The project I started just about 2 years ago has been finished. The outer, protective layer went on this weekend. It consisted of a 1:2 clay:sand mix and was probably the easiest part of the entire project. To give it a textured finish I lit a drying fire and waited a few hours then made imprints of my hands all over it. The photo below is before that final touch.
As this dries it will form small cracks. I’ve saved back some of the clay/sand mix to fill in those cracks. It may very well crack when it heats up too. It doesn’t hurt the oven any but is just a natural effect of the expansion that happens when it heats to over 600F.