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Specifying stone

Introduction
The natural stone industry in the UK
Identifying natural stone
The problems of "Third World" stones
Specifying natural stone
Further reading

Introduction

This page is not intended to assist in the selection of suitable stone for a given purpose, but to help ensure that once an apparently suitable stone has been selected then that stone, or a true equivalent, is specified for the project.

Identifying natural stone is not straightforward. Some idea of the complexity can be gauged by this quotation from the Introduction to BS EN 12440:

"The International marketing of natural stone has introduced a great number of names to designate the different varieties of stone. Most of them are traditional names and usually reflect the typical colour and/or other natural features and the place of origin of the stone, although this is not always the case.
Sometimes, the name of a variety includes terms related to the geological classification of the rock (such as granite, marble, quarzite, etc) that may or may not coincide with the accurate petrological name of the rock. Other times the same name is used to denominate different stones or similar varieties are denominated with different names.
The objective of this standard is to unify the designation criteria of natural stone varieties, maintaining the traditional names and introducing terms related to its petrologic nature, typical colour and place of origin."

The natural stone industry in the UK

Although the Stone Federation [www.stone-federationgb.org.uk] lists its members under 12 categories there are, essentially, three main areas of the industry relevant to the specification of natural stone - Quarries, Suppliers/ Importers, and Installers/ Contractors - and it is important to understand the essential differences between them, particularly as many companies work in more than one area. The Bath Stone Group [www.bath-stone.co.uk], for example, operates in all three areas offering an integrated quarry-to-completed-building service.

Quarries

At the very simplest, a quarry company may do little more than extract the stone as large blocks, in whatever size is appropriate to the particular quarrying method. It is, however, increasingly common for quarries to carry out some initial "processing" by sawing the quarried blocks into more manageable sizes, either as blocks or slabs as appropriate to the most likely use of the stone.
Some quarries will have the equipment to cut the stone to its finished size(s) for direct supplier to the installers/contractors. This supply chain is common in areas where stone is still a traditional building material, with local tradesmen going direct to their local quarry for the supply of finished stone, e.g. the Cotswolds, Peak District, Yorkshire, etc. It is also a common supply chain for stones which have traditionally been exported outside their local area for specific uses, e.g. Kirkstone slate for use nationwide as paving, or for decorative features.
Some quarries are now installing equipment, or upgrading existing equipment, to cut stone to very fine limits, with the intention of supplying direct to installers/contractors rather than through third-party suppliers as may have been their practice in the past.

Suppliers/Importers

A rather difficult-to-define grouping covering companies of all sizes, from major importers of non-UK stones down to small-scale stone "factors" who may buy just one block of a particular stone and process it for limited use in the residential market. The major defining characteristic is that they will be able to supply a variety of stones, often across a number of petrographic types.
Suppliers/Importers may also process the stone, cutting it to the finished size, but this is becoming increasingly rare unless these companies are also either quarries or installers/contractors.
Specifiers should be aware that suppliers/importers will often attempt to conceal the true source of their supply by giving stones a supplier-specific marketing name. This is mainly for commercial reasons, to prevent specifiers sourcing the same stone at a cheaper price, but can be extremely confusing if several suppliers are offering the same, or essentially similar, stones under different names.
Specifiers should also be wary of the many companies who are both suppliers/importers and installers/contractors of natural stone. It is not unknown for tendering contractors to offer subtly different stones dependant on their own sources of supply and, in many cases, a particular stone may only be available from one company making competitive tendering of the supply-and-installation a meaningless exercise.

Installers/Contractors

Again a wide grouping, from the one-man-band local stonemason buying his stone from the local quarry up to major national and international companies offering a complete sourcing, design, and installation service. In principle, the more sophisticated designs - thin stone cladding, for example - will probably require a design expertise found only in the largest companies even if the actual installation is comparatively small scale.
Specifiers should be aware that companies offering integrated quarry-to-completed-building services may not appear to be financially competitive when installing stone from quarries other than their own. They may, however, have a higher level of design expertise than installation-only companies sourcing the selected stone at an apparently cheaper price.

Identifying natural stone

BS EN 12440 Natural stone - Denomination criteria

This extremely useful British Standard (available, for subscribers, on IHS Construction Information Service) consists of just two pages of introductory text followed by nearly 90 Annex pages of "... a non-exhaustive list of the majority of European natural stones ... a first attempt to list the stones produced in Europe ..."
Although the Annex is extremely useful for the identification of European stones, the denomination criteria in section 3 are equally useful in providing a framework for the identification of natural stone from any international source.

Criteria for identifying natural stone

Essential criteria:

Name of the natural stone (traditional name) : (BS EN 12440 clause 3.1) : The name of the natural stone under which it is marketed corresponding to a particular type of rock and with a specific place of origin. Geographical names not related with the actual place of origin of the stone and company names shall be avoided.
Petrological family : (BS EN 12440 clause 3.2) : Scientific name of the rock obtained by petrographic examination according to EN 12407 and 4.2 of prEN 12670:1997. If the rock is not included in the classifications of 4.2 of prEN 12670:1997, an appropiate term from clause 3 of prEN 12670:1997 shall be employed.
Colour : (BS EN 12440 clause 3.3) : The range of colour that a stone variety shows. A visual impression on one or more dry bulk samples observed under shadow natural light. It is noted that moisture, dust and other features affect the visual colour impression of the stone, thus whatever the observation conditions might be, these shall be reported.
Place of origin : (BS EN 12440 clause 3.4) : The location of the area or quarry shall be as precise as possible, including at least, the city or village, municipality or community, county, province or department and country.
Finish : (BS 8298 clause 1.3.6) : This clause defines nine finishes: axed; dolly pointed surface; filled; flame textured; honed; polished; riven; rubbed; tooled. These terms should be considered only as the starting point for any discussions about finish as there are a wide range of techniques to achieve a particular finish on a particular stone and the finished appearance may vary depending on the technique adopted.

