RED Colours of ancient times

Post 430 – by Gautam Shah

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Red is the most fascinating colour in history. It is the colour of kill, blood and life. It was prominently used in prehistoric cave art. It was made with red Hematite or iron oxide, as red ochre. It is one of the oldest pigments and has lasted well for more than 70000 years. Red ochres are very stable colourant unaffected by alkaline or acidic effects, moisture or UV exposure. The red ochre is available in many colour variations, in almost all regions of the world, and usually at surface level. The minerals in the form of rocks, lumps or dust form can be ground to fine powder form.

Cochineal insects crushed

Red oxide or Red ochre colours of the primitive age were not the most brilliant colours if one were to compare it with some of the synthetic red pigments (cadmium, chrome, rubine, etc.), we use today. But in absence of the ‘brilliant reds’ the Red Ochre was a magical red. Primitive man, however had some ochres that were brighter then others. The brilliance of red was enhanced by mixing, or with topping with wax or oils.

Natural dyes on Skeins

The early civilizations used ‘red colours’ for different purposes such as body painting, ceramic painting, dyeing of leathers and fabrics. Root extract of the Rubia or Madder plant was used in ancient Egypt for colouring textiles. A red colourant made from an insect exudate called Lac, gave the term ‘Lake’ (a transparent dye-based colour). In Italy during the post mediaeval periods, the Lac was considered very expensive pigment. Henna leaves and madder roots were mixed with alum to create red shades. Another source of red colour was from insects Cochineal and Kermes vermilio. The Romans used a bright red or vermillion pigment made from a natural mineral called cinnabar. A warm ruby-red resinous exudation of Calamus draco was used by illustrators. Red lead or Lead tetroxide pigment was widely used in Persian and Indian miniature paintings. It was also used in European art by name minium. Sindoor is a brilliant red colour powder used by Indian (Hindu) women, on their forehead and for hair parting. It was Vermillion, but being a toxic material is now produced by reacting Turmeric with Alum or Lime.

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Sindoor Tikka Powder Hindu Women use in on forehead

In India, red dyestuffs were used from antiquity. These were plants of kampillaka (Mallotus phillippinensis), pattanga (Caesalpinia sappan), jauka (a species of Oldenlandia), and animal substances like indragopa (cochineal). The Sappan wood tree (Caesalpinia sappan from Asia and Brazil) based, red dyestuff is called Brazilian.

Vermilion Pillars

Red Ochre powders were placed with dead bodies (Neolithic) or heaped on burial mounds (S. India). The word magic =Zauber in German, =taufr in Old Norse, or =teafor in Anglo-Saxon >> all meaning Red Ochre.

Red Lanterns Shanghai

Red colour is associated with Egyptian God, Set. Set was a god of storms, unpredictable and associated with deserts and foreign places, meaning with chaos and danger. The word ‘desert’ has derived from the Egyptian ‘dshrt’ or ‘deshr‘ or ‘deshret‘=red place or Red Land. Red Ochre was sourced from desert lands. ‘The hieroglyph for red is the hermit ibis, a bird which, unlike the other ibis of Egypt, lives in dry areas and eats insects and small creatures. Writers of Egyptian papyri used a special red ink for nasty words.

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Red palette has offered a vast number of shades and chromatic variations. Red has been used as space making colour of walls and earth or floors, but not for aeriform figures like flying angels, heavens, skies or ceilings. It has been used in murals, paintings, statues, architectural interiors, manuscripts, dresses, adornments, foods, funerary, etc. Red has been a colour of piety, royalty as much as of death and fear. Colours have been codified but named differently in various parts of the world, roughly the time sequence was black, white, red, green, yellow and blue. Some of the ancient Reds were: Iron oxide (hematite), Ochres (burnt-calcined), vermilion, cinnabar (natural mercuric sulfide), realgar (natural arsenic sulfide), minium (heated white lead), natural dyes (madder, kermes, brazil-wood), sandarac. words.

