Post 546  by Gautam Shah



Modern coating technology aims at two distinct targets, improvements in formulations and the technology of application. The formulations have been multipurpose as well as specific. Coating application methods primarily aim at high speed-pressure deposition of maximum proportion of solids, while minimizing the wastage due to over spray, drips etc. and economic use of VOCs (Volatile Organic Compounds). As a result, many combinations of formulation and application have been devised.

Ceramic coating over Metal as cutting edge Wikipedia image by Author SlonikkinolS

Coatings, traditionally were made of natural polymeric materials like starch, shellac, amber, cellulose, and rubbers. Man-made polymers have been known since 1832. Coatings of synthetic polymers provide better adherence and protection. Polymeric substances have been used in paints and other means of covering such as extrusion and dispersion. Such coverings (e.g. Teflon coating, PTFE, PFA) have heavier films than paints. Polymer covering materials replaced tin, chrome and cadmium plating in many instances. Polymer coating materials include alkyds, vinyls, acrylics, epoxies and polyurethane.


Non-polymeric coatings or coverings are created through following basic routes.

1 Galvanic deposition: A metallic compound migrates from a high potential cathode, to the object, that is an anode. The migration is at molecular level with the formation of an alloy or like substance at the receiving end.

2 Oxidation: An oxygen donor fluid or substance is encouraged or facilitated to migrate and form an oxide coating to protect the substrate.

3 Melt Deposition: Metal, Ceramic or Polymeric substances in the form of particles, flakes, fine grains or foils are sprayed or fixed to the substrate, and are melted or fluidized with the help of heat (thermic, sonar, radiation, friction), before, during or after the deposition. The fluidized material forms not only a coating but integrates itself into the substrate.

Metalized polymer yarn Wikipedia image by Author rexess


These techniques of material application are used in commercial processes known as:

  • Electrostatic coating
  • Electrophoretic Coatings
  • Anodization
  • Electrolysis of organic substances
  • Diffusion Coating
  • Electro-deposition
  • Spark hardening
  • Conversion and Conversion – diffusion coating
  • Chemical Vapour Deposition
  • Electro-less and electrolytic metal deposition
  • Welding as surface coating
  • Printing as coating
  • Air and airless spraying
  • Flame spraying
  • Metal sprayed, polymer, impregnated coatings
  • Detonation coating
  • Arc plasma spraying
  • Electric arc spraying
  • Electric harmonic spraying
  • Vacuum metalization of plastics and metals
  • Sputter deposition
  • Ion plating
  • Ion implantation
  • Microwave radiometry coatings


Electrostatic process based printing-coating Wikipedia image by Author Guruleninn

Air electrostatic processes employ air forces to atomize the liquid material and then charge and deposit these particles using electrostatic forces. Articles to be coated are introduced to a space or chamber which provides negative charged environment. The atomized coating droplet on collision with ionized air picks up the negative charge and as result is attracted to the grounded article (to be coated). In another method the spray gun nozzle has an attachment to charge the coating droplets. In such processes, because of attraction there is no over spray wastage and deposition is very secure. Such processes are readily applicable for coating with suitable electrical properties and for articles that are conductive (metals).

Road Coatings Author English: Cpl. Ryan R. Jackson

For architectural coatings, polymeric formulations rule the scene. As majority of such finishes are based on emulsification with water, requiring negligible-amounts of volatile organic solvents. For wood and metals, modified alkyds that need organic solvents are being replaced with solvent-less formulations like powder-coatings. The products manufacturing at plant level, however are adopting metalizing, surface alloying, ceramic coatings, sputter deposition, Ion plating and Ion implantation sputtering technologies.





