A history of new ideas in tanning

1 September 2013

Dr Graham Lampard charts the history of major technological breakthroughs in the history of leather tanning and asks where the key areas of focus should be for innovation within today’s industry

The first IULTCS congress back in 1897, held at the Herold's Institute and the Leathersellers Hall, London, brought together 20 or so of the world's - or in this case Europe's - leading tanners. Note that it was the tanners, not chemists or scientists, who accounted for the vast majority of attendees - granted, Prof Procter and Dr Gordon-Parker were there, but they were very much in the minority.

They gathered to discuss how best to "bring about uniformity of method in the analysis of tanning extracts and materials." It was the "hot topic" of the day. The majority of leather was tanned with vegetable extracts and the tanners were concerned that they were unable to produce consistently good leather because the percentage of tannin in a batch was very much in dispute. It depended greatly on how the extract was sampled, whether it was a liquid or a solid and what the impurities were.

110 years later the "hot topic" would appear to be "tanning theory": we all tan hides and skins, and those tanners who do it as a business do it very well, meeting all the requirements put before them, both from the end-users perspective and from the legislative point of view. However, a comprehensive tanning mechanism still seems elusive. Even the greatest leather scientist of the current age with his Link- Lock theory still seems unsure whether it is a "dark art or a clear possibility".

Prof Dr A D Covington said at the IULTCS's Valencia congress in 2011: "Predicting the outcome of any technological change in complex systems of leather processing is not an exact science. Nevertheless, by adopting the kind of approach set out here, drawing on both scientific and technological principles, it is possible to go a long way towards accurate and useful prediction of the effects of concurrent and consecutive process steps. This can make developmental programmes much more efficient.

"Perhaps more importantly, the ideas proposed here will allow prediction of not only the processes designed to make leathers with desired properties, but also the constitution of reagents to achieve the end. This constitutes a paradigm shift in thinking and, hence, in creating strategies for advancing leather development and for defining and making new collagenic biomaterials."

Also in Valencia in 2011, Ellie Brown from the USDA's Eastern Regional Research Center, reviewed the development of tanning theory. She said early bark tanning likely started with the soaking of hides in pits lined with tree bark, and evolved into milling or crushing the bark to increase surface area and make the tannins more available. The remnants of mills for crushing bark prior to 1500 BC have been identified. The crushed bark was placed in a pit with water and skins were soaked with the tannins released from the bark to produce strong, pliable, durable leather. Similar processes are currently used in several countries.

By the Middle Ages, tanning was designated a craft, and tanners guilds (the model for IULTCS and the various local organisations) began to form. The next major period of innovation came with the Industrial Age of the 1800s when the tanning drum largely replaced the open pit and greatly increased the efficiency of tanning. By 1840, the medical community had adopted sutures that were stabilised by soaking in a Cr (III) solution. A few years later, it was discovered that soaking these chrome tanned sutures in glycerol made them more pliable, or softer. These two advances in medical technology were soon adapted by tanners and became chrome tanning and fatliquoring. By empirical methods, over millennia, several classes of tanning agents with a variety of properties have been identified.

She continued: the function of tanning is to stabilise the structure of the collagen matrix of the hide or skin, increase its hydrothermal stability and protect it from microbial degradation. Current commercial tanning agents, include mineral (mainly Cr(III)), vegetable (polyphenolic tannin), and organic (aldehyde) reagents. The product of each of these tannages is leather, despite the different chemistries. From a 21st century perspective, none of these processes is ideal, and a comprehensive tanning mechanism would provide a basis for the design of more sustainable and eco-friendly tanning processes. The goal of a comprehensive tanning mechanism that can explain the effects of current and past practices, predict requirements for new reagents and processes, and accurately predict the outcomes of proposed new practices is one that leather scientists have pursued at least since the early 20th century.