Red Colours in Egyptian Art

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PRIMITIVE COATINGS # 1

Post 411 – by Gautam Shah

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Coatings have been used for decorating and suffusing objects and surfaces for the past 60000 years. These were used for several purposes such as to add a colour, impart a protective layer, ‘plaster’ a surface, imprint a pattern or create an identity signage.

Stone and other artefacts requiring coating or decoration

A primitive person had many objects that could receive the coatings. These objects were natural, reformed or produced ones. The objects had different sizes, shapes and surfaces. The surfaces had characteristic textures, porosities, base-colours and patterns (grains, patches, stains, etc.).

Korea Neolithic age pot

The objects were own body skins, hairs, and nails. Animal products like teeth, animal hides, furs, bones, and teeth. Minerals’ items like sands, clays, stones, rocks, precious stones, and sea shells. Plant items such as dry leaves, grasses, seeds, dried fruits, fibres and woods. The manufactured range of items included clay products, ceramics and metals.

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The surfaces were prepared to receive the coating. Hides were cleaned and shaved by heavy rubbing. Body surfaces were oiled to receive the colourants. Walls were washed and wetted prior to coating application. Bones were ground to remove the sheen and make surface slightly rough and absorbent. Stones and woods were polished or scrapped. Raw and baked clay products were re-fired after coating. Leaves were rolled and flattened and dehydrated at the green stage by burying in layers of ash or sand.

The coatings’ materials were of natural origins, such as available off the ground, or from animals and plants. But the coating materials were processed by filtering or sieving, washing, cleaning, decanting, boiling, singeing, and sintering.

The act of coating was intentional, done with a sure purpose. But the resultant effects were wondrous, something that gave a new purpose to the artefact. The art of drawing and the technique of coating, was seamless process of magic. A process to express, what the postures, gestures or spoken language could not do.

Coating techniques and materials of the primitive age are still being used in many situations, and so continue to be relevant. Blood is perhaps the earliest colourant, as a fresh liquid it has very rich colour. It was a colour to represent the vibrancy of life and metaphorical power over the kill. Blood, however, is biologically degradable material, dries to a darker shade, and has very weak colour-integrity. Wood coal is a dry colourant, easy to handle. It requires a textured base for ‘rubbing-in’ or a binding liquid to form an applicable paste. Carbon (Lamp) black -a deposition collected over burning fat or oil, is much better due to oil content. Whites were procured from metallic oxides and carbonates. Lime is most common everywhere. Other whites included talc, whiting and barytes. Iron oxides are equally common, and have many different hues (such as yellow ochres, browns of sienna and umber, red and black oxides). Oxides are very stable, and have ‘deep’ saturated colours.

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Black Carbon of Soot or Lamp black

In this palette of colours, the notable absence was Blue and Green. Blue and green appeared very late in the form of Lapis lazuli and Malachite (copper carbonate hydroxide). Brilliant Red that could represent the fresh blood, and Brilliant Pink of the meat were also absent. Purple was nearly unknown. The absentee colours were sourced from plant juices and natural dyes, but had poor colouring strength or low opacity (transparent), sun light fading, and biodegradable colours so were not long lasting.

Sprayed colours

The primitive colourants were mostly of dry powders or soft rocks. These had no binding capacity. Mineral pigments were heavily rubbed on the surface to trap them in the micro cavities of the surface. Liquid juices could absorb into the surface. Some form of binding material or technology was needed. Water has temporary binding capacity and can be used as a carrier agent. To fix colours plant oils, mutton fats, fish oils, etc. waxes, and plant latexes were used. These substances except the wax were ‘non-drying’ and remained wet for a long time. The wet surface attracted dust and trapped insects. The oily substances biologically deteriorated, and on oxidation turned darker in colour.

Proteins-based materials like blood, eggs, milk, urine, and starches were also used as binding materials. Plant and insect exudates or natural gums had binding properties but were highly hygroscopic (affinity with water) materials. Plant milks or latexes, like materials, were also used. Wax was used to mix with pigments and as a protective layer. Wax and natural Creosote were used to protect wood and leather surfaces.

