STONES -viability now -1

Post 687by Gautam Shah

. Part 1 of Two articles


We today have greater capacity to search over wider terrains and also reach at sub surface locations. Exploitation of stones as collection from the surface or extraction from various depths is not a major technological problem. There are other issues that are forcing reappraisal of Stone as the viable material of construction. The issues are > economics of transportation, wastage in production, and reuse of the material as debris and production residues.


Stones, like the clays-soils are universal materials of construction and require very simple technology. There are Three essential sources of stone: 1 Surface collected stones, 2 Extracted stones, 3 Wastes and debris stones. Use of the surface collected stones in original size-form is easiest. Such stones, however, require down sizing and form dressing, before carriage to a place of use. Extracted stones are surface protruding and subterranean mass. These are often stratified or layered. Stone extraction causes ecological devastation due to removal of the top burden, large volume of reject-mass, and wastage of local cleaning, cutting and size dressing. Wastes and debris stones are man-made endeavours. Wastes occur at points of extraction and location of constructions, whereas debris occurs due to the demolition of structures. These need sorting, cleaning and transportation.


To make Stones viable now, Technological Developments and Materials Management are required. Stones are used for their mass, surface and structural strength. These can be exploited further by new design, joint technology, assembly methods, formation of composites, improved structural geometry and conversion to different materials (chemicals).



1 Extend the Surface Area: Stones are valued for their surface qualities and the prime need is to increase the surface area. The extended surface reduces the mass / weight of the stones. The surface area of the stones can be enlarged by two basic methods: by Thin Sectioning and by Amalgamation of bits and pieces, which otherwise end up as a collection and production wastes. Other methods of optimizing the surfaces are to endow new sensory qualities and surface properties. Many exciting technologies are now available.

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2 Exploring structural properties: Stones have certain directional structural properties which can be exploited and reinforced. The efforts start with new ways of excavation, extraction and conversion of the material. Other common processes are selection, orientation, rational sectioning and controlled aeration-seasoning. Structural potential of stones can also be exploited by developing new areas of usage and new techniques of construction.


3 Stone’s Combinative formations: Traditionally stone composites have had lime and cement as the matrix component. The explorations now relate to composites with new forms of filler arrangements and new types of a matrix.


4 Designing geometrical or spatial compositions: Stones shows great promise in offering radically different materials’ combinative formations. The formations include various ways of combining or ‘synthesizing’ materials of diverse nature, such as, with metals, polymers, ceramics etc.


Stones have naturally variegated constitution and surfaces. These, provide with inexhaustible opportunities to work to many different forms, sizes, and finishes. Though, qualitative consistency of man-made materials poses a great challenge to multifarious nature of stone materials. Stones have structural attributes, often called Engineering characteristics, which regulate their usefulness for conversion to: Building or Dimension stones, Veneered or thin slabs and for crushing. Similarly stones also exhibit very distinctive sensory properties that govern their use as a facing material in the form of building blocks, cladding and flooring slabs.

The opportunities of intervention operate on two fronts: Improvisations over existing methods and Adoption of radically different technologies.


Part 1 of 2 articles


STONES -materials of sustainability

Post 676 –by Gautam Shah




SindhuDurg Fort

Stones are procured through collection off the surfaces, by extraction (mining) from depths of the earth and reused (spolia) from existing or demolished buildings. The stones of these types are abundantly available. Major problems with sustainable stone exploration are the economics of transportation. Other issues are cost of size conversion, surface preparation and quality equalization. In future greater attention will have to be for management of stone-wastes at locations of mining and processing.


Stones are used for their surface quality and structural properties. And in spite of technologically greater capacity to search over wider and deeper terrains, stones always remain scarce or unviable at many places. At use-points natural stones must arrive in optimum mass-units and in forms that are viable for transport, storage and usage.


Stone resources are of basic two types: Surface Stones and Extracted Stones.

Surface Stones show many, but qualitative and size variations. Over a geographic region, though the quality is fairly consistent. Quality equalization can only be enforced through region-based sourcing, selection and separation. Surface-collected materials are naturally formed (boulders, pebbles, gravel, sands, etc.) or wastes of stone processing. Such materials are fractured along the plane of shearing force or across the weakest plane, and so show varied structural properties, colour and grain structure (texture) on different faces. These stones are equally weathered on all faces.




