Every piece of bookbinding leather arrives at a binder’s bench as a finished product. It has a color, a surface, a thickness, and most likely all the characteristics that go on a product catalog. What it doesn’t arrive with is any visible record of the decisions made to produce it – the sourcing choices, the tanning method, the finishing processes, the shortcuts taken or avoided at each stage of its production. Those decisions are invisible by the time the leather reaches you. But they determine everything about how it will perform.
Understanding what happens between the animal and the bench is not an academic exercise. It is the only reliable way to evaluate a bookbinding leather before the binding tells you what it’s made of – five years, ten years, twenty years down the line.
Where Leather Begins: The Raw Hide
The journey of a bookbinding leather follows a specific sequence, and understanding that sequence is the key to understanding what you are actually buying.
A hide begins in the hair; the animal’s full skin, with the hair still attached. At this stage, the grain is not visible to anyone. It cannot be properly assessed, graded, or evaluated. Buying a hide in the hair is a gamble, and has always been one. What lies beneath the hair in terms of grain quality, faults, and suitability for a given application is unknown until further processing reveals it.
From there, the hide is dehaired – limed to loosen and remove the hair – and fleshed. It then enters its first tanning stage, at which point it becomes what the trade calls crust. Crust may also be fat-liquored at this stage. This is the first point at which the grain becomes visible and assessable.
At this point, the moment the hide becomes crust, the leather has entered its first tanning stage, and this stage determines whether the material can ever be considered archival. Once collagen has been exposed to a tannage, that tannage is permanent. It cannot be reversed, removed, or “stripped out,” despite what some suppliers claim. Only excess, unfixed tannins and surface residues can be washed away; the underlying chemical modification of the collagen is fixed from the moment the first tannage is applied.
If a supplier makes the claim that leather can be stripped of its initial tanning agents and converted into something else, the burden of proof lies with them. In practice, they never provide laboratory evidence to substantiate such claims. Without rigorous testing, these assurances remain marketing statements, not scientific fact.
This is why the crust stage is the most consequential point in the entire supply chain for anyone who intends to produce archival bookbinding leather. If the initial tannage is unsuitable, nothing done later – no retannage, no finishing, no dyeing – can convert it into an archival material. The chemistry of the collagen has already been altered.
In regions such as India and Bangladesh, it is common for crust to be produced using white syntans, typically melamine-based syntans, phenolic condensation syntans, or naphthalene-sulfonate syntans. These materials are inexpensive, fast-acting, and designed for bulk commercial production. They create a pale, uniform crust that ships easily over long distances and accepts retannage and dye readily. But they also create leather that is not archival, and cannot be made archival later.
White syntans are synthetic replacement tannins. They do not form the long, stable hydrogen-bond networks characteristic of pyrogallol vegetable tannage. Instead, they create short, brittle crosslinks and leave behind reactive residues that drift acidic over time. Once these syntans have interacted with the collagen, the resulting leather is permanently constrained: it cannot be converted into a pyrogallol-tanned archival leather, no matter what retannage or finishing is applied afterward.
In addition to syntans, crust produced in these regions may also be tanned with local vegetable agents, often catechol-family materials such as myrobalan, babul, or other regional tannins. These are inexpensive and widely available, but they are not historically associated with long-term stability in bookbinding applications. The historical record is clear: catechol-dominant tannages oxidize more aggressively over time and are associated with end-stage failures such as red rot, whereas pyrogallol tannages demonstrate significantly longer lifespans.
For binders and suppliers who purchase leather at the crust stage, this is the critical point: the initial tannage defines the ceiling of what the finished leather can ever become. If the crust was produced with syntans or non-archival vegetable agents, the finished leather will never be archival, regardless of how carefully it is retanned, shaved, or finished afterward.
This is the constraint that governs the entire downstream process, and it is why sourcing in the hair, before any tannage has been applied, is the only way to guarantee that the initial tannage is appropriate for archival bookbinding.
It is also the point at which most of the trade enters the picture. The majority of leather suppliers do not source in the hair. They buy at the crust stage, from tanners who have already completed the initial processing. This is commercially rational – the grain is now visible, the hide can be graded, and the uncertainty of what lay beneath the hair has been resolved by someone else. From crust, the leather moves through re-tanning, additional fat-liquoring, and finishing before it reaches the binder.
This sequence matters because it defines what information is available at each stage, and therefore what decisions can honestly be made.
