I've got a little curl here, you can see. (In this case I think it's partly because it's been on a slightly warped wooden board.) It's not significant. At this point it's unlikely I can reverse it, but I can try to prevent any further curling.
This happens because the top, which is exposed to air, dries faster than the bottom. And clay shrinks as it dries. At Indian Hills, we'd dry our tiles on an open grid rack (which was actually the grid off one of those square box fans). At WaterWorks, we've got plaster boards, which will suck moisture out.
Here I've got the tile on a plaster board with wads of clay weighing the corners down. I know it looks silly, but I've done it many times before and my tiles are mostly flat with at most a tiny wobble.
I'm a little concerned this one's going to end up a little wobblier than my others.