Post 545  by Gautam Shah


Stairs and escalators are stepped and inclined-vertical transfer systems. Both provide uninterrupted transfer, unlike the modulated transfer by elevators. Traffic on stairs and escalators is restricted by the width of passage whereas the same on elevators is limited by the module size and its speed. Use of stairs requires some orthopaedic proficiency and cautious posturing, but automated escalators allow freedom to see around during the passage. Some flight of stairs can be used for accent and descent, but escalators require different sets. Though reverse escalator for descent may not be provided, as physically it is not very strenuous to climb down.

Streets of Cusco, Peru Wikipedia image by Author Rod Waddington from Kergunyah, Australia

All movements are essentially directional. A designated unidirectional system like an escalator is more efficient than a mixed movement system like a stair. This factor, however, is relevant for stairs with low to moderate traffic. There are several other transfer systems, such garbage chutes, emergency evacuation tubes, trunks or ramps, and fire-mans’ poles, where movement is unidirectional and generally downwards. The gravity accelerates the down-movement, and inclination retards the rate of passage.

Staircase and escalators at Cabot Circus shopping centre, Bristol, England Wikipedia image by Author Arpingstone

Stairs and escalators are point to point passageways, as there is no mid-way interference, except at landings. Mid-way disturbance occurs in comparatively flatter and very wide stairs over the mountain sides. Here, for ascent or descent, people cross the steps diagonally to increase the ‘tread-depth’ of the steps. Stairs generally have a pitch higher than ramps. Stairs are safer than ramps provided the person is fully mobile and orthopaedically fit, but to ascend or descend stairs are not as easy as the ramps. A ramp can have gradually variable pitch, but a stair has to have a one continuous grade of pitch.

The Grand Staircase of the RMS Olympic Wikipedia image

The inclination of steps is defined by the proportion tread versus riser of the steps. This could vary for stairs used for different purposes, ranging from steep ladders to flatter ramps like foot-ways. The dimensions of tread and riser are proportional and can be plotted on a hyperbola. Certain formulas also provide such proportions: 2T + R = 650 to 680 mm or R x T = 43000 to 45000. For steeper pitch the additional effort required to work against the gravity reduces the efficiency.

High pitch steps of ChetSingh Ghat Benaras India Wikipedia image by Author Patrick Barry from san francisco, ca, USA

Stairs have a pitch of not less than 17.30° (5:16), and of not more than of 48.30° (9:8). Below these limits it becomes a ramp or foot-way, and above it a ladder. A ladder is not a comfortable utility. Step-ladders are lower in pitch, less than 75° and require flat treads. Risers may be open or closed, for toe accommodation and handrails may or may not be provided. In the ladders’ class of stairs, some are easier to climb than others. Ladders are used for fire escapes, boiler rooms, fly galleries, attics, decks, etc. Rung ladders are pitched more steeply, above 75°, and have extremely narrow treads or round rungs to accommodate the foot. In certain cases, the space to accommodate the knee between steps may be necessary. Rung ladders usually do not require additional handrails as the side members of the ladder can be used for holding grips. Rung ladders are often caged for safety, though such cages are more useful for ascent then for descent. It is safer to climb down facing the ladder. Swimming pools, water tanks, and sewers have rung ladders. Manhole steps are very narrow in widths, but the width is otherwise compensated by its staggered placement. The same holds true, for climbers for bunk beds, whether in railways, buses, barracks or homes.

Alternating tread stairs Pinnacles National Park, California, USA Wikipedia image by Author Wing-Chi Poon

Minimum width required for low intensity unidirectional traffic is 600 mm, however most standards specify 900 as minimum width for escape in a hazardous situation. A two-way lane stair should be at least 11200 mm. Sufficient width space for movement is required at torso level, otherwise at feet level a minimum width of 250 mm is required. Where same step is to be used for placing either one of the feet, both, the step and passage widths of minimum width of 500 mm are necessary. Stairs less than 500 mm width are generally emergency stairs rarely to be used, or service stairs to be used by experienced persons. For single lane traffic 750 mm width is an accepted standard. Most of the building bylaws allow minimum 900 mm widths for private buildings. For public buildings a stair width of 1200 mm to 1500 mm is recommended. For pedestrian over bridges and other public thoroughfares, a stair width of 2400 is recommended. On public thoroughfares where traffic is totally segregated or is only one directional, the minimum stair width could be 1800 mm.