What Crust Becomes: The Decision Points
After the crust stage, the most consequential decisions about the leather’s long-term performance are made in tanning and finishing.
The tanning agent is the first decision. Pyrogallol-family materials – sumac, oak bark, gallnuts – produce leather that is associated with long-term chemical stability. The tanning process with these agents is slow, often measured in weeks rather than days. Catechol-family materials – hemlock, quebracho, gambier – are faster and less expensive, and they generally produce leather with a fuller body and better immediate physical properties. They are widely used in the trade for exactly these reasons. However, the historical record consistently shows that vegetable-tanned leather produced predominantly with catechol-family agents or other synthetic agents demonstrates significantly shorter lifespans than pyrogallol-tanned equivalents.
An important point to note is that all vegetable leather oxidizes over time as part of its decomposition process. One observed end-stage associated with catechol tannage and other non-pyrogallol tannage is what the trade calls red rot – where the leather turns progressively red and eventually crumbles – though the precise decomposition mechanisms are still debated and other end-stage failures are also possible. What is not debated is the observed difference in longevity between pyrogallol and other tannage families across centuries of documented bindings.
The second decision is acid. Mineral acids – sulphuric acid principally – entered the tanning process in the 19th century because they accelerated bleaching and color development. They are still in use today for the same reasons. The problem, established by the Society of Arts Committee in 1905 and confirmed by direct testing, is that mineral acid does not leave the leather. It fixes in the fiber and converts over time to sulphurous acid, which destroys the leather from within. The damage is invisible at the point of purchase. It is irreversible from the moment the acid is applied.
The third decision is finishing, and this is where the sequence matters most to the bookbinding leather buyer.
What Finishing Does to Crust and Why It Matters
After tanning, leather enters a finishing stage. This stage determines the surface character, the appearance, and the immediate usability of the leather. It is here that the material is made to look complete, but it is also here that decisions are made which affect its long-term survival.
The most significant finishing decision is the choice of agents and processes that will be applied to the crust. These determine whether the leather will retain its integrity or whether it will be compromised for the sake of appearance.
The problem is not the appearance of the leather at the point of sale. The problem is what those finishing decisions do to the chemistry and structure of the leather over time. A finish that looks convincing today may conceal damage that will only become visible years later, when the leather fails in service.
Embossing is the most visible finishing practice. It is used to press false grains into the surface, imitating historical patterns. The trade presents this as a way to match tradition, but it is a deception. Crust leather must be judged by its natural grain, not by embossing. Embossing is a falsification of history, obscuring the forensic record. The binder’s role is forensic: to preserve evidence, not to disguise it. Covington’s hybrid tannage claim is misleading; only pure vegetable tannage with natural grain meets archival standards.
Embossing destroys the grain-corium interface, the bond that defines durability, and produces leather that will not last (Reed, Ancient Skins, Parchments and Leathers, Seminar Press, 1972, pp. 145–147).
Embossing is only one of many artifices. The finishing trade has developed a whole vocabulary of grain assistance, a euphemism for procedures that displace the grain from the corium and treat them as separate layers. Once this unity is broken, the leather is weakened. Tumbling, used to soften and pebble the surface, loosens fiber bundles and breaks cohesion, producing a false pebble that shortens the life of the leather (Covington, Tanning Chemistry: The Science of Leather, Royal Society of Chemistry, 2019, pp. 312–315). Glazing with glass or agate rollers compresses the surface, altering porosity and fatiguing the grain until it loses breathability (Fletcher, Leather Manufacture, Constable & Co., 1915, pp. 198–200). Jacking stretches the grain unnaturally, destabilizing its bond with the corium, while boarding, with its repeated flexing, mimics historic hand-worked patterns but fatigues the collagen until the interface is compromised (Reed, 1972, pp. 152–154). Plating and hot pressing denature the surface proteins with heat, forcing contraction that separates grain from corium, leaving only a cosmetic finish (Covington, 2019, pp. 318–320).
Artificial shrinkage is another deception. Heat treatment denatures collagen fibrils at the surface, causing contraction without corresponding corium bonding. Vacuum drying exaggerates grain contraction by destabilizing hydrogen bonds, while swelling agents such as formaldehyde or melamine syntans stiffen the grain independently of the corium (British Leather Manufacturers’ Association, Technical Notes on Finishing Processes, c.1950s, pp. 42–45). Chemical grain raising, whether by acids that puff the grain through protonation of collagen carboxyl groups, alkalies that disrupt ionic bonds, or synthetic resins that deposit only in the surface layer, produces a raised grain that was never put there naturally (Covington, 2019, pp. 322–325). In every case, the grain is displaced from the corium, and the appearance of strength is achieved at the expense of substance.
