MATERIAL PROCESSES

MATERIAL PROCESSES

Surface Finish materials result from natural causes and human effort. Materials produced by natural causes are occasional, usually very slowly formed and with no quality control. Just the same, nature often presents spectacular and impossible to duplicate finishes. As natural finishes are rare and available in very limited quantity, some degree of quality generalization and equalization are required. Natural finishes require, minor to substantial modifications, such as physical conversions like cutting, dressing and polishing, to chemical alterations such as baking, bleaching and sintering. Materials modified out of the natural raw materials retain many of their original characteristics. Manufactured or man-made materials as raw materials have their own appearance, or after secondary processing, acquire a designed form and facet.

Since prehistoric times certain finishes have been collectively associated with specific materials, tools and techniques employed. A new product results with a variation in any one of these parameters. Materials provide vast options in their natural, processed and manufactured forms. Tools extend human limb abilities and also endow new capacities, so are continuously improvised through learning. Techniques of employing tools and processes relate to quality refinement, efficiency and productivity through scheduling, task management, planning, all leading to some form of automation.

Tools and techniques together create rational manufacturing processes, a strategy of for replicating specific end results. A manufacturing process could be very personal initially, but gradually spreads across the locality, and then becomes universal. Personal manufacturing processes reflect the personal skills of the originator or the inheritor of the knowledge. Craft-products as a result are very individualistic, local and ethnic. Industrial products on the other hand are universal, unless patented or copyright protected by the inventor. Industrial products come with assurance of consistency.

Creation of a surface finish is a cumulative consequence of all, the Processes, Techniques and Tools. There are many types of processes, techniques and tools. Some are more commonly associated with specific materials or end products, but many are common, i.e. applicable to many different materials and towards variety of end products. The difference between a process and a technique may not seem very obvious, unless when either of them, are specific to a material or end product.

Processes for processing materials for specific end product, e.g. surface finishes

Techniques for materials, using tools, etc. for surface finishing

Tools Hand tools

Power tools

Here we discuss about Material Processes.

PROCESSES:

Processing of Materials: Materials are processed with following three main objectives:

1 Shape and size formation

2 Alteration and induction of properties

3 Endowing finishes

1 Shape and size formation:

Body forming processes change the dimensional format of the material through phase-change of the material (solid to liquid to gas, or vice versa), within the same material phase (heat treatments, hot and cold working, and through mechanical work like mixing, stirring), and through material compositions (structuring, assembly).

Grains or dust mixed with a binder material can be cast into solids, or melted-fused as alloy, or sintered to a ceramic. Solids can be rolled into sheets or melt-spun into fibers, blown to powders, or drawn into wires. Fibres and wires are woven into fabric sheets.

Shaping processes can be categorized as:

Solid forming: Forging, beating, pressing, rolling, drawing, casting, extrusion, moulding.

Sheet forming: Bending, punching, stamping, cutting, seaming, forming, moulding.

Linear forming: Drawing, spinning, entwining, weaving.

Grain forming: Blowing or granulation, chopping, grinding.

These processes can also be classed as:

1a Flow processes: casting, moulding, extruding, drawing, rolling, forging, hammering, beating, powder-technology, material deposition, stamping, punching, pressing, bending, folding, seaming.

1b Additive processes: lamination, crystal growth, foaming (lower phase material dispersion in a higher phase material), plating, cladding, mounting, joining, material deposition, fabrication, joining, supporting, holding, keying, positioning, plugging, arranging.

1c Reducing processes: cutting, chopping, sizing, splicing, scooping, drilling, boring, machining, planning, chiselling, engraving, etching, de-layering, chipping, grinding, rubbing, sand blasting, cleaning, washing, melting, dissolving.

1d Other forming processes: spinning, weaving, knitting.

2 Alteration and Induction of properties:

Alteration and Induction of properties cause a change in the engineering or structural quality of the material, frequently accompanied by the modification of the surface characteristics. The alteration processes are also designed to regain the lost or the reduced properties during other processing.

2a Heat treatments: boiling, liquidizing, melting, softening, sintering, baking, drying, fusing, welding, soldering, forging, annealing, heating, hardening, crystal forming, blowing.

2b Non heat treatments: magnetizing, static discharging, infection proofing, cleaning, washing, earthing, wetting, drying, stretching, strenting, stressing, compressing, tensiling, twisting, filling.

2c Material applications: coatings, depositions, claddings, panelling, enamelling, inlaying, gilding, printing, moisture proofing, fuming.

2d Non material processes: embossing, engraving, polishing, burring, charring, burnishing, chasing, buffing.

2e Chemical treatments: dyeing, bleaching, etching.

3 Endowing finishes:

Finishes are provided: By altering the surface properties of the materials at a raw material stage and also after formation of the object, By reforming the objects, and By applying other materials at a raw material stage or at an object formation stage.

The need for a peculiar finish could be varied, but essentially for:

Imparting specific sensorial characteristics, for survival of the object in an environment, for changing the structural properties, as an aid in material processing and forming operations, for storage and handling of the raw materials or components, and for efficient operation of the system.

Processes for Natural Surface Finishes: Natural Finishes result due to many different factors, such as:

Elemental conditions of formation, subsequent responses like weathering, cognitive affectations, and later, natural or man-made interventions (angle of cut, tools and techniques used, etc.).