Tsarist soldiers march down the “Odessa Steps” from the Goskino film Battleship Potemkin (1925)

Same (filmed as above) Potemkin Stairs giant stairway in Odessa, Ukraine in 1990 Wikipedia image by Author Dezidor

Many stepping arrangements are used for emergency and special purposes. Simplest is a knotted rope or a rope ladder secured to a wall or column. In many countries older buildings were required to confirm to new bylaws, open iron stairways on the building’s exterior were placed. Open iron stairs, though are rendered useless by smoke from windows, so must be placed against a blank wall. One of the best fire escape stairs is a fully enclosed stairway in the building itself or in an adjoining tower. Uncoated or unprotected steel is highly hazardous during a fire as it expands and deforms the stairway. Wood though combustible catches fire slowly, and allows more escape time compared to an unprotected steel stair.

fire exit ladders in Soho. New York City 2005 Wikipedia image by Author Jorge Royan



Post 544  by Gautam Shah


Buildings are restored to retain and perpetuate select identities. The selection is a subjective process of a person, society or the political power (including invader or conqueror). The concept of restoration is comparatively easy as the retained identities seem to provide a link between the past and the present, whereas things that are ignored or removed, make the exercise easier, simpler and justifiable. The select identities are like values, utilitarian aspects, sensorial qualities, materials, technology, architectural character, spatial qualities, style, patterns, scale or proportions, antiquity, social-political-religious confirmation.

An early photograph of Stonehenge taken July 1877 Wikipedia image by Author Philip Rupert Acott

Pic from same position in 2008 showing the extent of reconstruction Wikipedia image by Author Mavratti

Buildings as affected by age, environment and human use become increasingly inefficient, irrelevant and unsafe. The environmental processes continue to change the fabric of the buildings. The human use, miss use and non-use, all reflects in the decline of the building. To terminate or arrest the affectations, several processes of change are required. Externally, the purposes with which a building was erected need a live sponsorship and continued participation of the society. The buildings’ location needs corroboration from its surrounding, which can be had through macro planning or re-validation of the functions. Internally a building is a visit-able entity or just relic, for both cases it should be stable and safe.

Partly-restored windows facing Pitched Stone Court, Raglan Castle in Monmouthshire Wikipedia image by Author Andy F

A restoration, cannot revert a building to its original condition. As there is no original condition of a building. What we perceive to day is an entwined mass of effects of age, environment and human use. Changes occur in time, and spatially across the fabric of the building. Buildings are nominally as well as intentionally altered, but these changes are never recorded. The evidence of previous conditions in absence of records is largely conjectural. A true restoration must faithfully use the original materials and technologies which however are unlikely to be available. The building loses its site related relevance and time related functionality over a period of time. One, cannot regress a building to a past state in isolation of its referential conditions.

Marcellus theater Rome

Theater of Marcellus Rome backside reconstruction

Medieval builders treated the works of antiquity as something to be extended. They knew that abandoning a well sited and a large sized building is a wastage of time. A large new building would take several generations, political stability and long period free of catastrophes and disasters. Restoration meant resurrecting a building in the shortest possible time, with an image or style of the time. Yet during resurrective restoration, there were several ‘corrections’, to suit the socio-political-financial conditions. Such a complex entity cannot have a particular or ‘original character’.