Number of cementing substances were used for coating or plastering. Mud plasters, slaked lime and Pozzolana (volcanic) ash, were materials that had binding properties. Colouring these substances, or coating over it (fresco style) required large quantities of materials, or concentrated pigments. Lime when mixed with a colourant imparted a white shade creating a ‘pale’ effect. Pozzolana had darker colour so made the colourant several shades darker. Plastering and daubing, were frequently used to prepare a better surface for a wall painting. Primitive binding materials can be categorized like, 1 Materials that are water reducible, water resistant and hygroscopic, 2 Air drying and non drying, 3 Non water-based materials.

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The primitive age craft of coating can be summed up as 1 Surface preparation, 2 Application of the coating, 3 Applying tonal variations or shades, and 4 Covering the surface with water protective coat, usually of oils or other transparent materials. The process of application of colourants or the coating system was adapted to the nature of the base surface, as much as to the type of colourants and binders.

Twig brushes

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Drawing points and crude shading brushes

Primitive coating applications are varied. The simplest way of marking cave walls art was to make finger-nail traces in the soft layer of clay covering the rock. Lime stone walls were engraved and filled in with iron oxide (Hematite, or ochre), or the black pigment such as the manganese or charcoal. Analysis of cave wall art pigments, reveal the use of extenders (dull or low opacity powders) such as talc or feldspar, to increase the bulk of pigments. The coating also shows traces of animal and plant oils, used either for binding or as a protective covering. The pigment in paste form was applied with fingers, and also tools like fiber pads, animal-hair brushes or crushed twigs. Lumps of pigment discovered on the floor of caves were perhaps used as crayons, or were grinding onto colour powder. Colours were often sprayed, from the mouth or through a tube. A network of ladder, supports and scaffolding was used to reach the ceilings and upper portions of walls. Light was provided by hearths, or portable burning torches. The coated surfaces were ground to achieve a sheen on the surface or re-coated with a protective layer of egg-whites, oils or fats.

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The earliest known use of colourants dates back to 70000 years, whereas Cave wall art is about 40,000 years or older. BC. Lascaux, an underground cave, 17300 BC, located in SW France, has walls and ceilings, decorated with some 1,500 engravings and about 600 paintings in shades of yellow, red, brown, and black. The subtle tonal gradations of colour on animals painted in the Altamira and Lascaux caves appear to have been dabbed in two stages with fur pads, natural variations on the rock surface were exploited to create the effects of volume.

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NATURAL IRON OXIDE PIGMENTS 4 # SIENNA and UMBER

Post 409 – by Gautam Shah

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Death of the Virgin. Louvre, Paris. By Caravaggio

An artist palette and building designers’ vocabulary invariably includes few rich earth colours such as red oxide, ocher, raw sienna, burnt sienna, raw umber, burnt umber and Indian red. These colours in various intensities, transparencies and reductions with white create a language that cannot be matched by synthetic pigments. The iron oxide colours are sun fast, and remain stable on alkaline (masonry or lime) and acidic (metal) surfaces. Browns colours cover a wide visible spectrum, ranging from yellow, orange, to red. The shades are also named using composite adjectives, like red-brown, yellow-brown, dark-brown, etc. Modern day browns derive by mixing green (blue with yellow) and red, or orange and black. Red colouration is linked to the presence of hematite, and yellow colouration to the presence of goethite.

Rembrandt van Rijn Self-Portrait (1659)

Rembrandt

Like the other earth colours, such as yellow ochre and umber and sienna are clays containing iron oxide, called limonite, which offers yellowish colour. These colours are a range of different colours. All standards fail to agree on precise shade for them.

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Red oxide usually contains about 70% Fe2O3 . Sienna is a brownish yellow containing about 60% Fe2O3 with some quantity of manganese oxide. Umber is a greenish brown containing some 45% Fe2O3 and 15% MnO2. Umber is named after a department in Central Italy where it was first tested. Umber of good quality, called Turkish Umber, is found on the island of Cyprus. Numerous deposits of colouring earths occur in various parts of the world.