Natural surface finishes have three main cognitive affectations: Colour, Pattern and Texture. The colours are of original formation, subsequent weathering, readjustment of stresses, or induced by physical and chemical changes. The patterns result from the stresses, mixing of constituents, weathering, and the varied reactivity of different parts and constituents. Patterns also result from granular or fibrous orientation, method of cut, cyclic nature of growth, formation of residual products, deposition of contaminants, and tools-techniques of handling and processing. Textures primarily result from the degree of homogeneity, angle of cut, differential weathering, and various formative processes.

Processes for Manufactured Surface Finishes

Manufactured Surface Finishes result at three levels: Raw material stage, Product formation stage, and later, through Application of surface finish on Completion of the system. In an integrated production setup all three could be a single stage or plant process, but in most other fabrication shops only the last two processes are combined. For a site fabricated systems like buildings, the last process, i.e. surface finish application, is a distinct process as it is carried out at a site. Manufactured surface finishes as a result are of two categories: Plant based and Site-based systems. Though lot of preparatory work may occur in the industrial plant for the site-based surface finish application. Products fully surface finished in industrial plants require very careful handling (transportation, loading, storage, delivery and positioning), and so may carry protective but removable coatings or shields.

Make-believe or pseudo finishes: These finishes are of two basic types:

1 Surface finishes that duplicate the natural materials, such as: wood figure or texture effects in polymers, stone like effects in ceramics, cotton like fabrics made out of polyester, or synthetic gems and diamonds.

2 Surface finishes that copy the effects of other manufactured finishes, such as: chrome polymers for metals, white metal ornaments for silver, or acrylic for glass. Future possibilities include flexible glass, elastomeric metals, polymers with programmable colouring or texture forming system, metals lighter than ceramics or polymers, biodegradable ceramics, etc.

Surface finishes can be considered as:

Inherent Finishes

Applied Finishes

Integrated Finishes

PROCESSES FOR SURFACE FINISHES:

Materials have some surface finish qualities at a raw material stage, and these are either maintained or changed during the conversion to a product. Shaping of solid materials is done primarily to reformat the shape, and to convert them into a planer or linear forms, but surface finishes begin to evident at this stage.

 

Hammering or Beating: This is a very ancient process for shaping materials. Chipping by hammering stones to shape sharp edges was perhaps the most primitive of all processes. Hammering was also used to grind food stuffs, ceramic forming raw materials and metal ores. Even today hammering and beating, are mother processes for surface finishing.

Hammering was used to flatten-out the natural pure nodules of copper, and shape them into ornaments, tools and utensils. Hammering a copper nodule made it brittle, but frequent heating and graduated cooling -annealing made the mass ductile. Similarly sudden cooling by quenching in oil or water, caused surface hardening of the metal. Annealed and surface hardened materials had not only different structural properties but also had special surface qualities, (e.g. colour). Beating was also carried out to grind, and wet-mix materials, such as for ceramics.

Forging: The ancient process of Hammering to shape a material is now a rational process. Forging is a process of shaping iron and other malleable metals by hammering or pressing them after making them plastic by application of heat. Forging not only provides a desired form, but also refines the grain size and arrangement and by that improves the structure of the metal. Forged metal is stronger and more ductile than cast metal, and exhibits greater resistance to fatigue and impact. Forging is also used to compact materials by removing gases and by packing the cavities.

There are six basic Types of Forgings:

Upsetting: Decreasing the length and increasing the diameter of the metal.

Swaging: Decreasing the diameter of the metal on concave tools called swages.

Bending: It is done by hammering the work around a form or by levering it against a support.

Weld forging: It joins two pieces of metals together by hammering them together at high temperature with the help of a flux such as a borax.

Punching: It forms small depressions or openings in the metal by a punch of the proper shape on a base of a ring-shaped piece of metal.

Cutting: It cuts out large holes, apertures or niches in the metal with heavy, sharp chisels.

Forging can also be categorized, depending on the equipment used in the process.

Hand Forging: Sometimes called smithing, or black smithing is the simplest and oldest of all forging techniques. It uses a hammer and a beating block, called an anvil. An anvil has slightly tapered or a convex surface that allows precise hammering angles. Often concave shaped tools like swages are used to cover or hold the material (to restrict the flow of metal). A drop-forging uses force of gravity to drop heavy weight over the work piece instead of hammering.

Machine Forging: various types of machine-powered hammers or presses are used instead of a hand-held sledgehammer. These machines provide, heavy and rapid blows for production of large size and high quality objects. Forging-compression is obtained through the entire piece, compared to the hand-forging where mainly the surface is deformed. Drop forging or Impact-die forging involves placing a ductile, or heat softened metal in a single die or between two shaping dies. The upper of these dies, is dropped onto the lower one forcing the heated metal into the shaped die cavities, as in coin-making. For reducing part of a piece of metal stock to a predetermined size, forging rolls are used. These consist of a pair of grooved, cam-shaped rollers through which the metal is passed. Machine-forging operations are frequently accomplished by use of a series of dies mounted on the same press or hammer. The dies are arranged in sequence to form the finished forging in a series of steps. After the piece has been partially formed by one stroke, it is moved to the next die for further shaping on the next stroke.

Shaping by Rolling: Materials are shaped by passing through rollers. Rolling not only compresses, levels and stretches the material but it can emboss patterns (e.g. checkered aluminium plates of bus floors) and textures, cut shapes, polish the surface. Hot rolling, joins or fuses the layers of materials. Hot and cold rolling of metal sheets are done to provide specific surface qualities.