Serbian monastery Gracanica Wikipedia image by Author Bujar I Gashi

From ancient times to first part of present century, restorations have always followed the sponsor’s wishes and restorer’s wisdom. Restorations have meant all types of changes, such as renovations, alterations, reformations, additions, and extensions, but rarely the ‘reinstatement of a previous condition’. ‘Restorations’ have been carried out by masters such as the professional artists, sculptors and builders and street level roving crafts-persons. The nature of ‘change’ interventions depended on the skills of the master. Such changes were primarily intended to upgrade the building to a better condition. It also meant ‘improvising or adapting a style or confirming to a contemporary taste’. Restorative changes have been carried out by experts to brand their capacity rather then any respect for the past. Restorers like Michelangelo, saw a building as a raw material and opportunity to organize it. Classical structures during Romanesque and Renaissance periods were as regarded pieces of admirable antiquity. Yet these were restored by retaining, enhancing and adding the perceived values, but ‘without any concern for the process or investigations’.

1 Ishtar-gate-بوابة-عشتار


Reconstruction of Tito’s Palace in Mostar. Part of film depicting several buildings and structures that have been damaged during the Bosnian War, wikipedia image

A restoration destroys something of the original character. It is irretrievably lost. The character of the building gets lost with too many restorations. Restorations without harming the substrate or basic fabric preserve them in their original condition. But, that does not mean a restoration allows one to mount a skin or a make-believe screen, to camouflage the original. Conservation is very restrictive process, but restoration could be conservative and preserving in nature.

restoration .jpg



Post 543  by Gautam Shah


This blog >>   was originally published 15 June 2015 at

Cement surface finishes occur in three different conditions:

1 Finishes generated over the cast cement products,

2 Rendered finishes of cements,

3 Cement coatings’ finishes.

The major problem with cement finishes is the consistency of colour over the surface. Other problems happen due to differences in application (casting, rendering or coating), mixing water content and quality and curing processes. The colour differences arise due to many reasons: ingredients used in cement manufacturing, particulate size and its distribution, size and colour quality, proportionate quantity of other ingredients, quality-conditioning additives and mixing procedures.

Centrifuge Cast cement concrete pods on Sea face at Mumbai, India

Cement cast products are plain or steel reinforced structures, pre cast blocks, units, etc. A cement finish of a cast product emerges due to the form-work, compaction, level of vibration (causing aeration), proportion of water and technique of casting (towards gravity, centrifugal or centripetal, short or long depth fall).

Cracks in Cement plaster due to uneven mixing of ingredients

Cement Concrete Surfaces can have finishes depending on several factors, such as:

1 Form work surface, joints and continuity, use of a release agent, absorbency of the form work surface and setting or hardening enhancer and retarders used.

2 Concrete mix proportions, ingredients’ colour, size, and texture (lighter toned aggregates and sands produce light-coloured concrete (colour of cement is variable not only from plant to plant but often batch to batch). Degree of mixing and air entrapment affect the colour. Free lime in water creates a soapy foam which also affects the colour.

3 Insufficient or uneven curing affects the hydration and eventually the colour of the concrete.

4 Inadequate vibration causes minor pockets of air bubbles, which affects the texture.

Weathered Cement Concrete at Sanskar Kendra by Le Corbusier Ahmedabad India

Rendered finishes of cement include plasters, sprays, guniting, masonry pointing, screeds and daubing. These are comparatively of thinner mass. The compaction, if any is part of the application or rendering-levelling by trowels, plasters boards, etc. For rendered finish the surface quality is fairly consistent for small-extent surfaces. For very large surfaces, such as multi-storeyed buildings, the surface differences are noticeable. These anomalies can be reduced by dividing the surface with wide intervening elements of different colour, texture, projections or depressions. Same technique is used for pointing to the masonry faces of stones or bricks. Pointing is designed to enhance the joints’ pattern, as strongly horizontal, vertical, or both. The joint is usually of a contrasting colour and of finer texture, then the main material surface. A raked joint or protruding joint looks much darker than a flushed joint or flat joint.

Masonry bricks of Cement with Fly-ash

Cement is used for creating in-situ and precast floor blocks. Use of fine sands increases the air entraining effect and reduces the work-ability. Angular or flaky sands are difficult to use in sand face plasters.

