Thirty years ago, America found itself in the throes of a heated public debate about the merit of a provocative photograph by a taxpayer-funded artist, Andres Serrano, who had plunged a crucifix into a beaker of urine to capture the eerie atmospherics of refracting light on the sacred object.
A keen collector and art historian, Forbes embarked on a global search for far-flung pigments in order to better appreciate the raw materials that make up the masterpieces we see and to guard against the counterfeit allure of forgery. The cornerstone of the Forbes Pigment Collection is the countless phials filled and labelled by Forbes himself.
Bovine dung, to be precise. According to Finlay, the British writer Rudyard Kipling remembered his uncle, the Pre-Raphaelite painter Edward Burne-Jones, conducting a funeral procession and solemn burial for his earthen tube upon the appalling discovery that the colour had been harvested from the bodies of the dead.
However, organic luminescent pigments are more popular in ink. Pearlescent pigments are used in ink to obtain a pearl-like appearance. They mimic the reflection mechanism in natural pearl. Pearl has multiple layers of calcium carbonate and protein. Incident light undergoes multiple reflection between these layers, resulting in the special appearance.
Similarly, in pearlescent pigments, flakes of the mineral mica lower refractive index coated with layers of titanium dioxide higher refractive index create the impression of luster and sheen, on reflection of light. By manipulating the thickness of coating on mica, a range of colors can be achieved.
Other pigments such as gamma Fe 2 O 3 and CrO 2 magnetic pigments that are used as magnetic recording media in the tape and disk forms could find imaginative application in ink also. Some of the physical properties of inorganic pigments include particle size, shape, size distribution, specific surface area, sieve residue, hardness, density, refractive index and oil absorption.
The important crystal structures generally found are cubic sphalerite lattice as in CdS , tetragonal rutile lattice as in TiO 2 , rhombic geothite lattice as in alpha -FeOOH , hexagonal corundum lattice as in alpha -Fe 2 O 3 , and monoclinic monazite lattice as in PbCrO 4.
Chemical parameters such as acid value, alkali value and pH of the aqueous suspensions are useful tools in characterizing the pigments. The optical properties emanating from the interaction between light and the particles produce various colors due to selective absorption of light, whiteness due to nonselective scattering, and blackness due to nonselective absorption. The important theoretical treatments correlating the reflection spectra and optical properties of a pigment are based on colorimetry, the Kubelka-Munk theory, the theory of multiple scattering, and the Mie theory.
The refractive indices of pigments, especially white pigments, are important as they decide the hiding power of the pigment, based on the Lorentz-Lorenz equation where the difference in the refractive indices between the pigment and medium is of consequence.
Synthetic organic pigments are the most generally used pigments in modern-day ink. Even high school students, who often perform paper chromatography demonstrative experiments to separate colored pigments from ink samples, appreciate the role of these pigments in ink. These pigments are comparatively costly. But, their superior beneficial properties such as lightfastness, tinctorial strength and low toxicity outweigh this weakness. This type is useful in waterborne ink.
One advantage is that the pigment dispersion problem is minimal. This is produced from the press cake quality and is very dusty. Much energy is expended to disperse this quality, as the adsorbed air and other gases must be completely displaced before the wetting needed for the dispersion process is achieved. Flush quality that is made from press cake quality by displacing the water content with resin containing vehicles.
This quality is compatible with many coating formulations. The ink maker can bypass the dispersion stage by selecting appropriate compositions of these dispersions. Microencapsulated quality that directly interacts with the polymer matrix in which they are usually embedded. They are generally prepared by precipitating preformed polymers onto the particle surface or by adsorption of gaseous monomers followed by polymerization. The problem of nomenclature of pigments was a thorny issue. Manufacturers named their products using their own attributes such as shade, color strength and chemical source.
These organizations have suggested a Colour Index name to reflect the hue as in the case of PB 15 Pigment Blue for the copper phthalocyanine blue, and a Color Constitution number to associate with the structural features as for PB CI index numbers point to the chronological and structural details of the pigment.
Fluorescent organic pigments, especially those that fluoresce in daylight, have gained prominence these days, owing to a variety of applications in security ink to prevent forgery, traffic light signals, safety applications, poster boards, and advertising. Many of these pigments are available as polymer composites.
Fluorescent pigments are currently used in flexography, screen, lithography and letter-press ink, as well as in paint and plastics. Organic pigments are available with superior fastness, a property that is becoming important in ink applications. There are many situations in which ink coatings are being exposed to factors like light, weather, moisture, heat and solvents. Chemically and photochemically inert pigment grades have been developed for outdoor applications.
One approach is to control the particle size of the pigment to reduce the detrimental effect of solar radiation.
Thus newer technology has helped ameliorate the color-fading problem even in relatively high dilutions. The ultimate color producing effect of pigments is related to the constitutional structure.
Molecular structure is the prime factor that is further influenced by the solid state properties. The structural feature — called a chromophore — is responsible for causing color that results from the absorption of near ultraviolet and visible region of light. For this, the electronic transitions should occur at wavelengths corresponding to the UV and visible regions. Inorganic pigments have weak colour strength. Apart from other uses, pigments are also used in cosmetics. Both organic pigments , inorganic pigments are used in the manufacture of cosmetics.
Organic pigments manufacturer makes lakes, inorganic pigment manufacturer makes iron oxides. Organic and inorganic colours are used by the cosmetic pigment manufacturer in their cosmetic manufacturing. Most of the cosmetic manufacturer uses these in their makeup since there is no chance of skin allergies. Iron oxides come in a range of colours such as black iron oxide, yellow iron oxide and so on. The colour tone depends on factors such s pH, temperature, concentration and so on.
Lakes are produced by soluble dye with metallic salts. While organic colours are certified, Iron oxides are none certifies but are widely used in makeup products such as foundations, lipsticks, eye shadows etc. Inorganic pigments include white opaque pigments that are used to lighten other colours and also to provide opacity. The most significant member of the class is titanium dioxide. White extender pigments are added to the paints so as to improve their properties as well as lower their cost.
This class includes calcium sulfate, calcium carbonate, china clays and diatomaceous silica. Black pigments are mostly created from particles of carbon.
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