When your peel, bruise or cut fruits or vegetables, the plant tissue releases some enzymes; there is one called Polyphenol oxidase (PPO, phenolase) which in the presence of oxygen from the air, goes into chemical reactions of plant compounds to give brown pigments known as melanins. This reaction is known as enzymatic browning.
An enzyme is a protein that accelerates the rate of chemical reactions.
Enzymatic browning can be a significant problem, because it limits the shelf life of many fruits and vegetables. However, enzymatic browning is not always unwanted. The browning reaction contributes to the desirable color and flavor of raisins, prunes, coffee, tea, and cocoa.
Although enzymatic browning causes changes in flavor and taste (bitter, astringent), and may reduce quality, the melanins formed are not toxic. Brown fruits are safe to eat for some hours after cutting.
Bananas like other fruits are ripened due to a hormone called ethylene.Ethylene breaks down complex sugars into simple sugars and breaks down pectin, a substance which keeps bananas hard.In addition there are hormones that break down green pigments which you see in un-ripe bananas and replace them with yellow pigments. However as the banana continues to ripen these yellow pigments are broken down and not replaced at all producing the brown color in a process much like that of leaves in deciduous trees (trees which lose their leaves annually) during fall and winter. So brown bananas are a result of over-ripening.
Decreased color vision. Cells in the retina that are responsible for normal color vision decline in sensitivity as we age, causing colors to become less bright and the contrast between different colors to be less noticeable.
In particular, blue colors may appear faded or “washed out.” While there is no treatment for this normal, age-related loss of color perception, you should be aware of this loss if your profession (e.g. artist, seamstress or electrician) requires fine color discrimination.
At the age of 70s & 80s, most people in this age group already have or will develop cataracts. Cataract surgery is the only option for correcting cataracts. Color vision declines, and visual fields begin to narrow. Ask your eye doctor about eyewear or lenses for increasing contrast vision. Use extra caution while driving.
Fra Angelico blue pigment extraction (DIY)
How It’s Made – Inorganic Pigments (Commercial, Factory)
The pink colour in a flamingo’s feathers results from the pigments in the spirulina algae it eats. The colour is leached out of the bird through its feathers as a waste product. The Spirulina algae are rich in beta-carotene, which turns them pink. The carotene is toxic to flamingos so they leach it out of their feathers to get rid of it.
During the printing process, solvents and dyes may cause the risk of damage to the environment or may be a health hazard to workers. Many types of solvent are used as bases for inks for gravure, flexographic and letterpress printing; their evaporation ensures that the ink dries quickly. But solvents can be dangerous to health if inhaled in too high a concentration, causing potential problems for print workers, and their emission into the atmosphere contributes to the build-up of unwanted gases.
The alternative is to use water-soluble inks. These are cleaner and vapourless, but tend to take longer to dry, which means that drying equipment has to be modified or the printing slowed down. Efforts are being made to develop new additives which will speed up drying process, but these may affect the quality of the end result.
Vegetable oil-based inks are being introduced in some newspaper printing as an alternative to petroleum-based inks. The main motive for this is the better color brightness and sharper dot achieved, and their improved rub resistance, which prevents print coming off on to the hands. At the moment, vegetable oil-based inks tend to be used as an addictive rather than on their own, but there may be increasing acceptance go them as complete alternatives to solvent-based inks.
The use of heavy metal-based pigments in inks, such as cadmium and lead, has been criticized because of their pollutant effect in effluent, and alternatives are being sought. However, it is difficult to match their unique color and opacity properties.
The environment-conscious option in printing appears to lie with water-based and vegetable oil-based inks, and the designer should request these wherever possible. However, new equipment may be required to handle them, and many printers are therefore likely to use solvent-based inks for some time yet. If solvents cannot be avoided, care should be taken to ensure that the chemicals are disposed of carefully, and emissions reduced to a minimum by using solvent burners.
Source: D, Mackenzie. (1997) Green Design: Design for the Environment p. 123, 124
Dyes and paint pigments are the two main colorant materials.
A dye is absorbed by the material to be colored, whereas a pigment is applied to the surface. Pigment are usually highly insoluble substances. The natural pigments are nearly all minerals found in the earth. The earliest natural pigments, found in prehistoric cave paintings, were the yellow, brown, and red earth colors. These materials are found on the modern artist’s palette as yellow and red ochre, raw and burnt sienna, umber, Venetian red, and Indian red, which are all forms of clay containing compounds of iron.
In sites of the Upper Paleolithic period in France, dating from about 40,000 B.C., caches of iron oxide powder were found that had been heat-treated to modify its color. This marks the earliest known example of pyrotechnology from which have emerged techniques for producing a rich variety of pigments and glazes for pottery.
As in prehistory, some pigments are simply rubbed onto a surface to color it. A pencil consists of a core of compressed graphite that breaks up into fine particles when rubbed on a rough surface. Graphite, being pure carbon, the major constituent of coal, provides an effective black because the small particles reflect light between each other many times before it can reflect back to the viewer. Some pigments are applied to a surface as suspension of fine particles in a liquid carrier, and the particles remain weakly attached when the liquid evaporates. Watercolors are examples, but the water carrier also contains a glue to help attach the pigment.
Source: Samuel J. Williamson, Herman Z. Cummins. (1983) Light and Color: In Nature and Art p. 343- 350