Extracted Stones materials are loaded (buried) with varying depths of overburden, of the same or different nature of materials. The over burdening mass, protects as well as contaminates the stones. The water passing through the organic soil burden is nominally acidic, and so affects the alkaline stone mass. Fresh lime stones are soft and porous, but when exposed to Carbon dioxide begin to change, harden due to the aeration.



Typically, igneous (granite, trap) and metamorphic rocks (marble, schist, slate) have nearly crystalline compounds, and are not stratified so do not present any layers or strata. Sedimentary rocks (lime stone, sand stone, soap stone, travertine) are formed of uniform constitution, though stratified, often in inclined and curved formations due to movements in the earth mass. Sedimentary rocks show grains intervened by a cementing medium.


All stones collected from the surface or mined, must go through some primary processing.

■ Subtractive processes remove excess mass for surface cleaning, sizing, cleaving and pattern sculpting. The processes are, chipping, splitting, cutting, dressing, sculpting, engraving, grinding, polishing etc.


■ Formative processes do not add any mass but change the spatial or physical characteristics of stone such as its sensorial, structural and environmental behaviour. The treatments include impregnation, edge reinforcing, various types of chemical treatments through acid, alkali, solvent and other oxidative compounds, heat and flame treatments, sintering, spluttering, dying, bleaching etc.

Stacked Stone Forms Pretty Stones Wall Stones

■ Additive processes add to the stone mass. Till very recently technologies involved were of Surface layering by way of coating or cladding. But now ceramic formation, metal alloying and deposition, surface synthesis, surface molecular treatments are being used.


The sustainability of stone is dependent on basic three aspects: 1 Minimum mass for largest possible surface extent, 2 Reuse of all waste products, 3 New uses for very small sized materials (sand, gravel, pebbles).

1 Stones are valued for their surface qualities, and we need to extend the Surface area. The extended surface reduces the mass / weight of the stone. This can be done by thin sectioning, and by techniques of amalgamation of bits and pieces.

2 Stones have certain structural properties which we can be altered and reinforced. This process starts with new ways of excavation, extraction and conversion of the material. And can be extended to new forms of usage.

3 A new field is emerging on materials’ technology front. This is about creating new materials combinative formations. The formations include various types of composites, geometrical or spatial compositions and combining or ‘synthesizing’ materials of diverse nature. These reconstructive processes include using particulate matter (various grades of fineness such as dust, sands, gravels, pebbles, chips and lumps) as fillers with a matrix of resin or cement. Forming layered composites with sheets or slabs of stone and other materials (polymer sheets, fabrics). Forming amalgamated materials by lamination, co-extrusion, sheet forming, metalizing, ceramic forming, etc. and chemically converting stones into byproducts like minerals and chemicals.


Spolia (Latin, ‘spoils’) are repurposed- recycled building stone used for new construction, or decorative sculpture reused in new monuments. These stones are from existing or demolished buildings or building workshops.




Sustainable Strategies for Stone

Stone is the least of bio-degrading materials, so not a ‘recoverable or ecological’ material. It can be recycled through reuse processes. Sized blocks of stones for masonry and flooring, have been reused since Egyptian and Roman times. But stone-waste dumps at mine heads and workshops are causing environmental problems.


Stones are broken or crushed from larger stocks for many purposes like roads, embankments etc. which is an avoidable practice. Stones like gravel and boulders (from river beds and old glaciers’ paths) are some of the toughest stones, left over after natures’ processes. But these rounded stones are not used in masonry work, or broken down to smaller sizes. River and seacoast sands are becoming scarce in supply, and could easily be replaced with ground stone, at least in mass concrete plants.



STONES -Opportunities of Intervention

Post 329 –by Gautam Shah 



Stones have naturally variegated constitution and surfaces. These, provide with inexhaustible opportunities to work to many different forms, sizes, and finishes. The qualitative consistency of man-made materials though, poses a great challenge to multifarious nature of stone materials.


