||ENGLISH STONE FORUM|
> Issues > Geologists guide to English building
basis for the study of building stones
One very sound approach, is one which looks at use through time. In any community, the church, the alms-houses, the town hall, being the oldest local structures may reflect the good stone most readily available in the district in medieval times when transport costs would rule out most distant sources of stone. In this way, unique and somewhat unlikely stones such as Hertfordshire Puddingstone figure prominently in the churches of South Hertfordshire, with sharp limits to its extent corresponding closely to its occurrence sub-surface. The slaty ashes of the Lake District, or the slabby greywackes and slates of Central Wales, can be equally 'local' when we look at what would be called vernacular architecture in the countryside.
Possible sources grew with the expansion of the canals in the eighteenth century (Midlands’ rocks south to London via the Grand Union Canal), while the railway network of the nineteenth century had the effect of making Scottish and Cornish Granites liable to turn up in towns only at the opposite end of the country at a modest expense. For our present times, the world is literally open to the trade and architects are able to select from a wide range of colours and textures drawn from either Brazil, South Africa, or India whereas Scandinavia or Italy were ' exotic' but sixty years ago. Such histories may be traceable where you live.
|In quite a different way, building stones can be regarded as an open-air museum of geological structures and textures often discussed at length in textbooks are less easy to see in natural geological outcrop. In the weathered or polished surfaces of building stones, it is often possible to observe features such as sedimentary structures (graded-bedding, cross-bedding, slump structures, erosional surfaces, intraformational conglomerates and desiccation structures). In carbonate rocks, the contribution of organisms to the build-up of the limestone can be assessed through the recognition of shell banks, incipient reef associations, and the inter-relationship of different grades of lime muds. Tracks and trails left by sediment-eating organisms (trace fossils) are often best seen in the polished slabs of limestones (the ‘marbles’ of many foyers) where several metres of surface are available for study.|
|.||In the igneous rocks, we are looking for crystal growths, twinning in crystals, and relationships between early-formed minerals and what became the groundmass in the final cooling. For igneous rocks in particular, it is important to look at surfaces in different light and from oblique directions. In this way, it is possible to see ghostly outlines in recrystallised and altered minerals, which would not be seen at first in direct scrutiny. It is surprising how effective a damp cloth can be in heightening the clarity of textures, and if it isn't actually raining, it can be useful to carry a garden hand spray to wet dusty and dry surfaces. Equally useful equipment would be a reading glass-type magnifier, the larger surface area being more useful than the normal compact hand lens which geologists usually carry.|
with natural stone
Architects and their clients may seek to express the company image through their buildings, producing a tendency to go for striking designs as well as colour and textural contrasts in stone, which may not turn out to be sound choices in terms of lasting qualities. This brings us to a consideration of long recognised and relatively new problems as stone weathers in buildings.
Good building practice always insists that bedding or any strong banding within a stone should be handled so that such 'grain' will always be set horizontally within the building. Up-ended blocks, or odd blocks out of such consistent alignment, are always liable to suffer excessive weathering damage, wind or particularly rain selectively opening up planes of weakness. This failing is termed face bedding. Of equally harmful consequences can be the setting of stones of different qualities and character side-by-side. Interaction at its simplest could produce decay at contact surfaces between porous, absorptive stone (such as most sandstones) and dense, compact limestone with low absorption of moisture. The contrasts can promote the leaching of cementing mineral matter from the porous rock, as well as frosting of the saturated stone of the pair. Hard Portland Stone string-courses in buildings of softer limestone or sandstone often produce interaction effects, as do isolated blocks brought in to repair damage.
Equally damaging can be infill repairs using mortars or stone pastes which inevitably have different porosities and mineral character from the stone which they purport to repair. There is seldom a good colour match, and often materials which commence harmonious, grow less so with age. In all of this, it would seem better policy to patch and repair from stockpiles of mature stone perhaps recovered from demolished buildings built from the same stone, rather than fresh new-quarried blocks.
Weathering in stone can vary considerably from place to place in the country, and from point to point in any one building, being a complex response to aspect and microclimate of the site, and the quality of the atmosphere. Many stones with an excellent reputation in the countryside surrounding the quarry, failed dramatically in the polluted air of city centres when coal smoke produced an early form of acid rain. At its simplest, the attack weakens the cementing matter, allows deeper penetration of subsequent rain-washing, and results in a rain of loosened grains from the surface of the blocks. At ground level, capillary action may draw up into the stone, mineral salts from the soil, which may in turn promote further break down of the stone through its cement.
Most visible of all the weathering effects, however, are the sooty crusts which may form upon projecting surfaces and overhangs not directly washed by the rains Apart from Direct deposits of carbon, crusts of gypsum may form from a chemical reaction between oxidised pyrite and the calcium carbonate of limestones. Such crusts of gypsum can actually become protective against further erosion and penetrative weathering, unless the crusts blister and break to expose unweathered limestone at the broken surface. This brings us to the question of stone cleaning.
clean or not to clean?
In recent years, there has often seemed to have been an unnecessary association of darkness of tone in stone with pollution and potential damaging decay. This has promoted extensive schemes of stone cleaning which have made this a growth industry. Once one building has been 'cleaned', those adjacent become candidates for similar treatment of necessity, whether they need be or not. Old established stone firms possess the basic knowledge and skills to clean in appropriate ways which differ according to the stone and the age of the building, but such has been the pressure of work that many new companies have sprung up to meet the need, often without the fullest understanding of the problem or the varied techniques. Some results have been more damaging than any thought to have existed in the first instance, although it has to be said that agreed standards set up by the Trade have now brought some sense into procedures.
Quite simply, darkening in colour is often a natural weathering response in stone cut from the quarry bed and maturing in the exposure given it in buildings. For example, sandstones usually come from the quarry a pale corn-colour, only for he air to oxidise the iron cementing minerals to produce the darker tones. Coal Measure sandstones, amongst the best freestones available, become velvety black with time without that signifying decay or potential weakness. In spite of this, beautiful black buildings such as St George's Hall Liverpool, and the very heart of Victorian Newcastle have been sand-blasted into an anaemic yellow on the pretext that such treatment was ' necessary protection'.
Sand-blasting, bombarding a surface with dry sand under pressure, must be one of the sternest cleaning techniques, only just acceptable for sandstones deeply engrimed, defying milder treatments. It should never be employed for any limestones or softer cemented sandstones, simply because this could destroy the protective 'skin' which cut stone acquires when allowed to mature between extraction and placement in a building. For the softer stones, spraying with a haze of water, followed by bristle brushing are the kindest cleaning operations, with precautions taken that water does not penetrate weakness of the fabric to any serious extent:
It is worthwhile remembering that like you or I, buildings 'age' and in that process, change their appearance. It is difficult if not impossible to arrest the effects of time, or turn back the clock. If we look at as well-known a building as St Martin in the Fields church in Traflagar Square, when new, it was as white as Portland.
|..||1 | 2|