Shaping by Drawing and Stretching: Materials are drawn out through apertures or orifices to reduce the cross sectional shape and size, but elongate its length dimension. Wires, seamless pipes and filaments are drawn products. Drawing is often accompanied by twisting-spinning, entwining, thread or rope forming, etc. Extrusion of plastic is also a drawing process. Opposite to this, materials are stretched to increase their sectional shape and size, often slightly decreasing the length dimension. During drawing and stretching constituents like molecules, grains and fibers get rationally realigned, providing different surface qualities. Drawing and stretching, are also carried out to mix new ingredients such as colours, liquids, plasticizers, lubricants, etc.

Fabrics, Paper, Leather, Glass, Elastomers (rubbers), Polymers, Filaments, etc. are some of the materials receiving specific surface qualities (creased-crinkled, corrugations, gloss, matt, hardness, softness, evenness, etc.) through Drawing and Stretching processes. Most of the commercial metal forms (annealed, surface-hardened, ribbed, fluted and corrugations) are produced through such processes.

Shaping by Shear cutting, Stamping (die forming), Punching: In spite of simpler options these processes are used to achieve special surface qualities. Ordinary cutting by toothed saw leaves cut marks but shearing provides a smooth cut. Ruptures can be performed on materials with substantial stiffness (density) but softer materials shear cutting is required. Stamping allows stretching the material into a shape cast. The quality of a surface is determined by: nature of die surface, lubricants, force, nature of material (cold or hot workable, for stainless steel austenitic or martensitic), etc. Punching is faster than drilling, but stresses the edges of the hole rim.

Bending and Folding: are processes used for shaping ductile materials. Bending and folding can be done by controlled hammering, but today mechanical systems are used. Edge shaped tools provide linear shaping (almirah -cupboard shelves), solid shaped-ends create shaped deformations (spoons, frying pans, automobile bodies), balls are used for forming curved shapes (concave-convex as in wok or chemical vessels bottoms). Bending and folding operations are also used for shear-cutting. Bending relates to smooth curving whereas, folding relates to a sharp or angular turning. In Sheet metal fabrication bending and folding generally increases the planner stiffness, as in AC ducts, cans and tins, barrels, corrugated sheets, purlins, pipe etc.

 TECHNIQUES

SURFACE FINISHING TECHNIQUES:

Many techniques of achieving surface finishes in use today are essentially the same as those employed in ancient times. These have been refined in terms of the tools used and rationalized in terms of procedures. Many processes are now highly mechanised saving time and energy, and some even are fully automatized, using robotics that allows faster, accurate and safer production.

Some important techniques of associated with surface finishes are briefly discussed in following four groups.

1 Techniques for surface finishing objects' own material.

2 Techniques for surface finishing with foreign materials.

3 Techniques of material deposition for surface finishing.

4 Surface modification techniques.

1 TECHNIQUES FOR SURFACE FINISHING OBJECTS' OWN MATERIAL.

Cutting: Cutting is the oldest of all techniques. First cutting edges were made of sharp edged flint stone chips. Cutting stone chips were tied to wood or bone handles which not only improved tool holding, but leveraged greater force. Cutting Axes were reasonable for medium strength materials like wood, but for harder materials like stone, cutting and beating separated into hammer and chisel (for tools: see next chapter). Cutting is used for fast division of materials and quicker removal of parts of materials (skinning, debarking, chopping, mining). Cutting is the crude but primary technique of material processing.

Carving: Carving is a controlled cutting technique requiring use of a chisel and only occasionally pounding by a hammer. Carving is associated with fine but soft grain materials like ivory, horn, bones and wood. Carving is also done to pliable metals like copper, silver-based alloys, and also hard but fracture-able materials like stones.

Engraving: Engraving is a delicate and shallower material removal technique than carving, using a chisel or sharp pointed tools with hand pressure, or very light pounding of the hammer. Engraving is today done by fine rotary tools similar to the dentist's pneumatic drill. A computer controlled, diamond bit engraving is now done to ultra thin materials also.

In Intaglio, or Gravure, printing, the image to be printed is etched or incised into the surface of the printing plate or cylinder. Gramophone records have engraved and embossed grooves.

Chasing: For chasing, the material is depressed or displaced by a fine tool as dots, small length linear-strokes or in continuous linear patterns. Wet ceramic pieces, and plastered-surfaces are patterns rendered by chasing. Braille writing on a thick paper sheet is a form of chasing.

Engraving and chasing techniques are frequently used to provide a matt finish, onto normally very glossy stainless steel surfaces. Chasing techniques are also used for relieving as well as introducing stresses at the surface section, allow moisture and heat transfer, and improve the ductility of the material.

Embossing: Embossing introduces a texture through pounding, beating or by pressing of the surface. The pressure may be applied from one face or both faces of a sheet, locally as spots, or continuously under a plate or roller, creating repeat patterns or random designs. Pounding or beating, compacts the surface-sections of the material, and thereby increasing its density and integrity. Embossing techniques are used to reduce the gloss by matting the surface. Embossing is done to increase the thickness bulk of very thin surface materials and make them apparently stiffer. Synthetic fabrics and fibers are emboss-deformed and permanently set (perma-set and texturizing) through heat or chemical treatment.

Repousse: Repousse is a method of raising a design in relief from the reverse side. The design is first drawn on the surface of the metal and the motifs outlined with a tracer, which transfers the essential parts of the drawing to the back of the plate. The plate is then embedded face down in an asphalt block and the portions to be raised are hammered down into the soft asphalt. Next the plate is removed and re embedded with the face uppermost. The hammering is continued, this time forcing the background of the design into the asphalt. By a series of hammering and re embedding, followed finally by chasing, the metal sheet attains the finished appearance. There are three essential types of tools used: -for tracing, -for bossing, -for chasing.