The Opportunities of Intervention for stones are of following types:

  1. Stones alone
  2. Stones with other earth-based materials
  3. Stones with natural organic materials: such as plants
  4. Stones with man-made materials such as Ceramics, Metals, Polymers (plastics and elastomers)



Stones represent, one of the largest resource of earth-based materials. We have not touched even a small fraction of its top layer of mass. Ecologically its use or disposals are manageable. Only problems with stones supply are its inconsistency of sensorial and other surface qualities, and difficult to predict structural properties. This is where man-made materials prove to be superior and reliable. Man-made materials require complex and costly processing whereas stones as a natural resource though unlimited in supplies have high costs of extraction and transportation. Man-made materials are highly custom created and so are not reused extensively, but stones have nine lives and can be used till conversion to form of a dust particle. Man-made materials are produced through multiple-processing, making them difficult to recycle or dispose off safely.




Stones combined with other earth-based materials provide many opportunities of usage. However, stones by themselves or with other earth-based materials have limited scope for combinations. These are mainly by positioning such as spreading, layering or stacking with gravity, by using electromagnetic forces or by kinetic method of tying-knotting. Few earth-based cementing materials such as mud, pozolana or plant gums are insufficient in supplies and technically inadequate. Yet use of natural materials with very small proportion of man-made of joining materials and technologies can achieve outstanding results.





Use of organic materials such as plant-based resources (Jungle, Farm produce) has not been explored adequately. Primitive man started using wood in combination of stone, which has been extended to buildings. Its use is limited, as wood is a scarce resource (not easy to replenish). Other organic products require several levels of processing before qualifying their application with stones. Every single new application is worth its wait and expense.



Stones have been used with man-made materials like metals etc. But most technologies involve non-mixing combinations, such as mechanical joining, adhesion fixing or coating. Stones and earth-based materials have been used in many synthesizing processes. Stones in their physical form and characteristics have been exploited, as fillers, for creation of composites. However, stones have been less frequently synthesized with man-made materials such as ceramics, metals and polymers. These are going to be the opportunities for the next generation.

Digital StillCamera

The inspiration derives from the successes achieved in combining Ceramics with Metals. Ceramics and metals individually have diverse temperature of forming. At a temperature a ceramic begins to evolve some metal either evaporate, liquidize or form oxides. A combination seemingly impossible is now being achieved, for example in electrical transmission equipments, electronic components, tools and cutting edges making. Similarly stones can be combined with many other materials.

Metal application technologies provide exciting results here. Metalizing a stone surface with metallic particulate or molecules, by plating and sputtering techniques is not farfetched. Synthetics are mainly made with organic (carbon-based) monomers in polymers but chaining. These have been used both as the matrix and fillers components in composites. And can we visualize stones, not in the role of filler, but of a matrix in composite forming.




STONE WORK – Quality Parameters

Post 228 – by Gautam Shah



Quality of stonework depends on not only the technique of dressing, cladding and fixing, but also on the stone material itself. A building stone material to be useful requires specific type of extraction, handling skills, seasoning and curing processes.


Most of the stone materials are extracted from a depth. Such materials, since their age of formation, have remained buried under heavy over loads. In many circumstances these have been devoid of oxygen, other gases, moisture, and light or radiation exposures. They may have stayed with entrapped moisture and gases.



On extraction the stone material is brought to a totally different environment, and certain inevitable changes set in. The lessening of pressure allows the entrapped moisture and gases to escape, such as in sedimentary deposits. Some materials absorb fresh moisture and atmospheric gases to fill in the voids. Due to reduced pressure, and exposure the mineral structures begin to alter, causing expansion or contraction stresses in the mass.

Carcassonne City Stone Wall Medieval Castle

The environmental changes continue long after mining, sizing and dressing of stones. Rain water, is slightly acidic, and causes a calcium carbonate to change into bicarbonate. In industrial areas atmospheric sulphur and carbon dioxide enhance the acid action on stone. Chlorides on sea front and in industrial areas can get converted into weak hydrochloric acid and dissolve the carbonate rocks. Nitric acid produced from oxide or Nitrogen also corrodes stone faces. All corrosive mediums depend on supply of water or moisture, so care of fresh-cut stone is essentially a moisture management exercise.


Seasoning: Seasoning involves both the drying and wetting on one hand, and airing of stone, on the other hand. There is an increase in strength due to the re-deposition of percolated minerals, surface carbonation, transmission and deposition of minerals on the surface of a stone, by both the evaporating moisture, and addition of water. Dehydration, during seasoning, is more or less an irreversible process and subsequent artificial saturation of stone with water, often lowers its compressive strength. Seasoning is more relevant to soft limestone whereas hard limestone seems to be less affected by it. Immediately after quarrying, a stone is soft, and it is easy to work with it, but polishing to glossy finish requires a fully seasoned and hardened stone.


