Weathering

Jess erosion

Products such as cut stone, metals, ceramics and glass will interact with the environment in which they are placed. The term ‘corrosion’ is applied to metals and is very definitely chemical or electro-chemical in origin. Window glass may be said to ‘weather’ but this does not seem to be a good way in which to describe the chemical and physical changes glass experiences when buried. ‘Deterioration’ is probably a better word to use under these circumstances. Stone is long-lasting but is not absolutely durable; it will erode or ‘weather’ when placed outdoors, although the rate at which this happens is subject to considerable variation. The two most important discriminating factors are the type of stone concerned and the environmental conditions to which it is exposed.

There will almost always be a chemical element to stone erosion since dissolved carbon dioxide will attack limestone, as will ‘acid rain’ containing oxides of sulphur and nitrogen. Acid rain is also destructive to those types of sandstone in which the quartz grains are cemented together with calcite. The presence of soluble salts will be destructive if they repeatedly dissolve and re-crystallize in rock pore spaces as the rock is wetted and then dries out. Sodium chloride will cause this effect which is naturally obvious on sea-coast buildings, and those lower courses of stone-work exposed to rock-salt during snow clearance. Many of the minerals forming igneous rocks, quartz being the exception, are not fully stable at surface temperatures and to some extent will even react with unpolluted water. Biological agents will aid erosion by a process of ‘biodeterioration’. Tree roots are commonly seen splitting boulders but, on a very much smaller scale, bacteria may be also be involved. Fungi are of course very important causes of the deterioration in organic materials such as wood and leather.

A good place to observe the weathering of stone is in a cemetery. With any luck the monumental masons will have used a variety of rocks: granite, basalt, sandstone and Portland limestone being common. In Suffolk recently we noticed that even cast iron can be used for inscribed tombstones. Naturally some grave markers will be more exposed to the elements than others and the products of weathering will be removed by wind and rain. Since the stones are usually dated they will give a general indication of the rate of erosion in that particular area. I must offer a note of caution since memorials may be added to a grave at a later date than the burials. A working hypothesis would be that the memorial is at least as old as the latest death date recorded. Generally it is rare to find a readable outdoor inscription older than 300 years. In general igneous rocks, like granites, are inert, hard, and impervious; they take many centuries to weather. On the other hand they may be more difficult to quarry or carve and will be expensive to obtain. Portland limestone is a good compromise in terms of expense, ease of working, availablity, and stability. It was for these reasons, presumably, that it was chosen for The Cenotaph in London and for hundreds of thousands of individual Great War grave markers.

In the UK today the action of water, both in liquid form and as ice, is responsible for most physical weathering but in arid conditions wind-driven particles of the hard mineral quartz can be very erosive. Sandstone and gritstone are the common stones in the Bradford area. The quartz (silica) granules, which are their main constituent, are stable under ordinary conditions, even in the presence of water. These particles are cemented together and this cementing mineral, calcite, limonite or quartz itself, will be more susceptible to attack than the quartz grains. Poorly cemented sandstone is said to be ‘friable’. Some types of sandstone (arkose) contain a relatively high percentage of pink feldspar which is more readily weathered than quartz.

Some sandstone is ‘massive’ without any obvious bedding planes. When cut into blocks repeated heating and cooling of the exposed surface will eventually cause sheets to erode off, a process known as exfoliation. Sandstone containing clay minerals has exceptionally thin bedding planes. These may not be obvious when the stone is first quarried but will be quite clear in time as the softer material is weathered. The grooves produced are called laminations and offer ‘targets’ for further damage from water during freeze-thaw cycles. If this change is obvious then the mason can place the laminations horizontally (‘normal bedding’) or either parallel to the joints or wall face. I believe ‘face bedding’ is considered to be the most damaging for the stone’s long term survival.

Bradford stone is commonly blackened by exposure to a once smoky and polluted atmosphere. The general view seems to be that cleaning this patina off can be aesthetically pleasing but may damage the stone by increasing its porosity to water. It is not surprising that stone facing the predominant wind direction is damaged more rapidly than those in the lee side of a building. Inevitably there will be weak spots in exposed stone faces and the small hollows that result will develop a micro-climate which is likely to encourage salt and water retention. The hollows will deepen leaving a ‘honey-comb’ appearance or leading eventually to the destruction of the central portion of the stone. This ‘alveolar weathering’ can produce some extraordinary appearances.

The weathering of cut stone is not simply a matter of geology since building techniques will make a significant contribution. Stone cracking may follow the corrosion of metal fixings. To prevent this wrought iron gate hinge pintles used to be sunk in lead. Poor water run-off arrangements may result in portions of a wall remaining permanently damp. A more knowledgeable friend has pointed out the importance of mortar. Modern Portland cement produces a hard and impervious pointing. Stone will increase and decrease in size as the external temperature changes, and damage can occur at the junction of a soft rock with a hard mortar. An impervious mortar will drive water to escape through the stone itself which will delay drying. Older lime mortars didn’t suffer from either of these disadvantages.

If you would like to see pictures of some very sick stones let me suggest:

http://www.stone.rwth-aachen.de/atlas.htm

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