Vieja friendo huevos by Diego Vel

Earth colours Sienna and Umber, are mix of limonite with some amounts of manganese oxide, and so both are darker then ochers. Senna is darker, more towards orange and so thought of as reddish, compared to Umber that is closer to yellow and thought of as brown. Sienna has a higher content of manganese which makes it greenish brown or darker brown. Raw Sienna is transparent and warmer whereas Raw Umber is opaque. Raw Sienna when calcined becomes richer and darker as the burnt sienna. Raw Umber becomes the rich deep or dark brown as burnt umber.

Johannes Vermeer Milkmaid in Umber shades

Sienna was mined originally mined near Arcidosso, formerly under Sienese control, in southern Tuscany. It was also called terra rossa (red earth), terra gialla, or terra di Siena. Umber is linked to Umbria in mountainous region in central Italy, and so called terra di Ombra or Umbria. Ombra literally means ‘shadows’, for the purpose for which it was used, shadows or for shade toning other colours.

Caravaggio flagellation Musée des Beaux Arts, Rouen

The creative exploitation umber for creating dark shades happened during the baroque period, in the chiaroscuro (light-dark) style of painting. It was the palette of Caravaggio (1571-1610) and Rembrandt (1606-1669). This nearly ended during 19th C, when Impressionists dazzled with modern brilliant synthetic pigments, rebelled against the dark and dirty umber and other earth colours.

The Calling of Saint Matthew by Caravaggio (1599-1600)

NATURAL IRON OXIDE PIGMENTS – 2 # Red Oxides

Post 407 – by Gautam Shah 

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The first primitive colours, ranged from yellow to brown to red to near black. These came from three basic materials that are Oxides of Iron, Calcium and Carbon, respectively for Red-brown, White and Black colours. The ‘earth colours come from inorganic minerals like, Iron oxides with its various stages of hydration, and Manganese oxides.

Iron Oxide Red

The chief earth colour constituent, Red hematite powder, a form of iron oxide (Fe2O3), was found scattered around the remains at a grave site in a Zhoukoudian cave complex near Beijing. The site has evidence of habitation as early as 700,000 (?) years ago. The hematite might have been used to symbolize blood in an offering to the dead. It was also used in powdery form, 164,000 years ago by the caveman of ‘Pinnacle-Point’ (caves in South Africa), possibly for body painting. Hematite residues are also found in old graveyards from 80,000 years ago. Hematite as a mineral, is coloured black to steel or silver-gray, brown to reddish brown, or red. The various forms hematite show variegated rust-red streaks or bands. The word Hematite is a Greek word haimai that alludes to red colour of the blood.

Iron bands

Hematite is harder than pure iron, but very brittle. In steel-gray crystals with metallic lustre, hematite is called specular iron ore whereas thin scaly ones are known as micaceous hematite. Most hematite occurs in a soft, fine-grained, earthy mineral form of red ochre or ruddle. Red ochre has been used for body coating and cave art painting. It is now used in paint pigment, for primers, and as a polishing medium or rouge for finishing plate and spectacle glasses.

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Venetian red

Rusted Iron colours

Limonite (hydrous ferric oxide) ranges from yellow to brown. These are known as Ochre, Sienna, and Umber. Ochre is clay-coloured with hydrated iron oxide. Tuscany, famous for its ‘Terra di Siena’, is a hydrated iron oxide with silicates and aluminates that endow some transparency to the pigment (Silicates and Aluminates are extenders, with a lower refractive index, so add to the bulk and increase the transparency). When limonite is heated for calcination, the water part is removed to form ‘Burnt Sienna’.

Oxide colour painted Barrackpore Railway Station North 24 Parganas W Bengal India > Wikipedia Image by Biswarup Ganguly

Venetian red is a slightly darker than a scarlet red, but less intense colourant from purer form of hematite. It was also called Sinopia due to its origin from Sinop in North Italy. Similar colourants (of pure hematite) were called Ocra rosso or red ochre. Due to its pure quality, the pigment, when mixed with white, produced a likeable pink. The red Sinopia or Sinoper and its lime mixed pinks were used in Italian Renaissance paintings for body tones. It formed the major ingredient in the colourant called Cinabrese. Sinopia was used for the cartoon or under-painting for a fresco.

