Plaster mixture

Could you speak to the ratios, contents and mixture of the lime plaster please?

Hi Pacer,

Entire books have been written about lime plasters, but in a nutshell, lime is the binder in a lime plaster.  The plaster will also contain an aggregate--usually sand--and sometimes a fiber, which could be straw or a synthetic material like poly fibers for tensile strength.  Those are the contents.  

There are different kinds of lime suitable for plasters.  Type S (stands for "special," and it has been partially re-hydrated) lime, quick lime (lime that hasn't been partially re-hydrated), or NHL (natural hydraulic lime).  Much longer discussion in the book and in many other places both on lime and in other books about various types of lime suitable for plastering.  

The sand is usually sharp (angular shapes when viewed with a hand lens) and well graded, meaning that the sand has a range of particle sizes from  fine to course, depending on the application.  Course sand with particle sizes ranging up to 1/8" are great for scratch and brown coats, but may not be suitable for finish coats since that layer can't be thinner than the largest particles or you get lots of drag marks while plastering.  We usually want the sand particle size to be no more than 1/4 the plaster thickness, so thicker layers like scratch and brown coats have larger particles, while finish coats use finer sands.

The lime binder fills the voids between the sand particles, and the ratio for most plaster coats (scratch, brown, and finish) tends to be around 1:3 lime:sand.  For ever one part lime you'll need 3 parts sand by volume.  You can conduct a void ratio analysis on your sand to determine if a better ratio might not be 1:2.5 or 1:3.5, etc., also described in the book and other places.  And you can raise or lower the amount of lime in the mix.  Finish lime plasters sometimes have more a higher ration of lime in them.

How to mix depends on whether all the material ingredients are dry, or whether the lime is a putty, and what kind of mixing equipment you're using.  Some plasterers prefer to dry mix everything for optimal distribution of ingredients, then add water.  Others start with the water, add half the sand and binder, add more water, then the rest of the sand and binder.  When starting with lime putty you don't need as much water, so we add the sand until we reach the ratio, then add just enough water for a workable mix.

There's much, much more to this subject!

Great overview, thank you for the information! Have you had any experience with heat as in around a wood stove or rocket stove? Will the plaster take the heat without cracking?

Hi Pacer,

The book has a chapter that addresses fire in a straw bale wall, and I think that’s where your question is going, unless it was just about the specifics about whether the plaster would crack?

I recommend that when placing a stove near a plastered straw bale wall that you maintain the stove manufacturer's clearances, e.g. 12" or 18" (or whatever) from combustible surfaces.  
That includes the plaster on a straw bale wall!  

I replaced part of a plastered straw bale wall on a building that caught fire because the stove had been located only 6” from the wall surface, when clearly embossed on the cast iron stove were the words “maintain 30” clearance from combustible materials.”  

Whoever installed the stove either didn’t read this or assumed that an inch or two of earth plaster was noncombustible.  While it’s true that it takes an awful lot of heat to burn plaster, it’s also true that plaster transmits heat to the straw behind it, which in this case was only ten inches from the back of the stove. Ten years of wintertime heating conditioned the straw immediately behind the stove to eventually ignite.  I forget what the process is called—pyrolization maybe?  

Regardless, because straw bales are quite dense and don’t burn well, the fire smoldered, crept along the seams between the bales, filled the house with smoke, etc., but didn’t fully burst into open flame until the fire department arrived and pulled plaster off the wall, allowing oxygen in. Appendix 1 in the CASBA book is titled “Fire and Straw Bale Walls.”  Bob Theis was the principal contributor to that.  There isn’t a lot of information about fighting fires in straw bale buildings because there haven’t been that many.  In this case the fire department used a Class A foam retardant to suppress the flames—that minimized the water damage to adjacent bales.

If this explanation wondered a bit off topic, let me know.  Clay and lime plasters have specific thermal expansion characteristics that makes them well suited for protecting walls against weather elements.  These same plasters account for a straw bale wall assembly’s excellent fire ratings as well (two hours for a lime or cement-lime plaster, one hour for a clay plaster).  However, while plasters may not readily burn, they will transmit heat.  As the plasters begin to fail under intense heat, you can expect to see some cracking.  That might be an indication of whether the heat source is too close to the plaster.

I wish I had any experience with rocket stoves to add here.

Thanks, Jim I am primarily curious about the crack resistance of the mix in comparison to clay or cement plasters.

Clay, lime, and cement plasters have different thermal expansion properties, so they respond differently to heat. I don't have enough experience with extreme heat conditions--e.g. plaster proximity to a heat source like a wood stove or rocket stove--to advise which might perform best.  I'm sure there are resources where you can find more information, if not an on-point answer to your question.  I'd start with books and articles about rocket stoves.  Good luck Pacer!