The mixed mortars are affected by the colour of the aggregates. So it is very difficult to produce a perfectly white marble mosaic tile or washed chips’ plaster unless only pure white aggregates are used. Cement and aggregate flooring such as IPS – Indian Patent Stone, Red Madras floor and Ironite (cast iron milling waste) are all affected by the colour of the constituents. coloured mortars have pigments of iron oxides (black, red and yellow). Green, blue and other colours (though not sun fast or long lasting) are achieved by use of chrome pigments.

Cement pointing over brick masonry

For vertical and ceiling surfaces ziki plaster formed with marble dust containing substantial amounts of fine mica and talc. Similarly pearl glow and a smooth surface can be achieved by including sea shell dust. Slow setting and engravable cement mortars require high workability, are achieved by addition of fully calcined gypsum or lime containing such compounds.

Cement coatings’ finishes are in the form of cement paints and high viscosity or bodied rendering formulations. Cement paints have limited life of 3-5 monsoons and re-applicability of 5-7 coatings. Cement paints are alkaline materials and applied on similar substrates, and as a result the colour range is mostly of Oxides colours. Some blues and greens of darker range are available. The chief problem is the process of adding the water, which creates flocculation and aerated mass. Next problem is application on a dried out substrates with chances of poor adhesion and difficult brushing. Rendering formulations have high viscosity, and the success usage relies more on the craft of application.

White cement + Marble chips Terrazzo


Cement Surface Finishes have few basic problems:

fine hair cracks

honey comb voids

unbounded loose particles

foreign particles stuck on the surface

foreign particles deposited on the surface.

washable salts leached out from the surface

salts and compounds formed over the surface by the constituents of the environment

mould and fungi type bacterial growth

disengagements from the substrate -peel off.



Post 542  by Gautam Shah



Craft processes have been closely linked to exploration of materials with hands or manual operations. Use of tools as reach-extension of hands is considered nominal. Equipments where manually manipulable are accepted, but any process operative on auto corrective feed-forward or feedback is challenged [1]. The skill of making things by hands (including with tools) evolves with experience. And as a result the product is continuously improvised. Every item crafted with hand and (non automatic) tools, is a unique piece. So are the intentions for good design and material manipulation with hand, sufficient to deliver a ‘craft’?

Lijiang, Yunnan, China: Exhibition of copper smith works Wikipedia image Attribution (required by the license) © CEphoto, Uwe Aranas / CC-BY-SA-3.0

If the crafts evolve with the exploration of materials, where does it start? Craft’s person has to procure materials that are substantially processed, and proceed from there on. In a modern world, craftspeople start their work with industrially re-composed and synthesized products. These materials till 1960s came along with specifications of quality and sometimes suggestive conditions for their use. But over the last few decades such specifications have been found to be very restrictive and futile. New modes by International standards Organization, ISO allows manufacturers to offer details on how the material-product is made, and the ‘quality culture’ the manufacturers follow. Further exploration of the same is left to the individual person, who may use it for a non traditional purpose.


Craft was also considered a pastime or a hobby. The terms a crafts-man or a crafts-woman, were long replaced by term artisan or crafts-people. The craft products missed this personal expression when materials from unknown places and in substantially processed form began to be used. The hands on materials, approach which was considered to be the basis of creative human expression began to be irrelevant. Craft was considered a reflection of a localized tradition, matured over a period. This was seen through the motifs, patterns other abstractions. The personalized expression when unique in content and high in abstractions became an art form. An artefact (Early 19th : from Latin arte by or using art‘ + factumsomething made‘) is a crafted item that has the essence of art.


Crafts have been called handiwork or handicrafts. This raises the question, Are products created using hands but by automated tools and equipments really crafts? Products whose formation is not exclusively dependent on fixed dies, jigs, fixtures, and other standard devices, has a greater scope of being continuously improvised. This cannot happen in a continuous line-production but in a single item production or a batch-based system. Line production does not allow human expression as reflected in some course corrections. This began to be seen when factories sponsored by royalties and big business group began to produce crafts-works. These sponsors established production centres that have produced some excellent crafted works.

