Slip for straw-clay walls is made with ordinary earth, albeit high clay-content earth. We have ours delivered by a local landscape contractor, who shreds it into easy-to-use granules with a topsoil shredding machine.
For the finer finish coats of plasters we use commercially bagged clays and finer, lighter-colored sands. We used a mixing head with smaller slots (above) for these lighter, powdered materials and other admixture ingredients.
The HydroMax's speed controller (above, at left) allows fine control over the mixing process. It also adapts the 220 volt 3-phase motor to 120 volt single-phase current.