The following paragraphs present a brief survey of the work of a few internationally recognised leather scientists who during the 20th century made major contributions to the understanding of the structures of collagen and tanning materials and possible interactions between them. Although the evolution of tanning processes occurred slowly prior to the industrial revolution, technical advances of the 20th century, combined with increasing use of collagen in medical biomaterials, began to provide a basis for understanding the relationship between collagen structure and function in both biology and technology.

The early 20th century was a time of rapid development in both basic sciences and applied technologies. This period saw a large increase in scientific publication to document advances in theoretical and applied science. The documented history of the quest for a comprehensive tanning mechanism is a bibliography of great scientists worldwide who applied their talents to this search. H R Proctor (UK) started the field with the 1914 publication of: "The Making of Leather", a book that detailed the then "state of the art" for leather technology and science and formed the base for future research.

Following Proctor, Brown stated, was J A Wilson (USA) who spent his lifetime merging the practice of tanning with the academic science of collagen, and served as an interpreter of basic science to the industrial leather chemists and technologists. Three of Wilson's publications were major reviews of the then current knowledge of the relationship between collagen function and structure. A 1919 publication applied the principles of colloid chemistry to the production of leather, proposing that tanning occurred primarily through electrostatic interactions between tannin molecules and charged sites on the surface of the skin or hide. The following decade was one of rapid advances in most scientific areas, and in the1928 Chandler Lecture at Columbia University, entitled "Chemistry and Leather", Wilson recognised that leather chemistry was very much concerned with the molecular structure of the protein, collagen, and that the interaction between collagen and chromium was not the simple binding of a Cr3+ ion to collagen.

In Wilson's final presentation in 1941, structures were beginning to be proposed for vegetable tannin molecules with the potential for multipoint fixation to the fibrous structure of collagen. The effect of these tanning materials on the shrinkage temperature of collagen was recognized as a reliable indicator of degree of tanning.

Brown noted the Swedish leather chemist, K H Gustavson was active between 1920 and 1969 when details of the shape and composition of collagen molecules became available. He brought to the attention of the leather technologists the unusual amino acid composition of collagen including the presence of 30% glycine, 25% proline plus hydroxyproline and a small amount of the unique hydroxylysine residue as well as a paucity of aromatic and sulphur containing residues. During this time, the molecular architecture of collagen with its periodic pattern of dense and more open areas, and the nonhelical telopeptides to guide the formation of fibres was also becoming apparent.

Gustavson's personal research, documented in more than 200 publications, provided a large data base of the reactions of chromium salts, polyphenols and aldehydes with collagen as a start to the molecular understanding of tanning. The Indian scientist G N Ramachandran, closely associated with the Central Leather Research Centre in what was then called Madras (now Chennai), determined many of the stereochemical characteristics of collagen that contribute to the success of tanning processes.

Brown explained that Ramachandran's work in the 1950-1970 period established that to explain the X-ray diffraction pattern, the single chain, left-handed helices must super coil into a right-handed triple helix. The presence of a glycine residue, lacking side chain carbons, at every third position in the chain, was the only option for the triple helix. Although nearly any other amino acid could be accommodated in the other positions, the presence of hydroxyproline, with its potential for hydrogen bonding to water immediately preceding a glycine, contributed significantly to the stability of the helical structure.

Finally, Brown reviewed the work of Heidemann, after whom the IULTCS Heidemann Lecture is named. She said E R Heidemann of the Darmstadt school was a leading educator on the roles of collagen structure and function as pertaining to tanning, in the latter part of the 20th century. Heidemann's career began with publications of fundamental studies on the nature and reactions of collagen in the 1960s, and culminated with the publication of "Fundamentals of Leather Manufacturing" in 1993.

He was among the first to explore the molecular level interactions of collagen peptides with tanning materials of various types. He clearly enunciated the long-term benefits of fundamental research into the structures of hide protein components, tanning and fatliquoring agents and the other materials that are necessary to produce a fine piece of leather. As did his predecessors, he applied the fundamental research results to the practical problems experienced by tanners. Thus, over the 20th century, innumerable academic and leather scientists, elucidated details of the unique molecular structure of collagen.