Wedgewood pottery Wikipedia image by Author Kpjas

Craftsmen till about middle ages were tied to a location and the guild. The guilds, protected as well as encouraged the crafts, by restricting new entrants, conditions of practicing the crafts and in few instances facilitating the raw materials. Factories of later periods enticed the protected crafts persons to move out of the restrictive and monopolistic guild environment. Craft products were not known by the craftsperson, the town or region, but by the production factories. The craftspeople were distanced from the user-clients and social connoisseurs of their skills.

Wood Block printing William Morris 1873 Wikipedia image

The factories supported innovative production processes that included partial automation. The factories subscribed to new designs for inclusion into now their standardized products. They offered large number of identical items like ceramics, vessels, utensils, tapestries, fabrics, furniture items, etc. with coordinated designs, patterns, or colour schemes. The crafts of the post middle ages began to reflect the style of the production centre.

Loan 74, f. Front Cover

The detachment of craft from material handling processes had already emerged before the onset of industrial age. The practical involvement hand with the material had now terminated. The craft factories were overwhelming involved with design. The new class of designers were only perfunctorily connected to the artisans. The semblance of human expression in unique creations, began to diffuse when large number of products arrived from automated industrial production. The Arts and Crafts movement emerged from exposition of ornate and artificial artefacts that disrespected the materials, at the Great exhibition of 1851. It sought to reform the design.


[1] In India Khadi (hand spun & hand woven) fabric is accepted as a craft product. The process is now being challenged by manual-machines as well powered machine spinning devices (called Amber Charkha). The raw material was mainly cotton (but occasionally silk, jute and wool). But now polyester fibres are being mixed with the raw material, resulting in better fiber strength, wear-ability, fineness and economics of costing. To complicate the scene further, since 1950s auto spun fibres are woven on hand-looms (manually operated weaving looms). So should these be a craft-product? Another variant, a Power-loom uses auto-machine spun fibres over automated looms (of simplistic design compared to air-jet and other weaving devices). The controversies of technological involvement occur for all types of products or artefacts.

Amber Charkha India, Automated spinning machine Wikipedia image by Author Sailee5



Post 541  by Gautam Shah

Pewter Trunkard Wikipedia image Author geni Permission (GFDL CC-BY-SA)

Pewter is a metal alloy with Tin as the chief constituent. Pewter denotes a vast array of alloy-compositions of many different metals in various measures. Pewter metal of the ancient times contained about 70% Tin and 30% Lead. Both were known as distinct metals since 3000 BC. (Tin shows a chemical similarity to the periodic table group-14 element -the lead). Pewter has been preferred material for craft and utility items, mainly due to its low melting temperature, and the fine finish, it offered. Pewter with high lead content, offered a shining but tarnishing black metal that darkened further on aging. Primitive pewter with lead was hazardous for food storage. And as a result the composition pewter has seen many changes, with efforts directed towards eliminating the lead.

Pewter Napkin Rings Flickr image by Didriks

Pewter as craft material offered certain advantages such as: low melting point, malleability, ductility, easy workability and highly crystalline silvery-white surface. Pewter can be crafted by cold-working, as it does not cause hardening like other metals, which often requires annealing. Pewter items are usually cast, and then further finished by hammering, turning on a lathe, burnishing, and sometimes engraving. Pewter alloys were rolled into sheets from cast blobs. Such sheets could be shaped, deformed, spun and welded with tin. The presence of tin provides affinity to all embellishing crafts metals like brass, gold, silver, etc.