Sinopia

Red oxide colours were not perfect red colours. These had either a yellow-brown tinge or blackish shade. The colour available in dyeing of fabrics gave that desired perfect red. Dyes were not preferred for paintings or wall art for two reasons, One, dyes were soluble in water and had lesser opacity (‘covering or hiding capacity’) Egyptians used the root of the Rubia, or madder plant, to make a dye, later known as alizarin. This was mixed with whites powders of low refractivity, and used as a pigment (known as madder lake, alizarin or alizarin crimson).

Ajanta Caves, India Oxide based earth colours

Ajanta Caves, India Oxide based earth colours

The earth colours have been extensively used in Ajanta and wall arts at several places. The early works show range of earth colours mixed with white and only occasionally greens. The mixing of ochre (in absence of pure yellows) produced darker or olive green effect. Pure pinks were absent.

Buddhist Monks on pilgrimage varied shades of earth colours Wikipedia Image Credit: Tevaprapas Makklay

In India the Earth colours have also been known as Bhagawa and Gerua. Bhagawa is brownish or more towards Ochre, whereas the Gerua is a red iron oxide colour. Both the colours show regional variations. Bhagawa colour has been colour of attire for Hindu and Buddhist monks. Geru or Gerua colour is used on water and plant pots and buildings. It is used as a decorative coating with lime white, on lower sections of tree trunks. Temples in South India have Gerua coloured stripes on external face. Buddha adopted the colour for own robes, because at that point of time it was the colour of prisoners’ dress. Sikhs adopted the Bhagawa colour, but now their choice leans more on purer synthetic orange or saffron, than earthy colour Bhagawa.

Varanasi (Benaras) India Gerua coloured Durga temple Image by Henk Kosters https://www.flickr.com/photos/68556734@N00/741206048

For translators, transliteration of colour, is always problematic. Colour names are closely linked to their cultural interpretations. So, for the Indian classical story of Nala and Damayanti, “the king loses the kingdom in gambling, and retires to forest. His wife queen Damayanti renunciates the world to accept the asceticism. She starts wearing Bhagawa or earth-brown coloured clothes”. A Russian translator perceives the mood, but reinterprets for his own culture and makes Damayanti wear Black, the colour of widowhood. Today, Western reporters claim the Gerua or Bhagawa to be colour of Saffron, and link it to Hinduism.

Typical Iron Oxide Red Shades

  •     Hex: #6E0303
  •     RGB: 110, 3, 3
  •     CMYK: 0, 0.973, 0.973, 0.569
  •     HSV: 0, 97, 43

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FLOOR PAINTS

Post 371 – by Gautam Shah

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Floor paints constitute a distinctive category of Architectural coatings. Floor paints are broadly of three classes: Clear coatings, Pigmented or coloured paints and Technical coats. Technical coats could be under coats, super coats or singular coats. Floor paints or coatings can also be differentiated in terms of their placements: such as for top of the roof, under coating for the roof, exterior and interior surfaces of wood or masonry materials, and technical coats for floors of water storage (tanks, moats) and channels (aqueducts and canals).

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Floor paints are used to improve the appearance, increase abrasion resistance, reduce moisture penetration and impart spillage proof qualities. Floor paints, nominally do not include coatings or applications with substantial mass such as various types of plasters or depositions. Some screed applications have dual classifications of floor plaster as well as a thin coating system. Floor paints are thin surface coatings, applied on variety of substrates and with equally varied methods.