Weathering: The term weathering denotes both desirable and undesirable changes. It may even enhance certain qualities, like colour, texture, strength, etc. Following are the main sources for decay or the unwanted effects, on a stone:


  • Penetrating water: Effect of acid in air and rain water, mainly sulphur dioxide derived from the combustion of the sulfur constituents of fuel.
  • Surface action of water, gases: Bleaching of colour, water soluble salts’ crystalline deposits, erosion.
  • Effects of temperature variations: Frost, Cracking,
  • Effects of foreign deposits or organisms: Bird droppings, roots of climbers and other vegetative growth weaken the stone and accelerate the decay. Fungus and mildew are other destructive agents.


Applied Surface Finishes on Stone: Water repellent or finishes that completely seal a stone surface should be used very judiciously. A sealed surface is likely to allow crystallization of salts beneath the surface instead of its natural leaching out. Water repellent finishes once applied are difficult to remove, and in many cases not re-coatable. Better design techniques, correct type of stone use and use of a biocide treatment give more satisfying result for problems such as algae, fungi, lichens etc. The decay of stone can be minimized by laying the stone with natural grain in horizontal orientation.640px-Olandalbymedbridge

Porosity, has no direct relation to the weathering resistance of stone materials, such as limestone. It is the shape, size and nature of pores, especially the degree of micro porosity that plays an important role in weathering. The action of carbonic acid, increases with greater micro-porosity, and capillaries prolong the dissolving action. Small cavities on the surface and rough finished textures trap dust, bacteria and retain water for longer period. Sand-stones containing colloidal minerals as a cementing medium has the most pronounced expansion due to weathering.


Efflorescence (also called wall white, stack white or wall or stone cancer), is the appearance of crystallized salts on the stone’s surface, and the cryptoflorescence, denotes crystallization of salts within the pores. The crystallization of the salts and their re-crystallization from a lower to a higher hydrate within the range of mineral stability may develop stresses of high magnitude with quite an appreciable qualitative change.


Strength of a stone is affected by three types of stresses:

Compressive stresses, decrease the volume of the material, causing breaks with a shattering effect. This is more common in non-homogeneous stones, and stones with inclined grains or stratification.

Shear stresses, move one part of a stone with respect to another, under certain conditions, inducing a permanent change of shape. These are best avoided by appropriate angle of extraction and cut, by careful orientation during coursing a masonry.

Tensile stresses, produce cracks and fissures and torsion (or twisting). Generally, fine-grained rocks are stronger than coarse grained. Rocks with interlocking crystals are stronger than rocks with poor interlocking. Stratified rocks have poor strength along the plane or strata. Stratified rocks as a rule have lower strength than igneous and non-stratified homogeneous rocks.





 Post -by Gautam Shah


Cararra-Steinbruch_retouchedA Rock is a very generalized term and can include any assembly of minerals such as stones, pebbles, sand and clay materials. Rock is also defined as any large and hard mass, composed of minerals that after its extraction and size reduction becomes stone. The term Stone is applied commercially to all natural rock materials quarried or mined for construction and industrial uses. Construction stone came from hard and consolidated materials of which there are three major classes:


  1.  Dimension stone
  2.  Crushed or Broken stones
  3. Naturally formed or Weathered stone


● Boulders are pieces of stones of various compositions buffeted by movements of snow glaciers, river floods or ocean waves and storms. Boulders are of tougher mineral compositions, because natural sorting has taken place. Only the strong and stable stones have survived periods of transportation and rolling which may have taken millions of years.




● Flat boulders occur on weathering of stratified rocks. Boulders are found not only on river beds and ocean beaches but also on mountain or hill tops and at places with no apparent riverine flows. This is so because, earthquakes and movement of plates displace such deposits, as in case of Tehri Gadhwal (Himalayan range, India) regions. People have been using such stones without any further cutting or dressing for constructing buildings with catastrophic results during earthquakes (e.g. Tehri Gadhwal, Latur in Maharashtra, India). Boulders are rounded shape and have smooth surfaces. In a masonry wall composed of boulders, it is not possible to ʽbreak the jointʼ or ʽcourseʼ the masonry. Walls are usually very thick, minimum being 400mm wide. Entire masonry structure, like a fluid mass, remains unstable. During earthquakes due to lateral movements, corners are displaced, and boulders fall off destroying the structure. Since walls of boulders are thick and seemingly solid, people have a tendency to place equally heavy roof structures on them, another cause for a heavy death toll during earthquakes.




