Petrographic examination is a requirement of quality assurance for stone but is also used for forensic engineering and conservation. This article by Jeremy Ingham reviews the application of petrography to stone for construction. Jeremy is the author of a book entitled Geomaterials Under the Microscope* published last month (February) in which the petrogropher’s specialist knowledge of modern and historic building materials is revealed.

*Geomaterials Under the Microscope – a colour guide by Jeremy Ingham (ISBN: 978-1-84076-132-0) is a 192-page hardback with 365 colour photographs and 26 tables. It is £50 from Manson Publishing and can be ordered from their website: www.mansonpublishing.com

Petrography is the systematic description of rocks as they appear to the eye and in thin section through the microscope. Petrographic examination is an important part of the assessment of stone for construction, requiring the use of high quality microscopes employing a variety of observational techniques. It must be conducted by suitably qualified and experienced petrographers.

The Applied Petrography Group (APG) of the Geological Society maintains a list of petrographers with appropriate experience (www.appliedpetrographygroup.com).

The production of current petrographic examination data is required for CE marking of a number of construction materials, including stone.

Around the world, thousands of petrographic examinations are performed on building stone and roofing slate products each year. In the UK, the petrographic examination of building stone is carried out according to BS EN 12407 (British Standards Institution, 2000a). For roofing slate, BS EN 12326-2 (British Standards Institution, 2000b) is followed.

For both standards, a sample submitted to the laboratory is first examined in the ‘as received’ condition, both visually and using a low-power stereo-zoom microscope at magnifications of up to, say, x60.

The features observed in stone samples at this stage include colour, relative hardness, fabric, grain size, open and refilled cracks, pores and cavities and presence of macrofossils. Features of roofing slate samples observed at this stage include colour, thickness, grain, relict bedding, open and incipient cracks, joints and veins, nodules and inclusions, and surface discolouration.

The initial examination is used to determine the most appropriate location for thin sections to be taken for further, more detailed high-power microscopical examination.

Thin sections are prepared by gluing the rock sample to a glass slide and then grinding the rock down until it is thin enough for light to pass through (typically 0.03mm).

A glass cover slip is placed over the top of the ground rock section to improve the optical clarity for viewing through a microscope.

Thin sections of stone samples are prepared from slices cut across any bedding, rift or grain. Thin sections of roofing slate are typically prepared from slices representing three different orientations, two perpendicular to the cleavage (at right angles to each other) and one parallel to the cleavage.

The thin sections are examined using a high-power polarising microscope at magnifications of typically up to x600.

The polarising microscope uses filters inserted into the light path to enhance the optical properties of the minerals within the rock, enabling them to be identified more easily.

The high-power examination allows determination of the mineralogical composition, condition (for example the degree of weathering) and identification of features that could cause problems in service, depending on the final use of the stone.

Quality assurance

Natural stone and slate resources are inherently variable in character and it is common to find the good stone of a certain part of a quarry passing into less suitable material within a short distance. Consequently, the fact that stone from a particular source has performed well in the past is no guarantee that stone currently being produced is of the same quality.

The quality of stone is also influenced by the methods of extraction, processing and quality control of the producer, all of which might change over the years.

In addition, stone is frequently imported from new sources across the world, primarily to exploit the cost savings of producing stone in developing countries. Therefore there has never been a greater need for assurance of stone identity and for dependable prediction of durability.

Petrographic examination data are required for stone and slate products to comply with the Construction Products Directive (CPD) and achieve CE marking.

To ensure that a stone or slate product is fit for purpose, the manufacturer is required to undertake a programme of initial type testing to evaluate the composition, strength and potential durability of every new source.

Once it has been established that the product is suitable for the proposed construction application, further testing of the factory product is required at regular intervals for quality assurance.

For natural stone products, petrographic examination must be repeated at least every 10 years as part of factory production control. For roofing slates, petrographic examination should be performed as part of factory production control at least annually (or every 25,000 tonnes of processed product, if sooner).

Further testing is sometimes required by the manufacturer, the purchaser, or third parties to investigate non-conformities / complaints.

Although rare, it is not unheard of for the wrong stone or slate product to be supplied to construction projects. By accurately determining the mineralogical composition (and other features) of a stone, petrography can be used to definitively determine the identity of a particular batch of stone whose source is in doubt.

Forensic engineering

Petrography plays a crucial role in the investigation of the causes and significance of stone defects and failure of stone construction systems.

Flaws found within stones that may be observed microscopically include fractures, shakes, joints and stylolites that act as planes of weakness and may cause stone unit failure.

Petrography can and should be used in the assessment of fire damage to stone structures to determine the heating history and depths of damage.

It was used after the IRA bombs exploded in London to determine whether cracking of stone facades was actually caused by the blast or if it was a pre-existing feature.

Fluorescence microscopy is particularly useful for highlighting cracks. This involves impregnating the stone sample with epoxy resin containing a fluorescent dye that is viewed through the microscope using ultraviolet light.

Fluorescence microscopy is also useful for assessing the pore structure of stone, which plays an important part in determining its resistance to frost action and other weathering mechanisms.

The depth of damage caused by weathering is readily observed through the microscope and observations of the presence and types of secondary deposits (such as salts) help in the diagnosis of deterioration from salt crystallisation and pollution.

