Led by Europe, more tests are being devised all the time to try to evaluate stone. In this column Barry Hunt explains the tests and discusses what the results show… and what they don’t. This time he continues his review of petrography.

The vast subject of petrography and the many ways it can be used to help stone assessment and problems arising was introduced in last month’s column. As with all tests, there are pros and cons and this month I want to explain a little more about petrography methods and their limitations.
First I want to concentrate on the most popular procedure: the microscopical examination of geological thin-sections.
A thin-section of stone is 0.03mm (30microns) thick, about the same as the finest human hair.
Imagine cutting a slice of rock this thin – so thin that it is almost transparent. It has to be this thin to allow light to pass through it so all the crystals and structures within can be seen. This is what is called a geological thin-section – a technique invented almost 200years ago by William Nicol, a Scottish geologist and physicist.
Nicol invented a prism that bears his name. It converts ordinary light into polarized light and as the polarized light passes through different crystals it is altered uniquely by each type, so that the differences observed can be used to identify the crystals. After 200years the processes and equipment have changed little and we now have a wealth of understanding thanks to the Nicol Prism.
A thin-section is created by sticking a sample of stone that has been polished flat on one face on to a piece of glass. The opposite side of the stone is then cut away and the remainder milled until the required thickness is achieved. Making a thin-section properly should take one-to-two weeks, which is why the test is never quick to carry out.
Thin-sections are standardized at 30microns. Any thicker or thinner and the crystal properties are altered. But it is not uncommon for non-standard thin-sections to be made that cause the microscopist to misidentify minerals.
Thinner than 30microns and there is the risk of the thin-section falling apart and in many circumstances minerals might be plucked out. There might also be issues with very soft minerals falling apart, while some minerals are soluble in water and simply disappear unless oils are used for the various milling processes.
Impregnating resins can be used to counteract some of the difficulties of preparation. These can be a variety of colours but commonly fluorescent resins are employed that allow voids, pores, cracks, cleavages and other features to be better observed.
Other techniques involve applying chemicals to the completed thin-section to cause different minerals to react and stain.
There are techniques to allow different carbonates and feldspars to be told apart, when otherwise they can appear extremely similar and difficult to differentiate.
A thin-section is still a very small slice from a potentially very large natural stone resource, so it is difficult to consider taking just one thin-section as representative. Often it is useful to prepare several thin-sections from different grades of stone and produced in different orientations because of bedding and cleavage features.
Poor sampling could miss important features such as veining, or maybe the presence of asbestiform minerals. The list is almost endless – and ideally the samples should be selected from a resource by a geologist to try to ensure they are truly representative, but this rarely happens. Often problem features are only identified using petrography once they have caused a failure.
Visual examination with the unaided eye and microscopical examination using thin-sections will typically provide 99% of the story of a stone, but occasionally other techniques are necessary to confirm mineral types and elemental compositions.
Next time I will discuss scanning electron microscopy and X-ray diffraction, and when and why they should be used.

Barry Hunt is a Chartered Geologist and Chartered Surveyor who has spent 20 years investigating issues relating to natural stone and other construction materials. He now runs IBIS, an independent geomaterials consultancy undertaking commissions worldwide to provide consultancy, inspection and testing advice. Tel: 020 8518 8646
Email: info@ibis4u.co.uk
The advice offered in answer to readers’ questions is intended to provide helpful insights but should not be regarded as complete or definitive. Professional advice should always be sought in respect of each specific issue.