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Ancient floor coatings were applications of oil mixed with wax, bitumen and creosote. These were used for treating floor decks and seats of seagoing vessels, seacoast wood structures, wood floor boards around wells, and underside boards of roofs over purlins prior to covering with metal sheets or ceramic tiles, all to prevent the rot. Oil-wax was used, for floorings and stage floor boards, to keep intact the visual appearance of the wood. Bitumen and creosote darkened the surface and often remained tacky (due to addition of tallow or fish-oil), but wax-oil coatings, with higher content of wax were non tacky. Wax was hot-melt in bodied oil (double boiled linseed oil or polymerized oil) or oils modified with Pine-rosin. These formulations were nearly like Varnishes used for coating furniture or covering the paintings. The surface had gloss and smoothness. The Varnish coatings lasted a year or one wet season, and required re-application.

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Ancient Floor coatings were mainly applied on wood surfaces, but occasionally dull floors of stones were given a coat to achieve a shiny surface. Dull stone floors of sandstone or lime stones were coated for festive occasions like coronation, marriage and religious functions. These were temporary applications, and no one was bothered about its durability or issues of re-application.

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The floor coating system saw renewed interest when large size merchant and cruise ships, railway coaches and passenger bus vehicles arrived on the scene during the Industrial age period. These utilities did not have masonry or wood floors. It was necessary to create decks of colourful ambiance, and of wear resistant, non-skidding, sound dampening and maintainable material. The first options then available were linoleum carpets and alkyd-based enamel paints. Both created floors with fewer joints, but paint was a re-applicable surface.

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Road and surface signage and functional graphics became a necessity, with the increase in road and rail traffic. There was no space in many urban areas to place vertical signages. The floor signs painted on roads and pavements were warnings in the movement spaces, passengers’ zebra crossing marks, edges of surface drops, road segmentation, danger zones, curvature limits. These were initially by contrast colour paving. Similarly barefoot walk-passages in temples, mosques needed a lighter colour treatment to keep them cooler. The only option, than available was to use lime whitewash or alkyd based white oil paints.

 

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Resort Santorini Greece Building Terraces

In post world war period plastic (or Latex, as known in USA) paints became available. The ‘latex’ paints were water-based emulsion medium. The field of marine coatings was offering many new technologies for water and UV light resistant, hard wearing systems. Rubber-based systems (chlorinated rubber coatings) became synonymous with road marking paints.

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Other fields of Indoor floor paints were emerging. Stage show floors, Dance floors, TV programme production sets, TV News room floors, sports arenas, required colourful joint-less extensive surfaces. Food plants, Pharmaceutical units, Hospitals operation theatres and critical care areas, needed not only a joint-less surface, but one that was dust and scratch proof and bio-friendly material. These were first provided by Polyurethane systems and later by Epoxy systems. Electronic assembly plants, computer rooms, electronic exchanges and data server and router areas, needed static proof floor coatings. Fire-prone industrial areas needed a spark erosion flooring systems.

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Outdoor Floor paint field also has flourished well. Sports facilities, stadiums, exercise areas, malls, food plazas need many different types of a joint-less floor of non skidding and spillage proof surfaces. The colour and texture requirements of these usages can never be found in any natural or manufactured materials. Thin coating systems of floor paints not only satisfy such unique needs but also offer pattern laying facility.

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Technological innovations include use of Florescent pigments, night glow compounds, high luminescence whitening agents, texturizing additives, wrinkle finishes, two or multi-tone effects.

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The ability of a floor-painted surface to virtually do everything expected of a floor system, has forced changes on substrate technology. There was a time natural floor materials like stone or ceramic tiles were ground to smooth level and then coated. Any joint gaps or remaining surface level irregularities were filled in. But now cast on site surfaces of many materials are preferred. The material options include Ironite (cement+sand mixed with iron turnings), Magnesium Oxy-chloride Flooring, Cement Concrete floors (Tri-mix suction system), etc.

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CORROSION PROOFING TREATMENTS

Post 367 –  by Gautam Shah 

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Corrosion is degradation of a material due to an electrochemical oxidation process with the environment. The process is more common with metals, but can also cause disintegration of a polymer due to sunlight exposure, or in case of ceramics and stones as seen as efflorescence.

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Stable metals like copper and precious metals like Gold, Silver, Platinum, are less prone to disintegration. Some metals form their own protective cover on the surfaces to prevent, or slow down the corrosion.