Detail on a pewter fork handle from Norway, showing three scenes: King Olaf II of Norway, his men, and a Viking ship Wikipedia image by Author Goldenrowley

Romans used pewter utilities like cups, plates, dishes, etc. and medallions, rings, armlets, etc. But the use diminished due to lead poisoning. Use of tin with natural ‘impurities’ like bismuth, antimony, copper and silver was known but not clearly understood. Pewter utilities never formed cooking vessels as tin and lead had low softening point of temperature, besides an increased health hazard of lead. Pewter, in spite of its easy workability was primarily a utilitarian metal, and less exploited for its ornamental capacity. Pewter work finished like silver, and was passed off as silver. It was used where precious metal items were too expensive and theft prone. In the 11th C poor churches began to use pewter in place of silver items. Lower classes across Europe, were still using it for eating and drinking. Trade guilds in the 12th C in France and elsewhere began to control the constituents of pewter.

Components and products of pewter manufacture Wikipedia image

By 15th C, the Worshipful Company of Pewterers began to standardize pewter. The first, known as ‘fine metal’, with tin and copper, was marked for tableware. The second, known as ‘trifling metal or trifle’, with fine metal and 4% lead, was designated for holloware. The third, known as ‘lay or ley metal’, with 15% of lead, was meant for non food or drink utilities.

Rockport Pewtersmith Wikipedia image

Modern pewter is without any lead, but with about 91% tin, 7.5 % antimony, and 1.5% copper. Alloying materials like antimony and bismuth make it more durable. The surface of modern pewter is bluish white with soft satin to high sheen finish. It resists tarnishing, while retaining its colour and finish. Without any lead, it is safe for food and drinks. Britannia metal consists of tin, antimony, and copper.

Teapot, Britannia metal, Wikipedia image by Author Daderot

Pewter items are cast, moulded, pressure die-cast or shaped. Pewter was fashioned by hammering (workers called Sadware men) or by casting (workers called Hollow-ware men). Pewter wares were earlier cast in metal moulds, but now Silicon, Teflon and rubber moulds are for mass production. Few items are painted, enamelled, gilded, and inlaid or embellished with other metals and materials. Pewter was the chief tableware until the making of porcelain. Pewter items include household goods, (porringer, plates, tea sets, dishes, basins, spoons, measures, flagons, communion cups, teapots, sugar bowls, beer steins, and cream jugs), candlesticks bases, church vessels, snuffboxes, personal adornments, organ pipes dishes, statuettes and figurines, game medals and presentations, aircraft and other models (replica), coins, support or inner structures for gold-silver presentation items.

Pewter manufacturing Wikipedia image



Post 540  by Gautam Shah


First iron used by ancient people was a meteoric source, an iron alloy with nickel. This was used several millenniums before the actual iron age. It was a natural Iron in metallic state and so required no smelting of ores. This nearly pure iron is softer than bronze, and therefore tools formed of it had soft wearing edge.

Ahnighito fragment of the Cape York meteorite. Wikipedia Flickr image by Author Mike Cassano

Primitive age iron was smelted by mixing iron ore with charcoal, and burning in bloomeries, a type of furnaces where bellows were used to force in the air. The carbon monoxide produced by the burning charcoal, reduced the iron oxide ore to metallic iron. The apparatus, however, did not achieve a temperature of 1540° C, to completely melt the iron. The metal collected in the bottom of the furnace remained as a spongy non homogeneous mass or bloom. It had high proportion of intermingled slag. The blooms were repeatedly heated, beaten and folded to remove the slag. This produced wrought iron (=worked iron), a malleable, but fairly soft material. Iron age Irons were not castable products but required hot forming (forging). This was mainly due to inability to fully melt the material. Hot forming was a labourious process, requiring skill and experience. In comparison to bronze, iron ore was procurable everywhere and cheaper to process.