Brown then went on to review the work looking at tanning baths. During this same period, tanning baths were being analysed to determine the composition of tanning mixtures and the structures of probable tanning agents. Early attention was given to vegetable tannins, where the process is still closely related to the bark tanning practiced 3,000-4,000 years ago, although polyphenolic tannin extracts from tannin rich plants including mimosa, quebracho, and chestnut are now in common use.

Lollar's comprehensive review at the mid 20th century concluded that the previous 50 years of research had revealed many facets of the chemistry of tannins, but the conclusions drawn by different investigators were inconsistent. Lollar was optimistic that the tools then available would shortly produce correct structures and chemistries. Considerable research since that time has confirmed the complexity and non-homogenity of vegetable tanning agents, and has led most researchers in this area to develop model systems to gather data from a few polyphenolic compounds and extrapolate the results to the tanning process.

Na's 1988 study using monomeric polyphenols showed that only those polyphenols with acid functionality could bind directly to collagen. However, when tannin solutions were allowed to stand, auto-polymerisation led to the formation of much larger species that could compete for hydrogen-bonding sites on collagen, thus stabilising the fibre structure. Haslam in 1997 first proposed that these larger tannin complexes contribute to collagen stabilisation by filling gap regions of the fibril.

Aldehyde tanning originated in the reaction of smoke with cured hides, and the use of formaldehyde as a tanning agent was patented before 1900. Theis and co-workers, in the late 1930s, published extensively on the chemistry of the reaction between formaldehyde and collagen, noting that even when the substrate was soluble collagen, formaldehyde could react with only a fraction of the amino side chains.

Brown concluded that little more than hundred years ago the question was whether the stabilisation of hides by treatment with mineral salts could properly be called tanning. The nomenclature question has clearly been resolved, and chrome tanning accounts for about 90% of the production of high quality leathers today. Less clear still are the natures of the tanning species and the tanning reaction, and the final product. Basic chrome sulphate (BCS), the usual tanning salt, is most likely a mixture of mono and polynuclear chromium species bridged with oxygen, hydroxyl and possibly sulphate groups.

The composition is fluid, and with addition of masking agents even less well defined. The chrome tanning reaction has traditionally been described as cross linking of carboxyl groups on collagen by multinuclear chromium complexes. The formation of both intra- and interfibrillar crosslinks in collagen when treated with different chrome tanning formulations has been demonstrated. Leather researchers at the Eastern Regional Research Center (ERRC), USDA, over the past 70 years, have pursued research that contributes to the search for a comprehensive tanning theory. One of the earliest publications from the centre dealt with the stabilisation of vegetable tanned leathers. In fundamental studies of collagen structure, ERRC leather scientists of the 1940s and '50s used electron microscopy, then an emerging technique, to obtain micrographs of cattle hide collagen in its native state and after the various steps of beamhouse processing. In the 1970s, they examined the effects of hydration on collagen structure and the flexibility of the non helical regions. The fundamental understanding of collagen structure was advanced in the 1980s through studies of fibril assembly.

The closest we come to a comprehensive tanning theory is Covington's theory based on the link-lock concept that requires an initial reaction to link the collagen into the surrounding matrix of water and a second reaction component to lock the linked structure together, creating a macromolecular structure around the triple helices, and is the most interesting current proposal. The application of theoretical and experimental tools to the exploration of this concept should provide valuable insights to the mechanism.

The whole concept of "breakthrough" is rather misleading. As the above shows, it is more a series of small steps over a period of time, by various scientists and groups. Initially, it was in the first world, now it is mainly in Asia, and particularly China, where the fundamental work will be undertaken.

HR Proctor laid many of the foundations for 20th Century research
Tanning circa 1880

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