Failures of Lime Plaster Over a Clay Base?

There is a lot of contradictory information on the internet, here is an extract from an article written by Andrew Morrison from www.strawbale.com which discourages using lime over an earth base and yet historically this is how most of our earth buildings in the cape have been protected for centuries:

Chris says:
"As I mentioned in an earlier post, I’m building a small straw bale home in western Oregon. Since I’m largely inexperienced in straw bale methods, I’ve used the literature (with the help from a builder or two) to help inform my decisions on specific details, and for our upcoming bale plastering, my consultations have led me to seriously consider the application of an initial earth discovery coat with a lime finish on the external walls. This method is widely touted by just about every bale book I’ve gotten my hands on, as well as in articles in The Last Straw However, I’ve come across a few cases (mostly in the archives of this listserv) of plaster failures at the earth/lime interface. It seems like these have been the result of one or two factors: improper keying of the lime coat to the earth discovery coat, and differential swelling of the clay against the lime when the wall gets heavily saturated. All this thinking about external wall renders has led me to wonder why there’s this disconnect between what the books/articles recommend and the experiences of a few unfortunate folks. We live in a very wind-blown, cold and wet in the winter location. We decided on earthen covered by lime. Seemed ideal for a SB in our area. Earthen plaster doesn’t stick to chicken wire over wood."

Andrew Morrison says:
"I have seen and/or heard of a lot of failures of lime over clay plaster. I have been recommending, for years, that people not do it. There are different reasons for the failures, but they all come back to the fact that there are two different materials that simply don’t easily work well with each other. I suggest to people that they either do an earthen plaster through and through and through or they use a lime plaster. Don’t mix and match is my opinion. I am a big fan of Natural Hydraulic Lime plaster, although it is expensive.
To me, especially in wet areas, lime is the way to go for exterior plaster. It is durable, relatively easy and fun to work with, and time tested."

As per lime plastering over a base coat of earth, as with everything context is everything and there are many variables and so it is always difficult to compare apples with apples. My approach to straw bale building has evolved over 16 years of designing and hands on experience of building with straw and earth. My way of building with straw bale is worlds apart from the way Andrew Morrison builds with straw and the way people build in the States in general. Mine is more a fusion of cob and straw bale construction....my earth pre-coating method is more like building with giant bricks of cob than straw bale...and gives the walls much more solidity than typical straw bale that is plastered after the walls are all up. 

Traditionally in the Cape nearly all our old buildings are earthen based and to protect the earthen walls lime was used...and has proved very successful, except now where builders tend to wrongly repair with cement based plasters...

Earth is inexpensive and locally available on or near most building sites, whereas lime is not and is very tricky to work with.

While I do not dispute that lime is tricky stuff to work with and that it is inherently different to earth and so can be difficult to get to adhere to the earth I must point out a number of things.
a) There are many different kind of lime ...and we in South Africa have some of the best quality lime that there is....so one has no idea as to what kind of lime they are using in the states that they are talking about.

b) Inherently a straight lime sand plaster mix is the most tricky to work with, because its tendency is to dry out too quickly. This greatly weakens the strength of the plaster and inhibits its bonding to the substrate. This is why one often finds that lime plaster recipes add some clay to the mix. Whereas in the States or the article on Andrew Morrison’s article one can assume they are only using a straight lime sand plaster mix. By adding clay earth into the lime sand mix, one automatically gets a better chemical bond with an earthen substrate. Note one should always uses the same clay earth as one uses in the earth plaster, cob or adobe that one is plastering onto. Precisely because of the difficulty of getting lime to stick to earth and the trickiness of working with it, we recommend the lime, cow dung, clay and sand plaster mix, (see our info manuals), which is much more forgiving and easier to work with than normal lime sand plaster. The dung is also unique in its qualities to improve the weatherproofing and curing or the lime. Most of North American have a timber building heritage, and so unfortunately don't have the benefit of unique recipes that have been handed down from a previous age, when the tradition of building with earth and lime was still standard practice. In other countries like Mexico for instance, prickly pear cactus mucus is added to the mix, and in Laos buffalo hide and a jungle vine are added to improve the workability of the lime.   

I must agree that one must always properly prepare the earth walls with a good scratch coat to give the plaster something to key into. By using the pre coating / dipping process one ends up with well cobbled bales that are full of cobbing holes, providing an excellent key for the first coat of lime plaster.

I also agree that one can simply avoid the lime and stay with an all earth plaster....however if you go this route, one must have a good breathable coating over the earth plaster to protect it from rain and on the inside one either has to get a perfect mix to avoid dusting of the plaster or use a flour paste mixed with earth so as to stabilize it to avoid dusting. I have seen numerous old buildings where particularly on the inside and even on the exterior no lime plaster was used at all, simply a lime wash coating to protect the earth plaster that was renewed regularly. These days we have other more durable types of breathable coatings that can perform a similar function without the need to renew it all the time...lime Breathecoat paint etc.

Furthermore two things they mention that immediately raises alarm bells for me:

1) They talk about applying earth to chicken mesh...we never recommend that you do that...if one uses mesh, which is not a bad idea especially around openings, it is firstly a fibreglass reinforced mesh – not metal – and it is embedded into the fresh first coat of lime plaster.

2) They talk about a highly windy area...that is certainly asking for trouble. Plastering lime in a windy condition is a really bad idea…lime does not like to dry out too fast and the bond will almost immediately separate if you try and plaster a basic lime sand plaster onto an earth wall...due to differential shrinkage of the two materials...so I always advise not to do lime plastering in windy conditions...or cover up to protect from the wind. So the fact that they had issues does not surprise me in the least.