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    Towards an assessment of the intrinsic value of building stones
    The English Heritage pilot study into the historic use of building stones and the need for particular stones for repair and conservation has raised the issue of how to determine the relative value of individual stones. This value derives from a number of intrinsic factors which will each provide a level of importance against which the stone could be assessed. Aggregating these levels for a particular stone will make it possible to rank its value in relation to other stones using a subjective grade similar to the listing grades of buildings.
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    Extrinsic value
    Extrinsic value is the property we would normally apply to a particular stone almost without thinking. It is the size of the market: the extent to which a stone has been used in the past and will be needed in the future for building repair. This is also the value which the commercial stone industry would use. The greater the demand for repair and new build the more valuable the stone is and the more economically viable any quarry would be.
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    The scale of past use is a multiple of the time for which it was worked, the area over which it was used and the density of its use within that area. Thus past use can be semi-quantified by field studies supported by documentary research covering the number and size of buildings, quarry size and/or number etc. The future market for repair and conservation (ie excluding new build) will be inversely proportional to the durability of the stone. Thus a stone which had a very large use in the past but a very long durability may be less in demand for repairs than a stone with moderate historical use but a shorter life.
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    Additionally there will be a new build market within conservation areas etc.
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    Intrinsic value
    The intrinsic value of a stone is determined by its technical suitability and cultural / heritage importance. The former is not necessarily directly related to its past extent of use although this will certainly be the case for some stones. 
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    Building importance. One way in which to articulate the importance of a stone would be in relation to a concept which we are all familiar with - the listing (or national/historic monument) grade of buildings in which it has been used. The presumption being that, provided all technical criteria were satisfied, the higher the grade of the building the more important it would be to use an authentic stone for repairs. A simple count for each building and a multiplier for its grade could produce a score for a stone.
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    Technical importance. The technical importance of a stone is the extent to which its properties are special in terms of its suitability to be used. Petrological and physical properties - strength; porosity, fabric, cements, colour etc - and its compatibility with the surrounding fabric are both important. Recent experience indicates that compatibility has a much greater importance than has been realised in the past. For some stones and some applications it may well prove to be the primary factor in selecting a stone for repair. It is probable that in many cases the most suitable stone on this basis will be the original material from a closely defined location - a particular bench in a particular quarry for example. Guidance on the technical specification for stone for repairs has been published in the English Heritage Technical Advice Note Sourcing Stone for Historic Building Repair. This provides a second importance parameter. Stones could be ranked on a very simple scale ranging from ‘essential to use’ to ‘could be substituted’.
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    Cultural importance. Cultural importance is largely unquantifiable because it reflects abstract concepts such as a sense of history, historical references to persons or former industrial, agricultural or cultural activities; one’s place in society; local or regional building diversity and the importance of the ‘particular’ - of beauty and eccentricity. 
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    A stone might have a high cultural value even if it has only been used in one or a small number of buildings. If these are of great architectural or historic value, there would be a strong case for repairing them with the original stone. Similarly, a stone which has been used for all the buildings within a village is essential to its sense of place and should be used to repair the buildings. It is all too easy to use broad classifications such as Jurassic limestone or Millstone Grit and in so doing to miss the variety of detail that changes from village to village because the individual, local stones are different. Colour, bed depth, joint spacing, hardness, grain size; all affect the appearance of the stone, the way it can be worked and ultimately the look of the village. 
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    The way a stone can be used in buildings will have been an important aspect of vernacular construction and detailing. In vernacular slate and stone roofing, for example, it is axiomatic that the slating methods and hence the style of the roof – planform, pitch, detailing of intersections etc – are primarily determined by properties of the slates or stones – flatness, surface texture, size and shape. The same will be true of masonry. Strength and bed depth for example, will determine the maximum size of lintels and therefore the size of openings or the use of arches.
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    The cultural value would constitute the third parameter. It could be related to:
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  • Distinctive appearance. Conservation planning guidance focuses on the appearance of buildings. The distinctiveness of particular stones will therefore be a fundamental aspect of their relative importance. Stones should be assessed against surface texture, colour, bed thickness and other dimensions for masonry or size range and thickness for roofing etc. The weathered appearance, which will be determined by the mineralogy as well as external factors, and the plants which grow on the stones are also critical to determining whether a particular stone should be used for repair. 
  • Local distinctiveness. This is about the diversity of stone types used in an area. Typically it will vary from village to village. This characteristic will be predominantly based on ‘primitive’ transport systems mainly horse drawn but also involving water transport and latterly railways.
  • Regional character and continuity. Different stones will often be similar in character creating a coherent style in a (geological) region. Within the region there might be an element of substitution over time as quarries opened and closed and transport systems developed. 
  • Thematic use. Landowners or industrialists often used stone from their own land holdings in preference to those with a lower transport cost. This is expressed in suites of buildings – model farms, estate villages etc – all constructed from the same stone.
  • Landscape character. Buildings in the open landscape - field walls, animal shelters, hay barns, sheep pens, shepherd’s shelters and buildings associated with summer steadings and transhumance, were inevitably built of stone obtained from the immediate vicinity. Because all these buildings are a product of the local farming system which in itself is largely a product of the natural zone in which they sit - the soil, climate, elevation etc, they are intimately linked to the natural environment.
  • Detail. Many non-building uses of stone are important features of land-, village- and town-scapes. Milestones, stiles, bollards, gravestones, river and canal banks, kerbs, copings, pavements and other flagging, steps and gate posts; their texture, shape and style are distinctive and local. They are the details which add character to their locality
  • An understanding of the importance of particular stones in respect of these criteria could be obtained by carrying out characterisation studies similar to conservation area and village design statements. From such studies it will be possible to state how prevalent is the use of a particular stone in respect of each aspect giving an overall score.
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    Integrated value
    Aggregating the importance scores for buildings, technical suitability and cultural significance would give the stone’s relative intrinsic value.

    Terry Hughes

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