Lecture presented by Gautam Shah at Stone Workshop Faculty of Design CEPT University Ahmedabad
Stones like many other natural materials are abundantly available.
Most rocks that we are likely to encounter 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: surface protruding and subterranean mass
● Waste and recycled stones
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 COLLECTED STONES
Surface collected stones from a single geographic region show only minor qualitative and size variations. Further quality equalization can be done 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 a difficult to access terrain. Collection unless manual involves a large amount of useless mass.
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 are not exposed to Carbon Dioxide due to the overburden and so are soft and porous when freshly extracted, 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.
WASTES AND RECYCLED STONES
Stone extraction or collection creates large quantity of rejected and broken mass. Site based dressing and downsizing, mainly done to reduce the mass for transportation, also 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 are located near urban localities, and 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’.
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.
STONE PROCESSES AND TECHNOLOGICAL DEVELOPMENTS
Stones, for cost of transportation, need to be in lightest possible units. Stones of only good surface quality and appropriate structural properties are brought at the point of use or application. However, quality parameters and economics of transportation rarely match at many locations. As such, whatever supplies are locally available must be exploited. Stones are exploited through following basic processes:
Subtractive processes are about removing material by sculpting, dressing, engraving, grinding, polishing, etc. The concept may be to ‘dress’ a surface or ‘sculpt’ a shape or size.
Formative processes are ‘non-mass adding’ procedures that change the spatial or physical character of the stone mass and alter its nominal 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.
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 on the horizon.
DRIVERS FOR THE TECHNOLOGICAL DEVELOPMENTS
The technological developments of these processes are driven by following basic issues.
1 Extend the Surface Area: Stones are valued for their surface qualities and 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 2 basic methods: by Thin Sectioning and by Amalgamation of bits and pieces, which nominally end up as a collection and production wastes. Other methods of optimising the surfaces are to endow new sensory qualities and surface properties. Many exciting technologies are now available.
2 Exploring structural properties: Stones have certain structural properties which can be improvised and reinforced. These 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 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. Designing geometrical or spatial compositions of stones shows great promise. Materials’ technology front is also offering radically different materials’ combinative formations. The formations include various ways of combining or 'synthesizing' materials of diverse nature. (Like for being attempted with ceramics + metals / metals + synthetics / ceramics + synthetics etc.).
OPPORTUNITIES OF INTERVENTION
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.
1 Different species of stones
2 Local variations
3 Exposure to environment
4 Depth of deposit
5 Position or integrity of the strata
6 Technologies for excavation and collection
7 Angle of cut or rupture
8 Tools and techniques of down sizing
9 Size and shape
10 Dressing and Finishing
12 Application or Usage
13 Life cycle
These opportunities of intervention operate on two fronts:
• Improvisations over existing methods
• Adoption of radically different technologies.
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.
Dimension stones are selected or converted from natural rock material for the purpose of obtaining blocks or slabs of different shapes and sizes.
Veneered or slab stones occur naturally as thin body materials or converted into thin sections for the purpose of cladding, surfacing and floorings.
Crushed stones are naturally found, separated as small sized pieces, manufacturing wastes or ground materials for use as aggregates or as additives.
● Determinants at mining or collection level: Condition of the stone mass (integrity, fractures, consistency of colour, grain and pattern), over burdening materials, access to the deposit, exposure to weather, possibilities of size extraction and carriage, applicable technology for sizing.
● Use related selection criteria: Durability, weight bearing capacity, shear strength, ability to maintain the distinctive sensorial characteristics, resistance to decay and appearance.
● Mine or collection level operations: The stone materials if only relevant are carried off the place. This requires elimination of crust or weathered surfaces, uneconomic fractured or separated mass, removal of odd portions (in terms of constitution, colour, grain, etc., form or shape regulation extras, quality equalization selection).
● Workshop level primary processes: These are carried out at places where power, labour, equipment and markets are available, rather then at a mine or collection locality. These include sizing, cutting, dressing, finishing, splitting, sectioning or veneering, polishing, forming of edge or profile.
● Workshop level Subtractive or Mass removal processes: Surface treatments such as: sculpting, crafting, polishing, honing, engraving, inlaying, etching, sand blasting.
● Workshop level Additive processes: Resin or cement impregnation, crack filling, reinforcing mass with applique materials, sandwiching, edge binding and reinforcing.
● Workshop level formative processes: Colouring, staining, bleaching, flame-burnishing, Acid and Laser etching, printing.
● Craft processes: This requires human ingenuity (design-concept) and intervention of men & machines. These are one or few pieces to mass-produced items. Craft pieces are inspired by the form possible through a material or its combination, potential finish that can substantiate the form, and expression through aggregation of form and finish.
● Reconstructive processes: These processes use stone as a component raw material while keeping its physical form in tact. Particulate composites are formed with a matrix of resin or cement, and fillers of stone materials in various grades of fineness and shapes, such as: dusts, sands, gravels, pebbles, flakes, chips and lumps. Layered composites formed with sheets or slabs and also with such forms made as of particulate stone composites. Amalgamation done by lamination, co-extrusion and sheet forming with the use of polymers, metalizing, ceramic forming, etc.
