Sunday, September 15, 2013

COATINGS -Iron age

COATINGS    -Iron age              from series Surface Finishes > Coatings > Iron age

Iron Age is a period that is very broadly defined, in time scale, intensity and geographical extent. But it represents a complete command over the fire, related handling processes and beginning of chemical analysis. In the early phase, it gave a birth to remote handling tools and techniques, like the arrows, spears and other projectiles used for hunting. Fuels, structures for hearths and kilns and vessels were devised to work with fire. These were paralleled with refinement of materials processing technology. A whole range of chemistry was developed through baking of ceramics, forming glass beads and food processing. Reduction of metal from ores and the secondary processes like casting, shaping, forging, alloying, smelting, calcining, etc. provided many other forms of materials. Richly hued and intensely toned colourants or pigments formed part of these discoveries.

  • A distinction is usually made between a pigment, which is insoluble in the vehicle (resulting in a suspension), and a dye, which either is itself a liquid or is soluble in its vehicle (resulting in a solution). A colourant can be both a pigment and a dye depending on the vehicle it is used in. In some cases, a pigment can be manufactured from a dye by precipitating a soluble dye with a metallic salt. The resulting pigment is called a lake pigment.

Rich colourants were very important for human body colouration, alive or dead. In the second line were the processes for colouring of ceramics through the application of slip-glazing, and colouring for personalization of tools and implements. Origin of the coatings is attributed to the ritualistic paintings in caves. However, caves became less important for rituals as the abodes were now built in non-hilly farm lands.

Agriculture and husbandry provided a greater range of oils and liquids to bind the pigments. Pigments and white powders were now more concentrated and ground much finer. Human body decorations had now brilliant colours. Ceramic colouring was mysterious as completely different tones developed on firing. Tools and implements were made from made from wood, plant products like fibers and grasses, horns, bones, hides, and metals. These were decorated for personalization (ownership branding) and for endowing magical powers. Decorations or patterning required variegated colours. Patterns had finer zones accomplished with other craft processes such as texturing and engravings. Colours were rubbed to impregnate the surface deeper and then rubbed over to remove or dilute the over-laying materials, thus creating differentiated foreground-backgrounds.

In the later part of Iron age formal buildings were of masonry work. The crafted patterns and the architectural elements were coloured. Wall murals with greater details and intensive spreads, were created mostly over the stone, mud or mud-bricks surfaces. For mural paintings the walls were rendered by a prime coat of daubing, plaster or gesso work. The daubing used plastic clays with fibrous additives, slaked lime, volcanic ash (pozzolana). The plasters were made with slaked lime, gypsum, etc. but finished to a smooth surface or even textured finely. Gesso work over the wall consisted of white or lighter colour mineral powder mixed with natural gums, plant’s latex like juices, eggs whites, urine and milk, as very thin surfacing. Lime was best cementing material. Clays of high plasticity were mixed with pigments and used for substrate creation (a primer and leveling coat). To achieve high opacity and colour saturation, several such layers were applied.

  • Egypt wall art work: Stone surfaces were plastered with lime or mud, and topped with smooth Gesso, that is a mixture of chalk, gypsum and pigment with a binder like natural gum, white of eggs. Fine finished lime-stones were coated with lime wash before painting. Pigments were mostly oxides. It is also clear that paint was applied on a dry surface unlike as in Frescos where it was applied on wet plaster. After painting, a varnish or resin (plant exudate gum dissolved in hot oil or alcohol) was applied as a protective coat.
  • The Egyptians used colours for expressions then realistic presentations: Such as Red skin meant youth and vigour. Yellow expressed the woman or ordinary citizen, Blue and gold was reserved for higher persons or immortals, and Black was for royals.

Many other objects, made of wood, ceramics, stones, and metals were also painted or decorated. These objects had surfaces with poor holding (binding to the substrate). Oils and waxes were used to mix pigment colours or absorbent dyes were used. Oils (plant or animal origin) have a tendency to remain wet (non drying) for a long time and collect dust on aging. Oil mediums though superior in fixing and longer lasting, yellowed with age, and destroyed due to fungus and algae. The chemical modification or additives for oils were available around 10th C. AD.

Papyrus and Fabrics of cotton, jute, flex, linen, hemp, etc. were extensively used. These were printed, painted and stamped with pigments or coloured (yellow, blue and red) with dyes.

  • It is believed that word Chemistry is derived from the word khemeia, which relates to the Greek word khumos, (literally meaning juice of plant or art of extracting the juice -or extracting liquid metal from rocks). The art of khemeia is the art of metallurgy, which was obscured in unexplained processes. In Arabic, khemeia became al-kimiya (Farsi, Gujarati Hindi Kimiyagiri=one who knows the formulation). It was adopted as alchemy.

Next few BLOG posts in this series will be :

THE CRAFT OF WALL PAINTING
THE ART OF OIL PAINTING
WATER COLOURS AS ART MATERIALS
THE MODERN TECHNOLOGY OF COATINGS
COATINGS: FILM FORMING SUBSTANCE
COATINGS: COLOURANTS AND EXTENDERS
COATINGS: SOLVENTS, THINNERS AND DILUTANTS
CLEAR AND COLOURED COATINGS
ARCHITECTURAL COATINGS
INDUSTRIAL COATINGS
SPECIALTY COATINGS
SOLVENT FREE COATINGS
COATING AS A MATERIAL DEPOSITION TECHNOLOGY



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