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There are several ways to stop or retard the corrosion. One, is to isolate the metal object from environment and other metal objects, and Two, is to constitutionally alter the metal to reduce its vulnerability to rusting. In both the cases the availability of electrons for displacement is reduced. Metals are protected by formation of a barrier. The barrier could be an applique coating like paint or an integrated one like plating or galvanizing. Integrated barriers are also formed by metalizing, surface alloying and ceramic formation. The barrier could be generated by the material itself such as the Patina on bronze. The Barrier could remain on the surface forever, or covered by other coating systems.

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Barrier protection: One of the oldest methods of protecting the metal surface is plating with a metal of stable nature such as Tin, Silver or Gold. Such plating processes were expensive and used for small objects. Post middle ages, metal household objects of iron were covered with a layer of coloured ceramic-glass called enamel. Enamel is inert, and adheres tightly to the steel, protecting it from corrosion while providing attractive appearance. Later chromium plating via electrolytic compounds began to be used as a protective-barrier coating on steel. To get better adherence, the steel is first electroplated with layers of copper or nickel. Today many other types of barrier protections of organic nature such as paints and polymers are used.

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The oxide layer that forms on metals when they are exposed to air also constitutes a protective barrier. Bronze, Stainless steel and aluminum form the most stable and protective of such films. The thickness of the oxide film on aluminum is often increased by making the part function as the anode in an electrolytic cell. This process, called anodizing, enhances the corrosion resistance and makes it easier to colour the surface. The films that form on copper and steel as a result of corrosion (commonly known as tarnish and rust) are somewhat thicker and show a characteristic colour that is often incorporated into the design of the part.

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Galvanic protection: Applique protective films (like paint) on steel are susceptible to being broken at scratches and sharp dents. This occurs in automobiles and other entities, as the applique films have no ability for self-healing. A protection application of zinc metal which has greater capacity to donate electrons then the steel, if forms a prime (first layer on steel) surface, then the objects can be protected from effects of corrosion. This is called galvanic protection. A layer of zinc can be placed on a steel surface by either by hot-dipping or electroplating. Galvanized steel is much more resistant to corrosion than un-galvanized steel. Where a galvanized coating is cut or scratched, the zinc flows in over the exposed area and provides continuous protection. Cadmium can also be deposited for galvanic protection of steel. Hot-dip aluminum-coated steel is used in the exhaust systems of automobiles. At low temperatures its action is galvanic, but at high temperatures the oxidation forms a barrier layer. Galvanic protection is also provided by imposing an electrical potential on steel structures.

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Other coating techniques

Among other methods for applying a metal layer to metal is thermal spray coating, a generic term for processes in which a metal wire is melted by a plasma arc or a flame, atomized, and sprayed onto a surface in an inert gas. A process similar to this is vacuum coating wherein metal is evaporated and deposited as coating in high vacuum. High-temperature bearing super alloy components, such turbines are given oxidation protection by annealing them in a chamber containing volatile aluminum chloride.

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Metal coatings are surface treatments that form the first coatings in a multi coat set. Conversion coatings are of basic two types: phosphating and chromating. These are  temporary, but provide an adequate substrate for subsequent applications.

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Phosphate coatings are used for ferrous, zinc metals, aluminium, tin and cadmium metal surfaces. It is a thin, porous, insulating and adherent application that allows keying of the applied paint film. The electrical inertness of the coating arrests corrosion spread to local spots. Phosphate coatings are applied by immersion, brush application or spraying. Zinc phosphate coatings are smooth and fine-grained treatments, used for reducing the corrosion creep under the paint. Coatings containing manganese phosphate are less widely used as paint pre-treatment because they have a large coarse crystal structure although these heavy coatings are very useful as oil-carriers and have good wear resistance, which is advantageous for engineering components.

Steel structure showing residues of inner coatings

Chromating is formation of a chromium oxide film on the metal surface. It is used to increase corrosion resistance of metals like aluminium, magnesium, tin, zinc and cadmium. It is also used to enhance the tarnish resistance of copper and silver.

