I know this is a kinda late reply but if you plan to silicone a baseball to a piece of slate, I think I have a better idea. Drill a small hole in the slate and use a galvanized screw to hold the ball to the slate.
Tony that is what I am planning to do, this a work in progress set up. I dont want to just slap something together to say its done, I am going to take my time and make this a Show tank, one the kids will talk about till there 90 LOL
They are huge into baseball, so I want to make it special for them.