Additional criteria:

Supplier: Identifying a supplier for a particular stone may be irrelevant if the stone can be sourced directly from a particular quarry (see "Place of origin" above). Identifying a supplier may be desirable, even if the quarry is known, where the supplier provides additional services such as guaranteeing continuity of supply to a given standard. For example, a supplier may hold large stocks of a particular stone with particular characteristics which would not be available from newly-quarried material from the same quarry.
Bed (or other specific identifier): Some types of stone are inherently inconsistent in appearance (colour, figuring, etc.) and quality. In these cases it may be necessary to visit the quarry, identify the desired characteristsics, and agree that only stone from a given bed is used. This is often necessary with Portland stone, particularly Roach bed, where the visual characteristics may be highly variable between different areas of the same quarry,

Examples of inadequate identification

Bath Stone: This is a meaningless term for a specification. "Bath Stone" is, petrographically, a limestone coming from one of a number of quarries in the area surrounding Bath. The traditional names Combe Down, Monks Park, Stoke Ground, and others, as listed in BS EN 12440, Annex A, clause A2.20, might all be considered as "Bath Stone".
Clipsham, Cotswold, Kerridge, Portland, Purbeck: These are traditional names for stones which are available from a number of quarries. Specifying the name without specifying the actual quarry is meaningless. The stone traditionally referred to as Guiting is also a "Cotswold" stone.
Portland: BS EN 12440, Annex A, clause A2.20 lists 17 combinations of name/colour/quarry. Specifying "Portland Stone" without referring to a specific combination of name/colour/quarry is meaningless.

The problems of "Third World" stones

Whereas the UK stone industry has traditionally sourced material from UK and European sources, there is now an increasing quantity of stone becoming available from "Third World" sources, with Brazil, China, and India the major source countries.
So far it has proved difficult, or impossible, to trace stones from these source countries back to their true "Place of origin", certainly with the precision now possible for most European stones. This is due to a variety of factors: the extended supply chain, with stone often passing through several "suppliers" between its original source and the UK stonework sub-contractor responsible for its installation; the lack of any definitive naming conventions equivalent to BS EN 12440; the inherent difficulties in working with sales-orientated Third World suppliers who may not appreciate the need to provide technical data, or evidence of an auditable supply chain.
This has led to a situation where the inexperienced specifier is now (December 2006) referring to "Chinese Granite" or "Indian Sandstone" in much the same way that "French Limestone" was referred to generically, and imprecisely, in the late 1990s.
It is therefore highly desirable to institute a proper test regime when considering the use of "Third World" stones, as this may be the only way of ensuring that the supplied stone is truly fit-for-its-intended-purpose. This will have obvious cost and time penalties. These will probably be acceptable on larger projects, where the saving in the cost of the stone more than offsets the cost of the testing, but the cost of a proper test regime may be punitive on smaller projects.
It is highly likely that the larger suppliers in the major source countries will "put their own house in order" over the next few years, much as those supplying French stones have already done, as they realise that this is necessary to secure the larger contracts in the UK, Europe, and North America.

Specifying natural stone

Natural stone may be specified in one of six CAWS/NBS Sections:

F20 Natural stone rubble walling
F21 Natural stone/ashlar walling/dressings
H51 Natural stone slab cladding/lining/features
M40 Stone/Concrete/Quarry/Ceramic tiling/mosaic
N10 General fixtures/furnishing/equipment
Q25 Slab/Brick/Sett/Cobble pavings

Each adopts a slightly different approach to specifying the actual stone to be used and some rationalisation may be desirable. The notes below relate to the current NBS Standard Version.

H51 General Guidance
By far the most comprehensive guidance within NBS and equally applicable to all the other sections in which natural stone might be specified.
H51/110 Cladding to ........
The "Stone" sub-clause lists all the Essential criteria above, with comprehensive guidance notes.
H51/325+326 Preliminary test information
Lists all the essential tests and can usefully be copied to other NBS sections where appropriate. The NBS clause Guidance links to General Guidance 6 on Testing.

M40 Stone/Concrete/Quarry/Ceramic tiling/mosaic

M40 General Guidance
General Guidance 1.4 provides adequate guidance, which can usefully be augmented by reading the H51 General Guidance.
M40/115 Natural stone tiling to ........
The "Stone" sub-clause lists all the Essential criteria above, with comprehensive guidance notes.
There is no M40 equivalent to clauses H51/325 and 326 and these clauses may usefully be copied to section M40 if appropriate.

N10 General fixtures/furnishing/equipment

Natural stone is referred to implicitly in several clauses, as a worktop material for kitchen units in clauses N10/150+155 for example, but there are no stone-specific guidance notes. The H51 General Guidance is useful background reading although testing is rarely appropriate for worktops.

Q25 Slab/Brick/Sett/Cobble pavings

Q25 General Guidance
General Guidance 3 covers stone slab paving. The H51 General Guidance is useful background reading.
Q25/200+205 Stone slab paving
These clauses, and their associated guidance notes, appear to have been written on the assumption that the stone will be one of the UK-sourced natural stones traditionally used for external pavings. These clauses may need augmenting, referring to section H51 as appropriate, if a non-UK stone is specified.
Q25/500+505 Stone sett paving
Setts are generally specified from a known UK source and naming the supplier will generally be adequate.
Q25/700+720 Stone cobble paving
Cobbles are generally specified from a known UK source and identifying the source will generally be adequate.