A problem particular to certain types of marble is non-reversible expansion caused by thermal cycling. In some thin cladding panels (around 30mm thickness) this has caused bowing or warping on buildings, which could potentially lead to panel / fixing failure. (See NSS November 2004 for a report from Tim Yates of BRE on the work of the EU TEAM project, of which he was part, investigating the bowing of thin marble cladding).

This type of failure is commonly visible 10-15 years after completion of the building and notable examples of where it occurred include the Amoco building in Chicago, Grand Arc de la Defence in Paris and the Finlandia building in Helsinki.

An example from the UK is shown in Case Study 1 (below), where panel deflections of up to 50mm were recorded.

In recent years there have also been cases of roofing slate failures owing to poor slate quality. Slates of low strength, some containing unsound materials (such as reactive pyrite), some turnng out not even to be slate, have all been unmasked using petrographic examination.

Conservation

Over time, exposure to natural weathering processes and in-service ‘wear and tear’ cause deterioration of stone buildings. Hence there is a requirement for conservation and restoration of historic buildings.

Petrographic examination is a useful technique for determining the historical source of materials such as building stone and the ingredients for the mortars used with it (see Case Study 2 below).

The most compatible building stone for repair and replacement is usually one sourced from the same quarry as the stone original used for the building, if it is available.

Historically, building stones were usually obtained from locally occurring sources, but as communications developed, stone started to be traded over considerable distances to suit particular aesthetic or durability criteria.

Many historic quarries are no longer working, in which case petrographical information is used to help locate an appropriate substitute stone.

An investigation to determine the source of stone on a particular building typically involves desk study of any available historic records and visual survey to determine the number of stone types present.

Small representative samples are taken of each stone type. Special consent (in addition to the owner’s permission) will be required before undertaking investigations of listed buildings. The visual characteristics and petrographic data are then compared with reference samples to determine the likely original source and to select appropriate replacement stone if necessary.

In addition to matching stone and the mortars used with it, petrography is also used to help determine the causes and extent of stone deterioration from mechanisms such as salt weathering, acid attack, frost action and biodeterioration.

Diagnosis of the cause of the deterioration enables selection of the correct conservation strategy to maintain durability of the existing stone. Petrographic examination is also useful for assessing the effects of cleaning stone surfaces and the effectiveness of consolidating treatments.

Conclusions

Building stone and roofing slate products being sold within the European Union require petrographic examination for initial type testing, which must be repeated at regular minimum intervals as part of factory production testing.

Hence, petrography is a fundamental part of the quality assurance procedures for both building stone and roofing slate production.

There are benefits of conducting petrographic examinations more frequently than the minimum interval requirement as it gives a good indication of the general stone quality in the absence of a full suite of tests.

Petrography can also be used to confirm the identity of a batch of stone by determining its ‘petrographic footprint’ to confirm that the stone delivered is actually the one that was specified. It has also proved to be invaluable when investigating the causes of stone defects and assessing extent of stone deterioration.

And as the definitive technique for stone and mortar matching, petrography is increasingly contributing to the conservation of historic buildings.

Case Study 1:

Determining the cause of the bowing of marble cladding panels

The white marble cladding to the exterior of a large commercial building exhibited bowing and warping of the panels, leading to concerns of potential panel and / or fixing failure.

A comprehensive condition survey was undertaken using industrial rope access, measuring the degree of bowing, sampling selected panels and testing the samples in the laboratory.

In recent years it has become known that certain marbles are susceptible to non-reversible expansion caused by thermal cycling.

Research has indicated that the problem seems to be closely related to the microstructure of the marble. Marble with irregular crystal boundaries (xenoblastic texture) seems to be less prone to warping than marble with crystals that are almost equal width in all directions (granoblastic texture).

Petrographic examination revealed that the marble of the cladding panels had granoblastic texture and thermal cycling tests in the laboratory confirmed its susceptibility to non-reversible expansion. As a result, the cladding was enclosed in a metal net and a programme of monitoring has been undertaken to ensure safety.

Case Study 2:

Matching masonry and mortars for conservation

An investigation was undertaken to identify the nature of the materials used in the masonry construction of a castle, to enable them to be matched for necessary restoration works.

The castle is a Scheduled Ancient Monument comprising six phases of construction, spanning from the 12^th century to late medieval times.

Small samples of masonry were taken from all the historic construction phases and examined petrographically. Stone samples from a nearby disused limestone quarry were also examined.

Petrographic examination found that the main stone type present in the building was from the nearby limestone quarry.

Examination of mortar samples indicated that the original masonry consisted of sand from the nearby beach bound by a matrix of non-hydraulic lime with mix proportions ranging from 1:1 to 1:3 (lime : sand).

The information provided by this petrographic examination is being used to inform decisions regarding the future conservation of the site.

Acknowledgements

The author gratefully acknowledges the contributions of Barry Hunt of IBIS Ltd and Manson Publishing for generously contributing photographs for inclusion in this article.

References

British Standards Institution. 2000a. Natural stone test methods – petrographic examination:

BS EN 12407. BSI, London.

British Standards Institution. 2000b. Slate and stone products for discontinuous roofing and cladding – Part 2: Methods of test: BS EN 12326-2. BSI, London.

Ingham, J P, 2010. Geomaterials under the microscope – a colour guide. Manson Publishing, London.