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.
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 5th
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 16th and 17th C. filigree was
extremely popular for decorating vases and drinking vessels, especially in Italy
and Germany, and in the 18th- and 19th 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-Ã -jour
(French=open braids): Plique-Ã -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-Ã -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 19th 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.
