After 29 years of working with weathering of cultural heritage, I have finally seen it “live”: How the force of ice can destroy plaster on stonework. It is more diverse than I though. A complex and fascinating world of beautiful forms of ice growth: Ice needles, ice whiskers, ice lenses. An entirely different thing than statements suggesting that the 9% expansion taking place when water freezes to ice breaks porous materials apart. Although supported by one case study only, what strikes me is the resemblance between ice growth and a more well-known weathering agent: Salt crystallisation – the growth of salt crystals destroying materials. My observations took place over the last few weeks, in Hyllestad in Western Norway, on the masonry of a copy of a historic lime kiln.
Salt crystallisation vs. frost weathering
There are hundreds of scientific studies on how soluble salts destroy masonry – stone, mortar, plaster, paint. It is rather easy to observe how different salts form and grow within the pores, along planes of weakness in the materials – breaking up, pulverising, lifting small pieces, larger flakes – simply destroying cultural heritage over time. And well-founded studies all note that the processes are complex, depending on a variety of factors, such as the nature of the materials, availability of the many types of salts existing, moisture, climatic conditions and so on and so forth.
But, most importantly, the complexity of salt crystallisation would never have been elucidated if it hadn’t been observed, looked at, described – the foundation of all science, as presented in one of the best studies on salt weathering ever, by Andreas Arnold and Konrad Zehnder.
Now, the problem with frost weathering is that it is very difficult to observe in “real-time”, live – on a historic building, on stonework and plaster. Like salt crystallisation, it doesn’t happen all the time. It even happens less frequently than salt crystallisation. It takes place in specific periods when conditions are “just right”. There is a need for susceptible, porous materials (materials that, in the long run, cannot withstand the forces of ice-formation), there is a need for enough water or moisture in some form, and there is a need for temperatures oscillating by the freezing point. Such conditions may be at hand only a few times over a normal winter at northern latitudes, or perhaps only once every ten years. Then you simply have to be there to observe what happens! On temperatures just slightly rising the next day, you won’t have a clue about what you missed over the night!
Difficult to observe, yet so normal!
Though the actual destructive forces of frost are difficult to observe, ice is a normal thing. We very well know it, also at historic buildings, as growing icicles hanging from cornices, or in the form of developing white frost when very cold masonry is suddenly exposed to mild and humid weather.
We can also easily observe the many beautiful forms of ice by taking a winter walk in the surroundings. For example white frost on cold metal surfaces, forming long and fine needles if conditions are “just right”. Or growing ice lenses and thick whiskers in the ground, exercising such pressure that the ground and even larger stones are lifted: The phenomenon is well-known among geomorphologists and is called frost heaving.
Ice growth on and within masonry
What we easily see in our surroundings also takes place, on a smaller and more subtle scale, at historic masonry. Ice, in a variety of forms, grow and exercise pressure. It breaks up mortar and lime wash. And the destructive forces are typically at work when ice starts to grow along existing planes of weakness. This is exactly what I was able to observe in two distinct periods in January 2018. At a copy of a medieval limekiln in Hyllestad, Norway.
A most typical plane of weakness is between plaster or limewash and stone. It is often difficult to get the mortar/wash to adhere properly, and minute fissures will inevitably develop. This is just the place where ice growth takes place. And the process is very similar to frost heaving in a soil profile, as the following images show.
The finer art of frost weathering
Frost is often considered as a more massive destroyer of materials and cultural heritage. But take a look at the picture below, and you will see the finer art of frost weathering: Minute “holes” in plaster.
Remarks on setting, climatic conditions and theory
As can be seen from the images, the term frost heaving in our case best describes how the action of frost lifts plaster and limewash covering stone. It is, probably, a phenomenon that is well known and has been observed by masons and other craftspeople over and over again. Craftspeople work at monuments over long periods of time, but they rarely write their observations down. I have searched the internet at length (English, German, Swedish, Norwegian), but not been able to find neither images nor descriptions similar to what is reported in this blog post.
In our case lime plaster and lime wash were prepared on an ad hoc basis using the hotmix technique in June 2017 in order to test freshly burnt lime while the kiln was still burning. The products, poorly mixed and made with random sand and clay in the surroundings, were then applied on the gneiss masonry of the lime kiln. All is described in a long photo story on the current website.
My observations took place from early January to early February 2018. From December 2017 on, the normally mild and humid winter weather I Hyllestad (West Norway) went “crazy” and there were ups and downs in terms of rainfall, snowfall, significant temperature drops and rises. Water necessary to form growing ice whiskers, ice needles and ice lenses were provided from a combination of direct rain wetting the masonry before temperature drops, small amounts of melting snow on the masonry and, presumably, humidity from the air (as when white frost forms on cold surfaces). Observations and weather conditions are described in more detail in this blog post (in Norwegian).
No attempt at relating the observations to current theories on frost weathering is made here. But it is obvious that the old, simplistic theory blaming the 9% expansion of water when it freezes, cannot explain the break-up of mortar and lime wash. Rather, theories from the field of geomorphology in cold environments, describing frost heaving and ice segregation need to be put at work. Such theories are described in three fine Wikipedia articles: