OR/13/015 Appendix 5 - Stone matching for sandstone: Difference between revisions

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Most quarried sandstone is permeable; in other words, moisture and air are able to pass through the stone via a network of connected gaps (pore spaces) between the sand grains. This character makes sandstone relatively light and easy to cut and carve, but it also makes the stone prone to decay as a result of processes like mineral dissolution, freeze-thaw, and salt heave. Characterising and matching intrinsic properties is therefore particularly important when finding the closest-matching stones for sandstone samples.
Most quarried sandstone is permeable; in other words, moisture and air are able to pass through the stone via a network of connected gaps (pore spaces) between the sand grains. This character makes sandstone relatively light and easy to cut and carve, but it also makes the stone prone to decay as a result of processes like mineral dissolution, freeze-thaw, and salt heave. Characterising and matching intrinsic properties is therefore particularly important when finding the closest-matching stones for sandstone samples.


Intrinsic properties
'''Intrinsic properties'''<br>
Sandstone consists of adhering sand grains with unfilled gaps (pore spaces) and/or a mineral ‘cement’ between the grains. Sand grains are small – between 2 and 0.064 millimetres in diameter – so many of the intrinsic properties of a sandstone, including the relative proportions of the constituent minerals, the grain-size and textural arrangement of the constituents, and the porosity (pore space) characteristics, can only be determined accurately by microscope examination. Some properties, including the colour and primary sedimentary structure of the stone, can be determined adequately with the unaided eye. Still others, including the cohesiveness and permeability of the stone, require a simple test to make an adequate evaluation. Each property can vary considerably from one sandstone to another, and no two sandstones are identical.
Sandstone consists of adhering sand grains with unfilled gaps (pore spaces) and/or a mineral ‘cement’ between the grains. Sand grains are small – between 2 and 0.064 millimetres in diameter – so many of the intrinsic properties of a sandstone, including the relative proportions of the constituent minerals, the grain-size and textural arrangement of the constituents, and the porosity (pore space) characteristics, can only be determined accurately by microscope examination. Some properties, including the colour and primary sedimentary structure of the stone, can be determined adequately with the unaided eye. Still others, including the cohesiveness and permeability of the stone, require a simple test to make an adequate evaluation. Each property can vary considerably from one sandstone to another, and no two sandstones are identical.
Each of the intrinsic properties of sandstone plays a role in determining how any one stone responds to the complex physical and chemical processes associated with weathering. The result is that no two sandstones respond to weathering in exactly the same way and at the same rate. If more than one type of sandstone is used in a stone structure, obvious contrasts in the condition of masonry blocks commonly become apparent over time. Furthermore, placing two sandstones of contrasting permeability next to each other in masonry can lead one (usually the more permeable stone) to suffer accelerated decay. For these reasons, it is generally considered good practice to repair or replace ‘original’ sandstone masonry with sandstone that is the closest achievable match in terms of the properties that govern how the stone responds to weathering (‘weathering properties’). This maximises the likelihood that the replacement stone will co-exist harmoniously with the original stone and will weather sympathetically. The poorer the match between the weathering properties of the replacement stone and the original stone, the greater is the likelihood that the condition of the two stones will diverge over time.
Each of the intrinsic properties of sandstone plays a role in determining how any one stone responds to the complex physical and chemical processes associated with weathering. The result is that no two sandstones respond to weathering in exactly the same way and at the same rate. If more than one type of sandstone is used in a stone structure, obvious contrasts in the condition of masonry blocks commonly become apparent over time. Furthermore, placing two sandstones of contrasting permeability next to each other in masonry can lead one (usually the more permeable stone) to suffer accelerated decay. For these reasons, it is generally considered good practice to repair or replace ‘original’ sandstone masonry with sandstone that is the closest achievable match in terms of the properties that govern how the stone responds to weathering (‘weathering properties’). This maximises the likelihood that the replacement stone will co-exist harmoniously with the original stone and will weather sympathetically. The poorer the match between the weathering properties of the replacement stone and the original stone, the greater is the likelihood that the condition of the two stones will diverge over time.


Petrographic examination
'''Petrographic examination'''<br>
A macroscopic examination of the sample of ‘original’ stone is performed with the unaided eye and using a binocular microscope. A microscope examination is performed on a thin section (a slice of the stone sample cut thin enough to be transparent), using a polarizing microscope. Before preparing the thin section, permeable stones (such as sandstone) are impregnated with blue resin to highlight pore spaces. The thin section is cut perpendicular to the bedding orientation in the stone (where this is visible), and is positioned to be as representative as possible of the sample. The thin section is typically cut to include the freshest part of the
A macroscopic examination of the sample of ‘original’ stone is performed with the unaided eye and using a binocular microscope. A microscope examination is performed on a thin section (a slice of the stone sample cut thin enough to be transparent), using a polarizing microscope. Before preparing the thin section, permeable stones (such as sandstone) are impregnated with blue resin to highlight pore spaces. The thin section is cut perpendicular to the bedding orientation in the stone (where this is visible), and is positioned to be as representative as possible of the sample. The thin section is typically cut to include the freshest part of the supplied stone sample, and also any weathered part and/or exposed (exterior) surface where these are present.
 
supplied stone sample, and also any weathered part and/or exposed (exterior) surface where these are present.
Observations from these examinations are recorded on a Petrographic Description Form designed for building stones, to ensure the description is systematic and consistent with the procedures set out in British Standard BS EN 12407:2000 (Natural stone test methods – Petrographic examination). The completed Petrographic Description Form for each sample is presented in Appendix X, with a set of accompanying notes describing each of the recorded properties. The description is accompanied by one or more photographs illustrating the typical character of the stone as it appears in the thin section.
Observations from these examinations are recorded on a Petrographic Description Form designed for building stones, to ensure the description is systematic and consistent with the procedures set out in British Standard BS EN 12407:2000 (Natural stone test methods – Petrographic examination). The completed Petrographic Description Form for each sample is presented in Appendix X, with a set of accompanying notes describing each of the recorded properties. The description is accompanied by one or more photographs illustrating the typical character of the stone as it appears in the thin section.


Stone matching
'''Stone matching'''<br>
The following factors are taken into account when comparing an original stone with a potential replacement stone.
The following factors are taken into account when comparing an original stone with a potential replacement stone.
1) Mineral and textural features – ideally, these should be as similar as possible in the replacement stone and original stone, to increase the likelihood that the two stones will respond in similar ways and at similar rates to the various physical and chemical processes associated with weathering, and will therefore co-exist harmoniously. Replacement stones are selected to match the original stone in its fresh (rather than weathered/decayed) state, unless otherwise requested. Particular attention is paid to those minerals and textural features that are known to play a significant role in sandstone decay and discolouration.
2) Permeability – ideally, the replacement stone and original stone should have similar permeability characteristics, thereby minimising the degree to which fluid (water and air) migration between adjacent blocks of original and replacement stone might be impeded. Accelerated stone decay can occur where fluid migration is impeded.
3) Appearance – for aesthetic reasons, the replacement stone and original stone ideally should look similar to the unaided eye in terms of colour and stone fabric at the time the repair is made. However, the closest-matching stones in terms of the properties that govern weathering performance (mineral-textural features and permeability) do not necessarily provide the closest match in terms of appearance. A repair using stone selected primarily because it is the closest match in terms of appearance may look good initially but could quickly show signs of decay or of being incompatible with the original stone. For that reason, priority is generally given to the properties that govern weathering performance, thereby maximising the likelihood of long-term compatibility of the original stone and replacement stone. A degree of compromise may in some cases be desirable and acceptable if the closest-matching stones in terms of ‘weathering properties’ are not a close match in terms of appearance. Immediately following repair, the fresh surfaces of a stone insert or indent will usually contrast in appearance with the soiled or discoloured surfaces of adjacent original masonry, but if the ‘weathering properties’ of the two stones are a good match the new stone should blend in over time and the contrast should become less obvious.
4) Functional and performance requirements – specific functional and performance requirements of a replacement stone are taken into account if requested. For example, if the original stone performed a load-bearing role, the choice of matching stones should include only those that are at least as strong; and if the original stone was carved or shaped in a particular way, the choice of matching stones ideally should include only those that can be carved or shaped in a similar way, with a similar level of detail and quality of finish.
One or more replacement stone types are proposed taking these factors into account. A brief description and a thin section photograph are provided for each.