Willamette Meteorite, the sixth largest in the world, is an iron-nickel meteorite Wikipedia image by Author Dante Alighieri

Wrought iron shows high resistance to corrosion due to the trapped slag in the metal. The presence of slag in the iron helps fusion joining by hammering or forging. Wrought iron is no longer produced commercially, because low-carbon steel is less expensive and is of more uniform quality. Wrought iron, however, is still produced for certain craft-based uses such as making intricate craft objects balustrades, gates, garden accessories, etc.

The Ashoka iron pillar New Delhi, India wikipedia image by Mark A. Wilson

Simulated form of wrought iron is made by melting scrap mild steel in small furnaces, blowing air through the melt to remove carbon, and pouring the molten metal into a ladle containing molten slag, which is usually prepared by melting iron ore, mill scale, and sand together. When the molten iron carrying a large amount of gas in solution, is poured into the molten slag (kept at a lower temperature than iron), the metal solidifies almost instantly, releasing the dissolved gas. The force exerted by the gas shatters the metal into minute particles that are heavier than the slag and settle at the bottom of the ladle, agglomerating into a spongy mass.

Silla iron armor, en:Three Kingdoms of Korea, 3rd century Wikipedia image

It was Chinese (1200 BC or earlier) who designed kilns that could raise the temperature for iron making. These kilns, used upgraded coal and had high volume air supply for efficient burning. Chinese were able to melt the Iron and cast it into desired forms. Casting was less labourious, and allowed multiple items with same die form. It was accurate than forging each piece. Chinese smiths melted wrought iron and cast iron together to produce steel -a material of controlled carbon content. The process was called ‘harmonizing the hard and the soft’. This was widely used for casting cooking pots and iron statuettes. A cast iron is harder than wrought iron, but maintains the cutting edge.

Casting pig iron, Iroquois smelter, Chicago, between 1890 and 1901. Wikipedia image

Perhaps as early as 500 BC, although certainly by 200 AD, high quality steel was also produced in southern India by the crucible technique. In this system, high-purity wrought iron, charcoal, and glass were mixed in a crucible and heated until the iron melted and absorbed the carbon.

Carbon content of iron is a major factor that creates harder material. It was necessary to absorb more carbon in the iron. This required higher ratio of fuel to ore, and push in a lot more volume of air. The strength of iron begins to increase with carbon contents of 0.5 percent. To heat treat iron a carbon content of 1.2% was necessary. Wrought iron which contained less than this proportion had no qualitative effect due to heat treatments. A higher carbon content creates a brittle material but allows heat hardening. ‘Iron hardening by quenching was not practised because it made iron very brittle, unless followed by tempering, or reheating at a lower temperature, to restore toughness’. Simple fire, 600-700° C, based technique of repeated cold forging and annealing was used.

cast iron columns line the Albert Dock’s quayside Wikipedia image

In the pre-Christian portion of the period, the first important steel production was started in India, using a process called Wootz steel. It was prepared as sponge (porous) iron. This was hammered while hot to expel slag, broken into smaller pieces, and placed with wood chips in clay containers, and heated. On melting, an iron composition containing 1 to 1.6% carbon was produced. The pieces were reheated to form articles that required a hard body and sharp edge. Such steel products were exported to Middle East and other countries. It was known as Faulad (Persian). (Faulad or wootz steel has a Kannada term, ukku, a Language of Indian region of Karnataka).

Elevator screen from the Chicago Stock Exchange cast iron electroplated with copper. Wikipedia image by Joe Mabel

Nowadays commercial steel plants produce ingots or pig iron. It has very limited use. It goes to casting foundries or to steel mills. At both the places it is remelted to reduce its carbon content and for allying by adding various elements such as manganese and nickel. Often scrap steels are also added for the same purposes.

Black Country Living Museum – Jerushah the Tilted Cottage – cast iron cooker Flickr image by Elliott Brown

Melting points for various forms of Irons

Iron, Wrought     1482 – 1593

Iron, Gray Cast   1127 – 1204

Iron, Ductile       1149

Steel, Carbon      1425 – 1540

Steel, Stainless   1510