Diffusion Coatings are also known as cementation coatings as part of the applied material interacts and forms alloys with the substrate. Cementation coating process is very similar to carburising of iron to produce surface-hardened steel (iron heated with carbon particles for the diffusion to occur). Common processes falling in this category, are: aluminizing (calorising), chromising and Sherardising (zinc cementation coating), siliconising and borating. Processes like hot dip galvanizing, tinning, aluminizing and terneplating also form alloy, but technology is different from diffusion coating. Such alloyed coatings are used where high corrosion and abrasion resistance, in very active environments are needed.

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Metal cladding can cover a metal or other surfaces to form a barrier against corrosion. The thickness of a cladding metal could be few microns (metal leaf) to few millimeters (metal sheets or plates).

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Chromising is the term applied to the formation of a diffusion coating on iron or steel by chromium to produce a surface with enhanced oxidation, corrosion and wear resistance. Gas phase chromising is performed, when the articles heated in a powdered mixture of chromium, alumina or kaolin and an ammonium halide in a hydrogen atmosphere.

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COLOURS -Perception and Expression

Post 356 –  by Gautam Shah 

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Colours have two relevancies. Some consider colour perception as biological phenomena, common to all human beings. Others perceive colour to be variable with social and a cultural facet. The social and cultural affectations of colour are observed in linguistic, ethnic, and aesthetic expressions.

Colouring_pencils

Perception of a colour, even if, a biological phenomena, one needs to convey that experience, usually by specifying the Hue and Tone of the colour. This may be done by comparison to some other colour of near hue and tone, and also by naming it in distinctive way.

Rangoli,kolam,chennai,Tamil_Nadu382

Nearly all languages unambiguously describe the black and white. Such unequivocal terms exist for few other colours, but not all shades. The third definitive term is for Red and fourth term could be either Green or Yellowbut not both simultaneously. The fifth definition may include either the Green or Yellow (excluded from fourth choice). The next, seventh descriptive choice is Blue. Other preferences in the choice sequence are for Brown, Purple, Pink, Orange, Gray.

Additive and Subtractive colours

Fluorescent_minerals_hg

Colours definitions other then these (11 or 12) common terms across different languages are more likely to be comparative idioms such as reddish, irrational value judgements like darkie or lighter, cool and warm tones. Metal and many materials have direct colour associations such as gold, silver, ash, orange (tangerine), KumKum (vermilion red), Turmeric (Haldi), etc.

Kumkum_By_Anis_Shaikh_31

The variations in colour terms across regions (cultures-languages) pose a different picture. In spite of scientific spectrum definitions, the cultural recognition and acceptance do not match.

Mountain-spring-redwhite

There was virtual lack of colour terminology in Homeric Greek literature, but it does not mean the Greeks could not perceive the colours as we do it now. We can now differentiate and define nearly 2.4 Mn colours. To distinguish a specific colour one needs to reference it in terms of hue, saturation, luminosity of the light, and context (contrast, background, reference -mental recall or spectrum definition).

Colour tonal variations Wikipedia Image by Phlake at en.wikipedia

We perceive colours and talk about it in entirely different context. We express colours through objects, surfaces, paintings, scenes, nature, fire, water, reflections, shadows, television, films, and other media. We also perceive colours with light and shade, textures, gloss, patterns, perspective, angle of vision, silhouette, visual aberrations and make-believe effects. In each case our connection with the colour is personal, and to recollect and replicate that experience into a scientifically coded vocabulary is difficult.

“The intensity of a spectral colour, relative to the context in which it is viewed, alters its perception  a low-intensity orange-yellow seems brown, and a low-intensity yellow-green looks  olive-green.”

Louis Anquetin, a French artist commented in his book that old masters’ palettes were small and set with a few, almost nondescript-looking colours, out of which they made jewels on their canvases. The contemporary artists’ palettes were enormous by comparison and set with a dazzling array of colours, and they produced drab paintings.

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