Lime Plaster Finish

Started Lime plastering one my adobe buildings over the earthen/clay plaster a couple days ago. The mix design is 1 part slaked Lime, 1 part clay (fine sieved) and 10 parts fine sand. The base plaster was applied 2 years ago and has become “furry” from wind driven rain, which provides good adhesion for the final finish lime plaster. The mix design for the base plaster is 4 parts water, 1 part prickly pear cactus juice, 1 part “green tea” (fresh horse manure soaked in 5 gal. of water), 4 parts aggregate (3/8″ sieved), 6 parts clay, and 36 parts chopped straw. I swear it does not smell. Some folks hand mix the stuff but I use gas powered mortar mixer.

If shrinkage cracks appear right after you apply it, there is too much clay and not enough sand. Adjust accordingly. Spray or mist with water the lime plaster as it cures the day you apply. Try to avoid application with a wall directly exposed to the sun, particularly to western evening sun.

The clay in a finish lime plaster provides a natural coloring agent. If you have access to natural purple or even green clay then you have a great opportunity to enhance a wall with a unique color.

I can understand how this old world plaster mix has all but disappeared. It is very labor intensive preparing all the ingredients and is physically demanding in it’s application. There are many variations of earthen plaster being used for finishing adobe buildings but the main point is the adhesion from earthen to earthen that cannot be achieved by dissimilar Portland cement based plasters. The dissimilar material cement vs adobe creates a cleaved condition that invites capillary flow of moisture which migrate up and to the side.

The National Parks Service now recommends for adobe buildings using “mud plasters”in lieu of cement plaster. See Preservation Brief 5.

A word of caution: the lime plaster is a very long term curing process so early in the cure it can be vulnerable to impact damage as I experienced with wind driven hail.