# Mineral and textural features – ideally, these should be as similar as possible in the replacement stone and original stone, to increase the likelihood that the two stones will respond in similar ways and at similar rates to the various physical and chemical processes associated with weathering, and will therefore co-exist harmoniously. Replacement stones are selected to match the original stone in its fresh (rather than weathered/decayed) state, unless otherwise requested. Particular attention is paid to those minerals and textural features that are known to play a significant role in sandstone decay and discolouration.
# Permeability – ideally, the replacement stone and original stone should have similar permeability characteristics, thereby minimising the degree to which fluid (water and air) migration between adjacent blocks of original and replacement stone might be impeded. Accelerated stone decay can occur where fluid migration is impeded.
# Appearance – for aesthetic reasons, the replacement stone and original stone ideally should look similar to the unaided eye in terms of colour and stone fabric at the time the repair is made. However, the closest-matching stones in terms of the properties that govern weathering performance (mineral-textural features and permeability) do not necessarily provide the closest match in terms of appearance. A repair using stone selected primarily because it is the closest match in terms of appearance may look good initially but could quickly show signs of decay or of being incompatible with the original stone. For that reason, priority is generally given to the properties that govern weathering performance, thereby maximising the likelihood of long-term compatibility of the original stone and replacement stone. A degree of compromise may in some cases be desirable and acceptable if the closest-matching stones in terms of ‘weathering properties’ are not a close match in terms of appearance. Immediately following repair, the fresh surfaces of a stone insert or indent will usually contrast in appearance with the soiled or discoloured surfaces of adjacent original masonry, but if the ‘weathering properties’ of the two stones are a good match the new stone should blend in over time and the contrast should become less obvious.
# Functional and performance requirements – specific functional and performance requirements of a replacement stone are taken into account if requested. For example, if the original stone performed a load-bearing role, the choice of matching stones should include only those that are at least as strong; and if the original stone was carved or shaped in a particular way, the choice of matching stones ideally should include only those that can be carved or shaped in a similar way, with a similar level of detail and quality of finish.


One or more replacement stone types are proposed taking these factors into account. A brief description and a thin section photograph are provided for each.
[[Category:OR/13/015 A survey of building stone and roofing slate in Falkirk town centre | 12]]
[[Category:OR/13/015 A survey of building stone and roofing slate in Falkirk town centre | 12]]

Revision as of 10:28, 7 September 2021

Gillespie, M R, Everett, P A, Albornoz-Parra, L J, and Tracey, E A. 2013. A survey of building stone and roofing slate in Falkirk town centre. Nottingham, UK, British geological Survey. (OR/13/015).

This appendix contains background information for stone matching in sandstone.

Introduction
Most quarried sandstone is permeable; in other words, moisture and air are able to pass through the stone via a network of connected gaps (pore spaces) between the sand grains. This character makes sandstone relatively light and easy to cut and carve, but it also makes the stone prone to decay as a result of processes like mineral dissolution, freeze-thaw, and salt heave. Characterising and matching intrinsic properties is therefore particularly important when finding the closest-matching stones for sandstone samples.