Boulders have been used as cobble stone for paving of courtyards, walkways and roads. Boulders are also used for lining of canals, waterways (though not very efficient from a hydraulics point of view), land slopes etc. Boulders are also used for garden decoration and as landscaping features.

● Pebbles are similar to boulders in formation and characteristics but of smaller size. Pebbles are used for lining with or without cementing materials, courtyards, parks, walkways, ponds etc. Pebbles are broken to form aggregates for road building and cement concrete works.

● Gravels are finer then pebbles (also formed by natural processes) are used as fillers and decorative aggregates for plasters. Gravels are also used for packing tube well bores and as a filtering media. Gravels are crushed to form high shearing sands with granular faces.

● Weathered sands are mostly composed of quartz with feldspar, mica, garnet, zircon and magnetite. A sand deposit is composed of particles of different sizes and shapes such as rounded, sub angular or angular. Most sands contain small to substantial amounts of clay particles. Sharp angular sand is more suitable for concrete work. A coarse and well-graded sand gives a better mortar. Fine sand is better for filling and reducing plasticity of soils like black cotton soils. Garden soil or river `Kanp‘ is a conglomerate of fine sand, silt and decomposed organic waste.


Boulders, pebbles, sands and clay type of materials are available mainly from surface or low depth excavations. Bedded (stratified) deposits, usually of the sedimentary variety, are next in importance for ease of extraction, although the efficiency of extraction depends on the tools available. These deposits have many advantages over boulder deposits, as size and shapes can be predetermined and formed as required during the extraction. Slabbed deposits with layers between 60 to 80 mm are the easiest to extract, and shape. Thicker layers are usually subdivided with pneumatic tools and wedges or with cutting and sawing machines Bedded deposits of suitable thickness do not require horizontal cutting, as the blocks can be lifted at the bedding plane, thus saving much time and hard work.



STONES for buildings

Post-by Gautam Shah



Stones like many other natural materials are abundantly available. Most of the rocks that we are likely to encounter and exploit are within the top 16 kilometres of Earth’s face. This mass is made up of 95% Igneous rocks, and rest consisting of widely spread cover of Sedimentary and Metamorphic rocks.


We today have greater capacity to search over wider terrains and also reach at sub surface locations. Exploitation of stones as collection from the surface or extraction from various depths is not a major technological problem, but economics of transportation limits its commercial usage.


There are 3 essential sources of Building Stone Materials:

● Surface collected stones

● Extracted stones: protruding and subterranean mass

● Waste and recycled stones

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The stones occur in many forms and sizes:

1 Large pieces which can be further down sized or cut into smaller units,

2 Units that are used without any other processing,

3 Pieces which are crushed or disintegrated into finer particles,

4 Rejected material from mining and collection processes,

5 Wastes from stone sizing and dressing operations,

6 Debris material recovered from demolition of old buildings and other structures.



Surface stones from single geographic region show qualitative and size variations of minor nature. Further quality equalization can be done by location-based sourcing, visual selection, grading, separation. Surface collected stones can be further quality equalized through many types of ‘processes’.

● Surface collected materials are naturally formed such as boulders, pebbles, gravel, sands, etc. These are very tough materials and equally weathered on all faces.

● Other surface collected materials are broken by natural disintegrating forces like weather, chemical reactions, land mass movements, internal stresses, etc. These stones may show up with varied weathering on their faces. Such materials are fractured along the plane of shearing force or across the weakest plane, and so show unpredictable structural properties, inconsistent colour and grain structure (texture) on different faces. These stones due to their long exposure are either the toughest remains or the weaker fractures. In the first instance further dressing or downsizing is difficult, and in the second case consistent shaping is not possible.

● Such materials are found spread or located over terrain difficult to access. Collection unless manual involves a large amount of useless mass.


s4 Naxos_Marble


Extracted materials are buried (loaded) under the same or different nature of materials’ mass. The over burdening mass protects, as well as contaminates the deposit. The water leaching through the organic soil burden is nominally acidic and affects the alkaline stone mass. Typically Lime stones when freshly extracted, are not exposed to Carbon Dioxide due to the overburden, and so are soft and porous, but begin to harden on aeration.