● Surface altering processes: Washing, waxing, sintering, burnishing, heat treatments, laser and other radiation treatments.
● Masonry constructions: Techniques of stone masonry are very mature and scope for improvisation is seemingly very low. Yet, new forms of buildings, parts and components, require new concepts for masonry design. New methods of stone works are also required to manage specific distribution and transmission of stresses, fitment provisions, carriage and placement conditions, condition of the stone units.
● Bonds and arrangements in masonry structures: Romans were the first innovators of stone arrangements in masonry structures, since then nothing much has evolved except re learning of the lessons post disasters and mishaps.
● Structural Joints and Joining: Cement less to cemented systems have been used for years. Cement-less joinery now involves metal, polymer and elastomer in the form of inserts, cleats and seam channels. Many of these items and technologies are proprietary (patented) designs. The range mechanical joining systems include auto-fit, perma-fit and re-use systems. New cementing systems are like: pressure injections, film bonding, gel gumming, powder melt adhesions, reactive and electrical charge bonding.
● Substrates: Substrates are very important in levelling, fixing and stabilizing stones along the plane of gravity (horizontal). However, stones with flatter (a larger surface) and levelled bottom is naturally stable, and this is now explored as a way of creating foundation bases, road beddings, gravity dams, retaining walls and porous structures. New substrate material combinations are available to replace lime and cement materials. These include cement and chemical foams, thixotropic compounds, expanded aggregates, a polymer sprays of organosols, and rubber and polymer underlays of plain, corrugated and bubble sheet formations.
● Cladding and Surfacing: Traditionally cladding and surfacing (veneer) stones have been adhesion fixed with cements and gums. The traditional applications were in the form of water bound pastes, but now solvent bound pastes, catalyst activated bonding compounds, heat softening materials, pressure sensitive film-gums are available.
● Surface Patterns: The patterns are natural on the stone, through the colour and grain variations, which are explored by regular stamping, mirroring, or randomization. Stone patterns are also formed by the techniques of dressing, material combinations, surface finish combinations, joints design, form direction and scaling.
● Patterns and their meanings: Natural patterns due to colour and grain variations are often unavoidable in large stone masses. The patterns, their contrast and intensity (frequency) of distribution, and the form of pattern body (granular or spotty, linear -straight, angular, curved, consistent or varying widths and massive) mean different things to different users.
● Plastering and Rendering: Stones in various form and sizes constitute a substantial and very important mass with the cement and polymeric media. The stone additives determine renderings’ effects such as colour, and textures like granulated, coarse, rustic, angulated, etc. Other effects are achieved by tools and techniques of application and post application treatments. Post application treatments include green washing, pre-set blasting, dry blasting, chipping, grinding, colouring, etc.
● Make-believe stone effects: Such effects replicate sensory qualities of stones, with and without the use of stone ingredients.
When stones are the constituent, these are used as fillers, or to take advantage of only one or few aspects of sensory qualities.
Stones may be used for texture, but coloured differently, patterns are masked by another design, feel is altered by a treatment or applique material, or water absorption is curtailed.
Similar make-believe effects are also created without the use of stone ingredients in any manner. Materials used for textural effects are mainly polyurethane-based polymeric compounds, which are hazardous in production, ‘life-cycle’ usage and disposal.
Wall papers, tiles, cladding panels, paper laminates, furnishing fabrics, textured painting compositions, etc. emulate real stone like effects. Palladio was one architect who extensively used stone like effects for building’s decorations from ceramics, and stones like masonry surfaces through plaster renderings.
● Plaster-based renderings are often created using tools and applicators’ skills. These are comparatively benign effects. However, such craft-applications over an extensive surface are not uniform.
● Effecting other materials’ through stones: These effects emulate other materials, but through stones as the ingredients. The stones are used for their structural properties, and physical characteristics, like grain form and size, low cost and easy availability.
● Dreaming wildly: The Opportunities of Intervention for stones are of following types:
Stones with other earth-based materials
Stones with natural organic materials: such as plants
Stones with man-made materials such as Ceramics, Metals, Polymers (plastics and elastomers)
Stones alone: Stones present 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 often 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 with other earth-based materials: 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 electro-magnetic 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.
Stones with natural organic materials: 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 wood, 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 with man-made materials: 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.
The inspiration arrives 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 matrix in composite forming?
A material technologist may further classify Opportunities of Interventions in terms of stones with:
Primary processed materials:
A brick manufactured from a soil or a fabric created from cotton or wool is primary processing
Secondary processed materials and composites:
A nitro-cellulose produced from natural material such as cotton is primary but a film out of it is secondary processing. Metal refined from ore and converted into an alloy is secondary processing.
Tertiary processed materials or multi material synthetics:
A monomer produced from petroleum is primary, a polymer out of it is secondary, a fibre spun out of a polymer is tertiary, and fibres fused as a layered sheet or used as a reinforcement make it multi-lateral processing.
Molecular level build-up of materials or sub-nano technologies: These are completely different methods of creating materials combinations. Here materials are created at molecular level combinations or molecules are implanted over a base. Many pharmaceutical drugs, chemicals, stem cell technologies are based on such works.