For all those who, through curiosity, have discovered or may discover their interest in the fascinating field of chemistry

Foreword

Nothing shows us the beauty of our world as vividly as its colors. For our distant biological ancestors, there were predominantly smells. However, at some point in our long evolution into modern humans, we dared to swap the dull magic realm of scents for the bright precision of our eyes. And yet colors are also a magical realm that holds many a secret. Unlike shape, density, or surface texture, color is not an inherent property of an object but only our perception of how the object reflects or absorbs visible light. Moreover, our eyes show us only a tiny fraction of the immense spectrum of electromagnetic radiation that fills our universe. The wavelengths, and thus the frequencies, of this spectrum span 16 orders of magnitude-from the 10 to 20?km long radio waves of some military transmitters to the gamma rays of imploding galaxies, which are only a thousandth of a nanometer short. Life on our planet mainly registers wavelengths between 300 and 1000?nm. This range includes the ultraviolet, with wavelengths below 400?nm, which unlike many insects, we cannot see; the range from blue to green to red, that is, from 400 to about 750?nm, which means light to us; and finally, infrared rays, with wavelengths above 800?nm, which some animals perceive as light but we perceive only as heat.

Ultraviolet was probably the first color that life on our planet saw. This spectral part of sunlight meant danger, as it destroyed many biological building blocks. Cells developed a sensor for ultraviolet and blue light that controlled the direction of rotation of their flagella to avoid these dangerous rays. Since these flagella act as propulsion propellers, the cells could now not only see the harmful short-wave light but also avoid it. A descendant of this blue light sensor is still found today in many primitive protozoa.

This ingenious blue light sensor probably also served the cells as a construction manual for a solar collector, thanks to which they could feed on the energy of sunlight. Cells shifted the blue sensor's absorption to yellow-orange to capture as much of the sunlight's energy as possible. The cells coupled this solar collector to a system that converted the captured light into chemical energy. With its help, the cells could now power energy-hungry processes such as growth, cell division, and movement, or synthesizing fat, sugar, and proteins. This primitive photosynthesis is still found today in some single-celled organisms that thrive in the salt-rich margins of the Dead Sea or spoiled cured fish. Ultimately, however, this form of photosynthesis proved to be a dead end because it did not efficiently convert light from the sun into chemical energy. When later cells used chlorophyll as a solar collector, ushering in modern photosynthesis, photosynthesis that evolved from the blue light sensor remained limited to a few primitive single-celled organisms.

So, life learned very early to see the world in two colors-blue and yellow-orange. And now that it had seen color, it no longer wanted to do without it. With their extensive and information-rich genetic material, the complex modern cells created three, four, or even five different variants from the primitive blue light sensor, which opened up a vast and differentiated color spectrum for them. Even more, these modern cells were able to couple the signals of these different color sensors separately to increasingly complex nervous systems.

Our eyes are equipped with five different light sensors, all closely related chemically and probably descended from the primordial blue light sensor mentioned earlier. One of these sensors is not used for vision but for the daily calibration of our "circadian" body clock. Another sensor is found in the rod cells of our retina. This sensor is susceptible to light and, therefore, we use it in dim light. However, this high light sensitivity comes at a price because our retinal rods do not detect color or fine detail. In bright light, we use three color sensors in the cone cells of our retina-one for blue, one for green, and one for red. These sensors are not very sensitive to light, but they show us fine detail-and color. Since each of these three color sensors can detect about a hundred different intensities of color, and our brain compares the signals from the three sensors, we can see not just three but one to two million colors. Older animals, such as insects and birds, have up to five different color sensors and cannot only distinguish many more colors than we humans can, but some can also see ultraviolet or infrared light to which we are blind. When the first mammals evolved, they mainly hunted at night, leaving some of their color sensors to atrophy, leaving only two of them. Almost all mammals-such as dogs, horses, cats, and cows-therefore see only about 10000 different colors-about the same as "color blind" humans. Only when intelligent apes wanted to distinguish ripe from unripe fruit against the background of multicolored leaves did they again develop a third color sensor, which allowed them to see the world in a new blaze of color. So, humans and our close relatives, the great apes, are the only mammals that can see millions of colors.

In this impressive book, Ingo Klöckl describes the magic realm of colors from a chemist's perspective. The synthesis of modern dyes with intoxicating color depth and impressive stability was one of the great triumphs of nineteenth and twentieth century chemistry, and the development of rewritable digital data carriers or catalysts for light-driven water splitting suggests that the time of color chemistry is far from over. Ingo Klöckl describes the bewildering variety of dyes available today and gives us detailed information on how they can be produced, categorized, and compared with each other. This book is a masterpiece, a true magnum opus that reveals to us in each chapter a new wonder from the world of colors. The wealth of information it imparts to us is almost mind-boggling, yet it is an exciting read for anyone who is no stranger to chemistry. The book also builds a most welcome bridge between science and art, which have become increasingly distant from each other in recent centuries, forgetting their common roots. May not only natural scientists but also painters and art scholars pick up this book and lose themselves in the magical realm of colors.

Basel

Dr. Gottfried Schatz┼

Foreword to the English edition

Dear readers, painting scientists, and researching painters, when the German edition appeared, I never expected it would appeal to so many fellow human beings since the balancing act between painterly observations and chemical-physical theories presupposes a profound understanding in many areas or at least interest. However, I was proven wrong, so this English edition will hopefully accompany and support your work in the vast, intricately interwoven field of art and technology.

Unfortunately, the unhelpful division into natural science and humanities also divides the view of our world as the gods of the Greeks divided the spherical people. Bucklow's work [10] showed me how a holistic, Platonically oriented understanding of the science of painting looked in the Middle Ages and embraced (al-)chemistry and painting equally.

My deepest hope is that you will also see this bridging of art and science as a contribution to an overall humanistic understanding of the world, as was familiar to many great minds of science.

Acknowledgment

My first and most important thanks go to my enchanting wife, Claudia, who once again had to spend a large part of her time with a fanatically lecturing author and who, through constant gentle urging, got me to finish the incubation of the book. Only this made the publication possible at all. Moreover, most important, she dedicated vast amounts of her time to translating the text into readable English, urging me to improve and clarify numerous statements. She introduced me to the secrets of translation-oriented writing and the benefits of a concise language.

Sadly, the author of the foreword, Dr.?Schatz, passed away, to whom I will forever be grateful for the insightful discussions and his foreword, which so well expressed the magic of color. Furthermore, I would like to thank Mr. D.?Widmer for important book recommendations and information on writing inks, Dr.?S.?Hunklinger for an engaging discussion on the color of semiconductors, Dr.?T.?Vilgis, Dr.?B.?Schneppe, and G.?Bosse for a discussion of the topic of egg white binders and clarea, Dr.?W.?Müller for his feedback on the composition of acrylic paints, Dr.?B.?Born and Mrs. R.?Ardal-Altun for a long, informative and entertaining telephone conversation on the production of artists' papers, Dr.?G.?Kremer for many valuable suggestions, improvements and information from the practice of a paint manufacturer, Dr.?K.-O.?Schäfer and Dr.?W.?Thiessen for information on ink production, and Dr.?G.?Schatz for sending additional material. I would especially like to thank Dr.?Kremer, and again Dr.?Schatz, for their positive evaluation of the manuscript. They encouraged me, once as a practical color chemist and once as a versatile natural scientist, to pursue the book's aim. They were right, as the friendly and positive letters from colleagues and...