Ornaments in relief are also produced by mechanical means. A thin, pliable sheet of metal is pressed into moulds, between set of dies, or over the stamps. Embossed utensils of copper and brass, statuettes of gods formed of thin silver and gold plates are very much part of every Indian house. Today Aluminium craft pieces are similarly embossed and black anodized.

Traditional Indian Brass and copper utensils have hammered finish on the outside. The same is often copied on aluminium utensils but reducing the strength due to 'cold working' of the metal. Leather and paper surfaces are rolled embossed to create textured patterns. Timber veneered surfaces are pressed for texture creation. Rendering of a wet plaster face by variety of pressing and chasing tools is very common. Chasing is very common with copper and brass pots (e.g. Peshwai Lotas and glasses).

Matting and Etching: These are mainly used for creating textured surfaces. Matting is generally a mechanical technique compared to etching where a chemically active substance is used. Mating and etching, are also achieved by metal removal processes (reversing the metal deposition by changing the cathode charge) in the final stage of plating.

Parallel, crossed, irregular, concentric, circular and other geometric configurations are carved or embossed on the surface. Line and spacing between them are often less than 1/100 part of a millimetre, depending on the compactness of the material mass. Another method of surface decoration is to impress it with repeating patterns of hatched lines ( used on precious metals), thus matting or breaking up areas to contrast with other areas left polished and reflective.

Etching: Etching is usually done by an active substance that will either eat away part of the surface or change its colour quality. Acid and alkali treatments also provide etched surfaces. Etching is also an artwork technique (see: chapter: Coatings) Glass surfaces are etched with Hydrofluoric acid.

Surface levelling: Surface levelling is a major field of surface finishing. Surfaces are levelled by chipping away very thin sections off the surface. The material must have layered formation (e.g. Kotah -ladi stones, bamboo, cane) or fracturable or brittle constitution (e.g. stones). Surfaces are ground and polished for a levelling.

Grinding: Grinding removes material from the surface to roughen a normally glossy surface like glass, or polishes a rough surface like stone. Grinding requires material of higher hardness than the surface material, and is done by rubbing down with a graded series of coarse to fine abrasives, such as carborundum, sandstone, emery, pumice, sand, glass and diamond powders. Where a material constitution permits, very fine grinding may polish the surface. Grinding is a cutting operation in which each grit that comes in contact with the material cuts out a minute chip, or swarf.

Grinding wheels usually consist of particles of a synthetic abrasive, such as silicon carbide or aluminium oxide, mixed with a vitrified or resinoid bonding material. Grinding can be coarse or fine, depending on the size of the grit used in the grinding wheel. Metal and glass can be ground to a mirror finish and an accuracy of 0.0000025 cm. Abrasive are used as grinding wheels, sandpapers, honing stones, polish, cutoff wheels, tumbling and vibratory mass-finishing media, sandblasting, pulp-stones, ball mills, and many other tools and products.

Stone surfaces are chiselled to split the material into thinner sections, to remove the weathered crust and also to level out the surface. Grinding and polishing is done to: Hard materials such as building stones, marbles, granites, metals, glass; Precious and Semiprecious stones like gems, diamonds; Animal products like ivory, bones, horns, teeth, leather; Plant products like timbers, seeds; Ceramics like pottery products, bricks, cement concrete and other cement products.

Polishing, Honing, Lapping, Buffing: Polishing uses extremely fine abrasive substances, such as jewellers rouge, Tripoli, whiting, putty powder and emery dust to rub or burnish an extremely smooth and brilliant finish on the surface of a material. The polishing materials are coated on the surface of cloth, felt, leather, rubber pr polymer wheels or as belts. Metal surfaces are levelled and finished by honing and lapping. Honing removes less than 0.0125 millimetres of material from the surface to eliminate micro scratches and machine marks from ground machine parts. It is done with bonded abrasive sticks or stones that are mounted in a honing head. Lapping is a process in which a soft cloth (wool, linen and chamois-leather) impregnated with abrasive pastes (rubbing compounds), is rubbed against the surface of a workpiece. Honing and lapping, are essentially metal finishing techniques. Buffing is a term used for polishing of metals. Buffing is done with polishing compounds and brushes of various shapes, and materials, like: (animal hair, synthetic fibers, plant fibers -coir), flex, wool and leather. Barber polishing the razor on a leather stripe is a buffing process that levels out small nicks on the blade.

There are four types of precision grinding machines: Center-type grinders used for tiny valve spools to steel mill roles. Center-less grinders used for bowling balls, surgical sutures, and tapered roller bearings. Internal grinders are employed for inside diameters of gears, bearing races, and similar parts. Surface grinders are used for die tops, bench surfaces.

Lapping is used to produce a high-quality surface finish or to finish a workpiece within close size limits. Dimensional tolerances of 0.00005 millimetres can be achieved in the hand or machine lapping of precision parts such as gauges or gauge blocks.

Shaving and Splitting: Shaving is done to remove material's components such as outward hair or fibers, layers, etc. Leather surfaces are shaved for thinning and to remove the surface hair. Leathers are also surface-split to separate leather suitable for uppers and soles. The palm leaves are shaved to remove the stems and make them smoother for writing. Tree-barks are removed by axes and choppers to retard insect attack and increase moisture removal. Timbers are re-cut or planned with finer tools to achieve a smoother surface. Timbers are split very finely to create veneers. Wood planning is also a shaving technique. Carpets and rugs require close shearing by scissors to shave of protruding fibers.

