Clay plaster is the oldest wall covering. The Amish use it inside and out.
Mix
Clay (binder) + sand (filler) + chopped straw or horsehair (reinforcement). Proportions found by testing: mix, apply to a board, dry. Cracks — add sand. Crumbles — add clay.
Application
Wet the wall. First coat (rough): 15–20 mm with coarse straw. Press into wall by hand or trowel. Second coat (finish): 5–10 mm, fine sand, no straw. Smooth with wet sponge.
Properties
Breathes: absorbs humidity and releases it — natural humidity regulator. Warm: clay has high thermal mass. Non-toxic: zero chemicals. Repair: fill a crack with the same mix — invisible.
Exterior
For outside walls, add casein (milk protein) or linseed oil — improves water resistance. Or cover with lime whitewash.