & while hot & workable put your ivory bark & tape side down doing your best to get the inside or top side as level as you can, let it set up. Like someone said earlier use good sharp belts, I use 36 grit, and start taking it down to your desired thickness. When down pretty close to the thickness you want, put on a sharp 100 grit belt and go down just enough to take your 36 grit marks out and you're done. I do this all the time and get great results. If the dome or curve of your bark side leans more to one side than the other make adjustments while hot glue is still workable to make up for it. Using the surfacing machine works great for me. One wood and micarta that I have to cut down I use double sided tape instead of hot glue, Also double sided tape for titanium on the surfacing machine. Works like a charm.
Dwayne