Burnishing: Burnishing is controlled burning (or a heat treatment) at the surface section to remove part of the material and to change the colour or texture properties of the surface. Burnishing is both a process of surface finish and surface cleaning. Most of the organic materials can be surface-treated directly with fire or indirectly with high heat to achieve a burnished or ironed effect. Textiles, paper, leather, leaves, wood, etc. are some materials that can be burnished. High temperature burnishing removes the surface fibers and hair, and chars or burns (sinter) the top part of the surface, creating a burnt colour + texture effect. Textiles are Ironed, i.e. de creased or perma-set, i.e. creased with pressurized heat treatment. Synthetics or composite textiles are selectively or locally burnished to fuse the fibers or filaments, create texturized effects and also alter the transparency, opacity, etc. Wood surfaces on burnishing, creates a dehydrated or an old shrivelled or shrunk surface, similar to an old wood. Metal surfaces also burnished not only to harden or anneal the top surface but to burn the oily residues, dehydrate, and descale the surface. Burnished metal surfaces often attain peculiar colour and pattern effects.

2 TECHNIQUES FOR SURFACE FINISHING WITH FOREIGN MATERIALS.

Surface finishing or decorating with a foreign material is a very ancient technology. Some important and traditional methods are described here. Other Material relevant techniques are dealt in chapters on Materials.

Damascening: Damascening is a technique of encrusting gold, silver or copper wire on the surface of iron, steel, or bronze objects. The metal base surface is finely chased or engraved with a sharp tool. The decorative metal thread is forced into the minute grooves by hammering.

Niello: Niello is made by fusing together silver, copper and lead, and then mixing the molten alloy with sulfur. The black product is powdered. Chased or the engraved metal surface is wetted with a flux compound and black powder is spread on it. On heating the niello melts and runs into the depressions or channels. Excess niello is scraped and the surface is polished, giving a dual metallic effect or pattern.

Granulation: Granulation is mainly used for gold jewellery. In granulation, beads of gold are soldered onto gold surfaces. Etruscans produced such jewellery in 5^th BC. The beads were minute and provided an effect of a bloom to the gold surface, rather than of a beaded surface.

Filigree: Filigree can be made of either gold or silver. Open-work patterns are worked from minute wires or cables made of two or three gold or silver wires twined or braided together. In the 16^th and 17^th C. filigree was extremely popular for decorating vases and drinking vessels, especially in Italy and Germany, and in the 18th- and 19^th AD in South America. In Russian and Scandinavian countries filigree has survived as a provincial craft and is used for boxes, mirror cases, and peasant jewellery. It is obviously very delicate and fragile work and, except jewellery, usually has a backing material. In India, Orissa is the main centre for filigree work in India.

Ajouré: Ajouré are achieved by cutting or piercing patterns in the metal. Raised patterns were also affixed by soldering small castings or cut out motifs onto a flat surface.

Embellishments with Other Materials: These include fixing or embedding precious and semiprecious gemstones, enamels, a variety of exotic substances such as rare woods, metals, ivory, horn, beads, sea shells, jade, and amber, and niello (a black finish on silverware) into chased cavities, heat or solvent softened materials. Fixing is also done by wire or thread knitting and knotting (Kutch Mirror work and embroidery on fabrics). In ancient times ceremonial furnishings were almost as exotically decorated as personal jewellery and cult implements.

Inlay: Inlay works are of many varieties, with metal into wood, stones and metal, and wood into wood, ivory into wood, more recently high grade plastics into wood, metal, and plastics, glass beads into ceramics, etc. Floral patterns and scripts on Taj Mahal are examples of Marble inlay work. Inlay work involves incising a pattern's shape and filling it up with a cutout of material to be laid in. The fixing is done with tight fitting, adhesives, or by hammering a ductile metal.

Gilding: is the art of decorating wood, metal, plaster, stone, glass, or other objects with a covering or design of gold in leaf or powder form. The term also covers similar application of silver, palladium, aluminum, and copper alloys. Thin sheets of gold and silver are beaten in leather sheet folders to create leaves (foils) as thin as 0.00001 millimetre. After being cut to a standard 90 to 100mm square, the leaves are packed between sheets of tissue-paper as small books, ready for the gilder's use. Gilding by gold or silver sheets requires as no adhesives as sufficient electrical charges attract the foil to the base, however for permanent fixing (exterior use) some form of adhesives are used. To day commercial guilders used aluminum powders in a variety of metallic shades such aluminium in different shades of oxidation, bronze, copper, gold, etc. These powders are dry sprayed on an adhesive or varnish-covered surface or mixed with a carrier varnish or lacquer.

Overlays: Overlays use slightly heavier sheets of metal than gilding, otherwise it is the same as gilding. Egyptian mummy cases and furniture were gold covered. The Chinese ornamented wood, pottery, and textiles with designs in gold. The Greeks not only gilded wood, masonry, and marble sculpture but also fire-gilded metal by applying a gold amalgam (gold+mercury) to it and removing the mercury with heat, leaving a coating of gold on the metal surface. The Romans acquired from the Greeks the art, and covered their temples and palaces with gold. Jain and Buddhist statues are covered by gold and silver foils and sheets. Ancient gilding shows that gold was applied to a ground prepared with chalk or marble dust and an animal size or glue. Today lacquers, epoxies and rubber-based adhesives are used to fix the foils and sheets. Temple statues are decorated by gold foil without any adhesive, by the electric (ion) charge, which makes it removable it. Silver gilding gets tarnished in moist weather. Gilding requires careful surface preparation. To day flat paints, lacquers, or sealing glues are used, depending on the nature of the substrate. Metals surfaces prone to corrosion may be primed (and protected) by red lead or iron oxide coatings. The area to be gilded is covered with an adhesive. When it has dried enough so that it just adheres to the fingertips, it is ready to receive and retain the gold leaf or powder. After fixing the foil, it is rubbed gently (burnishing) with a dry cotton swab, to achieve high luster. Other materials for burnishing include agate. Loose bits of gold, or skewing are removed from the finished work with a camel's hair brush.

