Alex-- My recipe used for the floor was silica sand (70 grit), red
art clay, wheat paste, horse manure, borax. I also used an aliz painted on top to get a smoother finish which had the same materials, but sifted very fine
straw instead of horse manure. After drying I finished it with five applications of linseed oil. After the linseed oil I topped with a coat of pure tung oil and a coat of Heritage Natural Finishes floor liquid wax. I highly recommend going to
http://www.heritagenaturalfinishes.com/ and checking out their products. If you have the cash to spend I would recommend getting all of your oil from them as they are really great and much better than the smelly boiled linseed oil I stooped to for the cost (also drop Cob Cottage Company's name and you might get a discount price).
If you are interested in floor I really recommend "Earthen Floors: A Modern Approach to an Ancient Practice" by Sukita Crimmel who helps run Claylin Earthen Floors out of Oregon. They even ship pre-made earthen floor mix right to your doorstep!
That slate floor looks great, I used some flagstone in my bathroom. Instead of building the floor around them which I thought might be a pain I just squished them into the earthen floor mix while it was wet and made sure they were level.