● Igneous and metamorphic rocks are not strongly stratified and do not present distinctive layers or strata. Sedimentary rocks are stratified, generally in horizontal layers. However, due to movements in the earth mass inclined and curved formations also occur. Sedimentary rocks show grains intervened by a cementing medium.

● Igneous and metamorphic rocks are often made of many different substances, some of these components, as remnants, are nearly crystalline compounds.

● Sedimentary rocks are comparatively formed of uniform constitution though with streaked colouration due to seepage of dissolved substances and stratification.

● Extracted rocks, require dressing, and often downsizing. The cleavage or fracturing during dressing and fracturing depends not only on the basic classification of the stone and also on constituent minerals such as silica, quartz, feldspar, mica, etc. These aspects also define the types of tools used for working and the nature of surface finish possible.


Igneous rocks, such as Granite and Trap are formed with the solidification of molten materials. Mineral gases and liquids penetrated into the stone and created new crystalline formations with various colours. Sedimentary rocks such as Lime stone, Sand stone, Soap stone Travertine, are formed from the bonding of deposition under pressure and heat over a very long period. Metamorphic rocks are formed by the transformation of igneous or sedimentary rocks, due to influence of heat or chemical action. Metamorphosed form of stones: Marble (of lime stone), Schist (of sand stone) and Slate (of mud-stone). Amorphous Solid is any material which does not have its molecules arranged in a lattice, or crystalline structure. Amorphous solids make up only 10 % of solids in the world. A well-known example of amorphous solid is glass, and that is why these solids are often termed glass. Amorphous solids’ structures have similarity to liquids, and so are also called supercooled liquids. Plastic is made from polymers, long strings of molecules purposefully chained together and is technically an amorphous solid. Crystalline Solids constitute nearly 90 % of all solids in the world. Crystalline solids have a lattice of molecules. The ordered pattern repeats substantially through the mass. Stones are classified as Siliceous when silica is the principal earthy constituent, Calcareous have carbonate of lime as the predominant material, and Argillaceous have alumina is the main component.






Stone extraction or collection creates large quantity of rejected and broken mass. Site based dressing and downsizing, are mainly done to reduce the mass for transportation, These generates large quantity of wastes. As stone sites are very remote from the point of use or application, it is uneconomic to transport and use such waste materials. Downsizing and cutting workshops located near urban localities, have an advantage that the wastes originating here have consistent one face or dimension. Machines that dress a block with rotary or stripe saws create wastes with smooth finish on one or more faces. Similarly slabs’ end or edge cuts have a uniform thickness profile. Stone polishing machines provide ground particles which are used as filler media.

Angular cut wastes can be tumbled with iron bits in a rotary drum to achieve rounded edged pebbles. Stone wastes can be used to create cement and resin-based composites, and for ‘synthesizing’.

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Stone buildings that are demolished in urban areas end up as debris for land fill for lack of man power required for separation and re-use. However, in rural area, it is possible to separate and reuse the material. Older stone flooring units are thicker in comparison to modern supplies. This can be split into two or more units and use the cut-face as the new face. Similarly masonry or building blocks can be cut to thinner blocks for use in cladding or surfacing. The advantage in reuse is free supply of mature (weathered-seasoned) stones.


Spolia (Latin, ‘spoils‘), repurposed building stone for new construction, or decorative sculpture reused in new monuments, is the result of an ancient and widespread practice whereby stone that has been quarried, cut, and used in a built structure is carried away to be used elsewhere. Entire obsolete structures, including underground foundations, are known to have been demolished to enable the construction of new structures. –Wikipedia


Interpretations of spolia generally alternate between the “ideological” and the “pragmatic.”

Ideological readings might describe the re-use of art and architectural elements from former empires or dynasties as triumphant (that is, literally as the display of “spoils” or “booty” of the conquered) or as revivalist (proclaiming the renovation of past imperial glories). Pragmatic readings emphasize the utility of re-used materials: if there is a good supply of old marble columns available, for example, there is no need to produce new ones. The two approaches are not mutually exclusive, and there is certainly no one approach that can account for all instances of spoliation, as each instance must be evaluated within its particular historical context. –Wikipedia