Enamelling: Enamelling is technique of providing a lustrous finish on any surface such as metal, ceramic or wood. A vitreous paste consisting of mixture of silica (from quartz or sand), soda or potash, and lead, is deposited on to metal objects such as jewellery, small metal boxes, utensils, ceramics or glass, and fused by heat. A resultant surface is chemically identical to glass or highly vitrified ceramic. The ingredients are made opaque and coloured by the addition of other metallic oxides. Enamel work is also known as Minakari in jewellery field. Enamel finishes were very popular as coatings for steel items when alternative rust-free materials like aluminium and stainless steel, were not available. So hospital-ware such as gandy, urine pot, kidney tray, instruments' tray, camping-ware like tumbler, bowls, dishes, and decorative items like ceiling panels, signboards, watch or clock panels, etc. were made with enamelling.

Bombay suburban trains' stations and street' name boards were created by ceramic enamelling on wrought iron plates (slightly puffed-embossed in the centre). Similar Multi coloured enamelled ceiling plates were used in many rich homes in Europe and India. Some such plates still survive after 100 to 175 years.

Five main types of enamelling are in use: Champlevé, Cloisonne, Basse-taille, Plique-à-jour, and Encrusted.

Champlevé (French= raised field): Champlevé enamels are done by scratching or etching a metal surface, usually copper, leaving hollows or troughs with raised lines between them. The hollows are filled with pulverized enamel that is then fired. The hard-finished enamel is subsequently filed down until the glossy surface and the metal surface can be polished simultaneously, with crocus powder and jewellers rouge.

Cloisonne (French= partitioned): In the cloisonne process, very small partitions, or cloisonne, consisting of thin metal strips are built up on the surface of the metal. They may describe a pattern and are fixed to the surface by solder or the enamel itself. The partitions are filled with pulverized enamel, and the subsequent procedure is the same as for champleve. The Cloisonne technique is usually applied to silver, although gold or copper may also be used as bases.

Basse-Taille (French= low cutting): The Basse-taille process is a kind of champlevé but is applied to silver or gold. The metal is engraved or hammered to various depths according to the design. The depressions are then filled with translucent enamel, through which the design beneath it can be seen.

Plique-à-jour (French=open braids): Plique-à-jour enamelling resembles cloisonne, but differs from it in that the partitions are soldered to each other rather than to the metal base, which is removed after firing. The remaining shell of translucent enamel gives the effect of stained glass. Because it has no metal base, Plique-à-jour enamel is exceptionally fragile.

Encrusted Enamel: Encrusted enamel or enamel en ronde bosse is prepared by spreading of an opaque enamel paste over slightly roughened surfaces of objects such as small figures.

Painted Enamel Painted enamels resemble small oil paintings. These are made traditionally on a metal plaque covered with a layer of white enamel and fired. The design, in coloured enamels, is then applied on the white ground, by painting, spraying, screen printing, or block printing. A separate firing may be required for each colour because each may fuse at a different temperature.

(Techniques of art work painting and other coatings are covered in Chapter: Coatings) (Techniques of knitting etc. covered in Chapter: Fabric Crafts) (Metal related techniques are further detailed in Chapter: Metal Crafts, and Chapter: Metal Treatments) (Wood related techniques are further detailed in Chapter: Wood crafts).

3 TECHNIQUES OF MATERIAL DEPOSITION FOR SURFACE FINISHING.

Material Depositions: There are many techniques of depositing materials. These techniques implant or deposit a material or combination of materials onto a substrate to make it an integrated surface. This is done without the use of mechanical joining or adhesive fixing. Chemical reactions, if any are only at the surface level.

Form of materials to be deposited: Metals, metalloids, alloys, ceramics, polymers, composites, and other material compounds in intermediate or a nascent stage.

Phase Stage of the material to be deposited: Gas, Liquid, Solid or Plasma.

State of the material to be deposited: Powders, granules, solids, liquids, plasma, gases and vapours, molecules and ions.

Production Status of the recipient object: Raw material, partially formed, fully composed objects, or operative systems.

Form of the recipient object: The recipient object and material to be deposited are of vastly different forms, such as: Ceramics deposited with metals or polymers, Metals loaded with alloying metals, metalloids, ceramics, composites, polymers, etc., Polymers receiving depositions of metals, metalloids, ceramics and composites. Biotic materials like bones, skins, and other natural materials are deposited with ceramic materials.

Electroplating: This is a method of electrically depositing a metal or a mix of metals (as alloy) on conductive surfaces (metals), as well as on nonconductive materials such as plastics, wood, leather, etc.; after these have been rendered conductive by processes such as coating with graphite, conductive lacquer, etc. Metal alloy compounds that nominally cannot be produced can be alloyed and electroplated on a surface (e.g. tin-nickel compound).

Modern Electroplating started with the discovery of battery, sometime in 1800. It all started when it was seen that a nodule of copper deposited on a silver cathode could not be easily removed. The battery current was used to deposit the lead, copper, and silver. During the same period zinc, copper and silver were deposited on themselves, and on other electroplating worthy metals including gold and silver. Commercial scale electroplating began in 1840s when cyanide copper solution was discovered. Iron, cobalt, nickel, copper, zinc, ruthenium, rhodium, palladium, silver, cadmium, tin, iridium, platinum, gold, and lead are commonly used for plating. During 1925, introduction chromium plating changed the face of many automobile and household gadgets completely. Chromium plating provided a permanently glossy surface. Soon nickel-chromium or copper-nickel-chromium strengthened the plating industry.