Intrinsic properties
Sandstone consists of adhering sand grains with unfilled gaps (pore spaces) and/or a mineral ‘cement’ between the grains. Sand grains are small – between 2 and 0.064 millimetres in diameter – so many of the intrinsic properties of a sandstone, including the relative proportions of the constituent minerals, the grain-size and textural arrangement of the constituents, and the porosity (pore space) characteristics, can only be determined accurately by microscope examination. Some properties, including the colour and primary sedimentary structure of the stone, can be determined adequately with the unaided eye. Still others, including the cohesiveness and permeability of the stone, require a simple test to make an adequate evaluation. Each property can vary considerably from one sandstone to another, and no two sandstones are identical. Each of the intrinsic properties of sandstone plays a role in determining how any one stone responds to the complex physical and chemical processes associated with weathering. The result is that no two sandstones respond to weathering in exactly the same way and at the same rate. If more than one type of sandstone is used in a stone structure, obvious contrasts in the condition of masonry blocks commonly become apparent over time. Furthermore, placing two sandstones of contrasting permeability next to each other in masonry can lead one (usually the more permeable stone) to suffer accelerated decay. For these reasons, it is generally considered good practice to repair or replace ‘original’ sandstone masonry with sandstone that is the closest achievable match in terms of the properties that govern how the stone responds to weathering (‘weathering properties’). This maximises the likelihood that the replacement stone will co-exist harmoniously with the original stone and will weather sympathetically. The poorer the match between the weathering properties of the replacement stone and the original stone, the greater is the likelihood that the condition of the two stones will diverge over time.

Petrographic examination
A macroscopic examination of the sample of ‘original’ stone is performed with the unaided eye and using a binocular microscope. A microscope examination is performed on a thin section (a slice of the stone sample cut thin enough to be transparent), using a polarizing microscope. Before preparing the thin section, permeable stones (such as sandstone) are impregnated with blue resin to highlight pore spaces. The thin section is cut perpendicular to the bedding orientation in the stone (where this is visible), and is positioned to be as representative as possible of the sample. The thin section is typically cut to include the freshest part of the supplied stone sample, and also any weathered part and/or exposed (exterior) surface where these are present.

Observations from these examinations are recorded on a Petrographic Description Form designed for building stones, to ensure the description is systematic and consistent with the procedures set out in British Standard BS EN 12407:2000 (Natural stone test methods – Petrographic examination). The completed Petrographic Description Form for each sample is presented in Appendix X, with a set of accompanying notes describing each of the recorded properties. The description is accompanied by one or more photographs illustrating the typical character of the stone as it appears in the thin section.

Stone matching
The following factors are taken into account when comparing an original stone with a potential replacement stone.

  1. Mineral and textural features – ideally, these should be as similar as possible in the replacement stone and original stone, to increase the likelihood that the two stones will respond in similar ways and at similar rates to the various physical and chemical processes associated with weathering, and will therefore co-exist harmoniously. Replacement stones are selected to match the original stone in its fresh (rather than weathered/decayed) state, unless otherwise requested. Particular attention is paid to those minerals and textural features that are known to play a significant role in sandstone decay and discolouration.
  2. Permeability – ideally, the replacement stone and original stone should have similar permeability characteristics, thereby minimising the degree to which fluid (water and air) migration between adjacent blocks of original and replacement stone might be impeded. Accelerated stone decay can occur where fluid migration is impeded.
  3. Appearance – for aesthetic reasons, the replacement stone and original stone ideally should look similar to the unaided eye in terms of colour and stone fabric at the time the repair is made. However, the closest-matching stones in terms of the properties that govern weathering performance (mineral-textural features and permeability) do not necessarily provide the closest match in terms of appearance. A repair using stone selected primarily because it is the closest match in terms of appearance may look good initially but could quickly show signs of decay or of being incompatible with the original stone. For that reason, priority is generally given to the properties that govern weathering performance, thereby maximising the likelihood of long-term compatibility of the original stone and replacement stone. A degree of compromise may in some cases be desirable and acceptable if the closest-matching stones in terms of ‘weathering properties’ are not a close match in terms of appearance. Immediately following repair, the fresh surfaces of a stone insert or indent will usually contrast in appearance with the soiled or discoloured surfaces of adjacent original masonry, but if the ‘weathering properties’ of the two stones are a good match the new stone should blend in over time and the contrast should become less obvious.
  4. Functional and performance requirements – specific functional and performance requirements of a replacement stone are taken into account if requested. For example, if the original stone performed a load-bearing role, the choice of matching stones should include only those that are at least as strong; and if the original stone was carved or shaped in a particular way, the choice of matching stones ideally should include only those that can be carved or shaped in a similar way, with a similar level of detail and quality of finish.

One or more replacement stone types are proposed taking these factors into account. A brief description and a thin section photograph are provided for each.