Here is a carved clay slab, and the textural mat that was made from it.
I roll out 1/2" thick slabs from LF06 clay, a nice low fire talc body that is smooth and grog free. When the slabs are quite stiff, I pattern them with a carving tool, and then bisque fire them.
An image may be taken directly from the bisqued slab by rolling conditioned clay - not sticky to the touch, but still flexible - onto the slab with even pressure from a rolling pin. This yields an impression that is just like the slab mat, and is very nice for the salt process.
It is actually much more convenient to put the slab mat through the slab roller, on top of the clay. The textured clay can be wrapped in plastic and used for drape molding or handbuilding when you are ready.
I make the mats from a 2-part latex material, Polygel 40, which I purchase at Stephenson Supply in Portland, Oregon. If people are interested, I will post instructions for working with the Polygel material.