Chrome plating on nonconductive materials is in use since 19^th C. However, in 1963 ABS plastic was found suitable for chrome plating. The plastic part is first etched chemically by dipping in a hot chromic acid-sulfuric acid mixture. It is then activated by dipping in stannous chloride solution and palladium chloride solution. It is then coated with an electro less copper or nickel before further plating.

Melt deposition: Melt deposition is a very broad term, and includes many different technologies of applying or depositing a material by melting through radiant heat, friction heat, pressure and solvents. Traditional processes use radiant heat, but today infra red, microwave, high velocity oxy-fuel-TIG, electron beam, laser and plasma systems are used for fast and controlled heat input. Pressure is caused by direct pressing, impact loading, pressure exerted through gas and liquid surroundings. Melt depositions are made in ambient environments, under pressure, in near vacuum conditions, electrically charged enclosures, and also in presence of inert gases and ionised materials.

Direct Material Deposition is one of the most common of methods to build up 3D objects, and also a method of surfacing. It uses feed from a wire or powder, which are melted before deposition. It, in a simple and economic way affords a very fast buildup, but causes distortions in the component. Melt deposition creates comparatively substantial (heavier-thicker) surface, or heavier-solid buildups. It is used to repair worn out surfaces, add features to an object, fill up cracks, level out the machined surfaces to a very fine finish. Metal deposition is also used in building up planner or solid prototypes.

Household tinning of brass and copperware is a metal deposition process. Gold and silver plating is a metal deposition process.

Deposition of the metal powders: Metal or an alloying-agent in the powder form, are sprayed through a nozzle that is coaxial to the CO₂ laser beam. The metal powder gets deposited on the molten mass on the object's surface. The process is fast and occurs on top surface section so does not affect the basic mass properties of the object.

Vapour Deposition Technologies: Vapour deposition technologies include processes that put materials into a vapour state via condensation, chemical reaction, or phase conversion. Manufacturers use these to alter the mechanical, electrical, thermal, optical, corrosion resistance, and wear properties of substrates. Vapour deposition technologies are used to form freestanding bodies, films, and fibers and to infiltrate fabric-forming composite materials. There are two main sub techniques: Physical vapour deposition and Chemical vapour deposition. In physical vapour deposition the object is placed in plasma (of active gas like nitrogen, oxygen, or methane) bombardment, whereas in the chemical vapour deposition, thermal energy heats the material to vaporise in a chamber, and initiates a deposition reaction. Physical vapour deposition involves dry vacuum deposition methods in which a coating is deposited over the entire object rather than in certain areas.

Metallizing: Metalizing is essentially a surface metal deposition technique used to change the appearance (gloss, colour, texture), to alter the surface properties (rust proofing, protective coating, spark or erosion proofing, wettability, conductivity, etc.), to fix or clad new materials, components, etc. Metalizing creates a very thin surface where inter particle boundaries are not very important. Though the density of particles spread is very critical.

Metallized Polyester films control the solar radiation. Gold Metallized glass, are used in windows of outer space vehicles to eliminate radiation penetration. Metallized films are used as mirrors. Metallized glass is used as mirror. Metallized films split into a very narrow-stripes are used in place of gold or silver Jari. Metalizing is also done through special mixtures -amalgam of gold or silver+ mercury in craft work such as gilding, Sankheda and Chinese lacquer work patterning.

Alternative Methods of Metal Deposition: Traditional methods of metal deposition such as electroplating (chromium, nickel, cadmium, copper) use toxic substances like cyanides that cause pollution problem. These alternative technologies include thermal spray coating, vapour deposition, and chemical vapour deposition.

Sputtering: Sputtering is used for etching and deposition of metals. In both instances it changes the physical properties of the surface. A gas plasma discharge is set up between two electrodes, a cathode plating material, and anode substrate. Positively charged gas ions are attracted to and accelerated into the cathode. The impact knocks the atoms off the cathode, which impact the anode substrate while coating it. A film forms as atoms adhere to the substrate. The deposits are thin, ranging from 0.00005 to 0.01 mm. The materials deposited are chromium, titanium, aluminum, copper, molybdenum, tungsten, gold, and silver. Three techniques for sputtering are available: Diode plasmas, RF diodes, and Magnetron-enhanced sputtering.

Sputter-deposited films are used in watch bands, eye glasses, and jewellery. The electronics industry uses sputtered coatings and films for thin film wiring on chips, recording heads, magnetic and magneto-optic recording media. Architectural glass, solar radiation control films, reflective films and food packaging films produced by sputtering.

Infiltration: Infiltration is a technique of filling in the cracks, pores and voids of various, micro to macro sizes. The infiltration is done with particulates of smaller size than the cavities that exist. Filling in of pores is caused by reaction with or deposition from a liquid or vapour. In the case of liquid reaction, the technique is called melt infiltration, and in the case of vapour phases, it is called chemical vapour infiltration. Many composites are formed by infiltration of the matrix forming compound, into the filler.

Typically in a cement concrete mass, the voids in the stone aggregates are filled in by sand, voids in sand granules are filled in by cement, voids in cement particles are filled in by water. Stone aggregates and sands stay deposited but cement and water react.