These manipulations are marketed as finishing, but they are compromises. They create faux historical surfaces that may satisfy the eye for a moment, yet they fail in service. Archival leather depends on the natural shrinkage grain formed during tanning, not on mechanical or chemical tricks applied after crust. Once the grain is displaced, the leather has lost its integrity, and no amount of finishing can restore it. Only when the grain and corium remain united through tanning can the leather be recognized as the correct leather.
There is also the question of shaving. Shaving reduces thickness and levels the hide, but it also exposes the inner structure. If done excessively, it weakens the leather by removing too much corium, leaving the grain unsupported. The balance between usability and preservation is delicate, and shaving must be controlled to avoid undermining the very strength that tanning has imparted. Beyond this, the grain layer and the corium are structurally distinct, and their proportional relationship in the finished leather determines how it behaves in a specific application. Leather shaved to bookbinding thickness that was destined for another application – belts, harness, upholstery – will have had its grain-to-corium ratio altered in the process. It arrives at the correct thickness but it does not arrive as the correct leather.
What the Binder Receives
By the time a finished leather reaches a binder’s bench, it has already been shaped by the decisions of the finishing house. The binder receives not raw hide but a commercial product, one that may or may not retain the natural unity of grain and corium. The binder’s task is to judge whether the leather has integrity or whether it has been compromised by finishing tricks.
The binder must look beyond appearance. A surface may shine, a grain may look convincing, but the true question is whether the leather has been preserved in its natural state. If finishing has displaced the grain or weakened the corium, the leather is already compromised. The binder must be able to see through the finish to the structure beneath, to know whether the leather will last in service or fail.
This judgment is critical. The finishing house may present leather that looks complete, but only the binder can determine whether it is the correct leather. False grains, embossed surfaces, artificially raised patterns must be rejected. The binder must choose leather in which grain and corium remain united. Only this leather can be trusted to endure, to serve its purpose in binding, and to preserve the book for generations.
The binder’s responsibility is not only to the present but to the future. The choice of leather determines whether the book will survive centuries or collapse within decades. The binder must act as conservator, choosing leather that has integrity, rejecting deceptive finishes, and insisting on the correct leather.
What Siegel’s Process Looks Like
What Siegel’s process looks like is a return to fundamentals. It begins with the recognition that leather must be preserved in its natural unity – grain and corium together, without displacement. The process rejects embossing, artificial shrinkage, and chemical grain raising, and insists instead on the natural shrinkage grain formed during tanning.
Siegel sources hair-on, before any tannage has been applied, which is the only point in the supply chain at which the initial tannage can be controlled and guaranteed. The tannage is pyrogallol-based – slow, vegetable-tanned, and free of mineral acid at every stage. Shaving is calibrated for bookbinding application specifically, preserving the grain-to-corium ratio the leather needs to behave correctly for its intended use. Full process documentation is available on the Sokoto™ product page.
Siegel’s process is conservative in the best sense. It does not impose cosmetic surfaces but preserves the leather as it was made. The finishing is minimal, designed only to protect and present the leather, not to alter its structure. The result is leather that retains its integrity, that can be trusted to endure, and that serves the binder’s purpose of preservation.
This process is not simply technical, it is ethical. It recognizes that the binder’s responsibility is to the book and to the future. By preserving the natural unity of grain and corium, Siegel’s process produces leather that is correct, that will last, and fulfills the true purpose of binding.
References
Covington, A.D. Tanning Chemistry: The Science of Leather. Cambridge: Royal Society of Chemistry, 2009.
Reed, R. Ancient Skins, Parchments and Leathers. London: Seminar Press, 1972.
Fletcher, J. Leather Manufacture. London: Constable & Co., 1915.
British Leather Manufacturers’ Association. Technical Notes on Finishing Processes. London: BLMA, c.1950s.
Hulme, E. Wyndham; Parker, J. Gordon; Seymour-Jones, A.; Davenport, Cyril; Williamson, F. J. Leather for Libraries. Published for the Sound Leather Committee of the Library Association by The Library Supply Co., London, 1905.
