Chemistry in Eighteenth-Century Theories of Coloration

Louis-Guillaume de la Follie's Theory of Coloration

Late in 1767, Louis-Guillaume de la Follie, a Rouen textile manufacturer, submitted a pli cacheté to the Paris Academy of Sciences.18 While this tactic of depositing a sealed envelope at the Academy was often employed to establish the primacy of a discovery, de la Follie's motivation is unclear. In any event, the envelope was opened less than a year later. A committee was appointed to examine what proved to be an essay describing de la Follie's theory of coloration; they reported on it in late 1768.

Telles sont mes sentiments sur la formation et sur le jeu des couleurs; c'est en suivant ces principes que je suis parvenu a simplifier plusieurs opérations de teinture, a produire quelques couleurs plus brillantes, plus solides et plus pénétrante dans la tissure des étoffes.

Louis-Guillaume de la Follie,Mémoire sur la theorie des couleurs,9 January, 1768, AdSpochette,p. 24.

In his essay, de la Follie used a combination of sources, drawn from the disciplines that occupied him as a manufacturer and as a man of the sciences, to create his theory of coloration. As such, the theory suggestes some typical mid-century concerns about production, processes, and their potential in the development of a theoretical system, as voiced by a knowledgeable and enlightened manufacturer.

The consistent behavior of nature was an obvious foundation for de la Follie's explanation of color and color change: In his theory of coloration he combined aspects of the physics and chemistry of textile and vitreous colors and he substantiated his points with examples from medicine and botany. Color differences within any object are a matter of porosity and pore direction, combined with the degree of vitrification of the coloring particles and tempered by airborne acids and alkalis. In his description of the hard, finely divided (and preferably metallic) earthy particles that create a permanent and beautiful color, de la Follie drew on enamel or ceramic coloring processes: His examples may have been found at the faience factories for which Rouen was famous and that he had investigated on behalf of French government agencies.19 It is a general law, de la Follie stated, that the more a body contains of these metal-like earthy substances, the more refractionary it is. This is why enamel colors resist fire better than vegetable colors, and why the key to good color is to find a way to make all coloring parts resemble mineral colors.

According to de la Follie's theory, color forms in bodies through several mechanisms simultaneously. First, there are the coloring particles themselves, small molecules of vitrified earth capable of reflecting the light of the bodies to which they adhere. Coloration occurs when these particles attach to an object. The exact color and its intensity are determined by the porosity of the object and its ability to accept the coloring particles, with the direction of the pores also playing a role in the result. de la Follie seems to build this notion from Hellot's description of color as lodging in the pores of fibers, and he exploits a macroscopic optical effect to explain his microscopic chemical relationship. Some color variations across the surface of a solid-colored textile depend on the position of fibers relative to the light source. Those variations, easily visible on heavily napped or piled fabric, are due to the direction and degree of spin in the yarns. Similar analogies exist elsewhere, perhaps most dramatically in the play of light on a colored wall.

In his chemical explanation, de la Follie identified phlogiston, the igneous principle, as an aid to the vitrification that creates good colors. Because heat made things more solid, phlogiston controlled both the color produced and its tendency toward permanence. Creating a good color required that quantities of phlogiston be fixed within the object. The same need to secure sufficient phlogiston also accounted for the color changes observed over time. Even after cooling, the pores continued to dilate and contract, altering the color on the object. With the introduction of more heat or salts, vitrified atoms break up, dissolve, or recombine, changing the degree of vitrification and so the color. In addition to fire, water and air also had specific and significant parts in de la Follie's theory. Water promoted fermentation or decomposition, and air attracted acids and alkalis. To appreciate the importance of the effect that these two elements could have on color, one need only think of the pattern of color change in plants as they are exposed to air, and in particular the color loss they undergo in the more acidic winter air.

Many of the ideas that de la Follie presented had already been suggested by others—in his mémoire, he noted often that his concepts were already well known. The report of the academicians who examined this theory, Pierre Joseph Macquer and Lavoisier, remarked on this when they denied institutional approval to de la Follie's article. reference Macquer and Lavoisier pointed to the greater simplicity found in Newton's system. They suggested that de la Follie may have misunderstood relevant passages in what was obviously an important source for his ideas: Nollet'sLeçons de physique expérimentale (1754-65).20 Another misunderstanding the reviewers cite concerns Dufay's statements about the three primitive colors: When Dufay suggested that red, yellow, and blue could be used to imitate all colors of the solar spectrum, he was not implying that there are, as de la Follie seemed to believe, only three kinds of rays. Macquer and Lavoisier noted other problems underlying de la Follie's efforts to join colors of light with colors of objects—his description of angles of refraction, for example, and his belief that coloration was due to light refraction from the many small prisms that cover the surface of bodies. For the most part however, the reviewers were most critical of de la Follie's understanding of the physics of color; their objections to his ideas about coloration were less sweeping. Concentrating on his acid-alkali explanation, they complained that this simple explanation was too simple. While it is probably true that the different salts, separated in the air and so able to recombine in different ways, had something to do with color, but it was difficult to believe this was the main cause of color. Certain phenomena, such as the formation of Tyrian purple, cannot be fit into the explanation. Other portions of de la Follie's theories, according to Macquer and Lavoisier, are simply gratuitous. His ideas were not worthy of publication.