Laser Surface Alloying/Laser Cladding Lasers are very suitable for surface modifications. Lasers can soften or melt a surface mass for a controlled depth, in a localized spot and very quickly. Laser generating equipments operate to synchronize injection of feed material. One of many methods of laser surface alloying is laser cladding. In this, a thin layer of metal or ceramic, or powders thereof, are bonded with a base metal through heat and pressure. Materials that are easily oxidized are difficult to deposit without using inert gas streams and envelopes.

Thermal Spray Coatings: Coatings are sprayed from rod, wire or granules or powder stock. The molten stock is heat melted and atomized by a high-velocity stream of compressed air or other gases, and sprayed on the substrate. The substrate is often both, charged and heated.

Ceramic coatings by thermal spraying: Ceramic forming materials are heated fused and sprayed, or are deposited and allowed to form a ceramic surface.

Ceramic coated surfaces are tougher, and highly scratch resistant besides are non static charging and better wettable than chrome sprayed surfaces. Ceramic rolls are now replacing the chrome rolls for offset and such printing techniques.

Electric Arc Spraying: An electric arc is formed between two ends of coating material feeds (wires, etc.). They continuously melt at the arc point is blown by a jet of gas like air or nitrogen, as droplets towards the substrate.

Plasma Spraying: A DC current passing between a water-cooled copper anode and tungsten cathode forms an arc ionizing to form plasma. The plasma heats the powder coating to a molten state. Compressed gas propels the molten mass at very high speed towards the substrate. Materials for plasma spraying are: zinc, aluminum, copper alloys, tin, molybdenum, steels, and ceramics.

4 SURFACE MODIFICATION TECHNIQUES:

Patina forming: Patina is a rust layer formed on bronze objects, either naturally over a period of time or intentionally by chemical treatment. Sculptures exposed to different kinds of atmosphere or buried in soil or immersed in seawater for some time acquire attractive patina. Bronze can have a wide variety of green, brown, blue, and black patinas. Iron is sometimes allowed to rust until it acquires a satisfactory colour, and then the process is arrested by lacquering.

`Anodising: Anodising is a technique of creating an oxide film on a metal surface. The film is intended for purposes such as corrosion resistance, electrical insulation, thermal control, abrasion resistance, sealing, improving paint adhesion, and as a decorative finish. Anodizing-process consist of electrically depositing an oxide film from aqueous solution onto the surface of a metal, often aluminum, which serves as the anode in an electrolytic cell. Plate properties such as porousness, abrasion resistance, colour, and flexibility, depend on the type, concentration, and temperature of the electrolyte, the strength of the electrical current and the processing time, and the type of metal being plated. Dyes can be added into the oxidation process to achieve a coloured surface. Coloured anodised aluminum is used in gift ware, home appliances, and architectural windows sections and trimmings.

Carburizing: Carburizing is one of the oldest methods of surface hardening iron-based metals, next only to fast cooling by quenching in oil or water. Iron or steel products heated to a very high temperature are placed in a carbonaceous environment for long duration. The carbon diffuses into the surface of the object, making it harder. The depth of the carbon penetration depends on the exposure time and temperature. In gas carburizing the parts are heated in contact with such carbon-bearing gases such as carbon monoxide, carbon dioxide, methane, or propane.

Carbunitriding: The same process is used in Carbunitriding except that ammonia is added.and it takes place at lower temperatures that produce less distortion in the steel. Gears, ball and roller bearings, and piston pins are among the products made by carburizing.

Nitriding: These surface hardening technique uses utilizes nitrogen and heat. Cam shafts, fuel injection pumps, and valve stems are typically hardened by this process.

Flame hardening and induction hardening: in which high heat is applied for a short time, (by gas flame or high-frequency electric current, respectively) and then the steel is immediately quenched. These processes are used generally for larger implements.

Peening: Peening is mechanical technique of hardening the surface by rolling, hammering, drawing or hammering of the surface at temperatures that do not affect the composition of the steel.

Sintering: Sintering is a high pressure and high temperature baking process. It is used for production of alloys, ceramics and composites. Pressure increases the densification and decreases the heat requirement by nearly half the melting point of ceramic. High pressure or impact load helps in shape formation. Simple hot pressing does not allow forming of complex shapes. In another process, called hot isostatic pressing (HIP), a ceramic is pre sintered to squeeze out the porosity (so that interconnected pores are eliminated) or encapsulated with a viscous coating such as glass. The ceramic is further processed under a high pressure fluid such as argon or helium, and at high temperature, so that residual gases from the object bubble out. Very complex objects can be formed by this method.

For sintering conventional radiant heat input are too slow. For rapid heating two sources are used: Plasma and microwave. In plasma heating energized and ionized particles deposit large quantity of energy on the surfaces of the ceramic being sintered. Plasma sintering takes place in an ionized gas. In microwave sintering, electromagnetic radiations at microwave frequencies penetrate and deposit the heat in the interior of a sintering ceramic first rather than on the exterior surface. A combination of radiant and microwave heating can be used to obtain thorough heating of the object.

Glazing: Glazing is a very common term, used for many different processes and purposes. In building construction it means providing and fixing glass, including roof lights, clerestory windows, curtain wall constructions and as figured-glass. Glazing is also used in ceramics as a process that provide gloss and colour through a pre-firing coating of slip and salt spray during firing. Glazing is used as polishing or asa glaze achieving process by coating (wax, oil, lacquer, silicon, etc.) and rubbing or roll pressing a surface. In textile glazing as a term is used for a process that provides slight gloss with sealing of the surface by a starch or polymer sizing composition. The term glazing is used with paper in the same manner.