August Bindery & Bespoke Atelier
Special Note – Leather

Special Note – Leather

A special note about the leather used here at August Bindery.

Siegel Leather


Leather & Parchment since 1550

A unique boutique tannery located in New York State, Pergamena has been producing leather and parchment since 1550. All of their leathers are produced using a refined chestnut vegetable tanning process and are sourced locally in the Hudson Valley.

They invite you to Learn About Leather From Farms, an initiative creating traceable leather from selected farms who use exemplary grazing practices. They work closely with farmers and abattoirs in the region to source hides from many of the excellent farms in the Hudson Valley. Farm suppliers are carefully chosen based on their positive practices which produce nutritious food, support ecosystems and improve animal welfare. This is leather from animals raised with respect and care, on farms who support biodiversity and ecosystem resilience. For anyone interested in ethical materials, Pergamena’s traceable leather from pasture farms is an offering not available elsewhere in the leather market.

Siegel Leather

We are pleased to offer pure vegetable-tanned leather from Siegel Leather, a family owned business and leather merchants since 1917. Siegel Leather has a world-wide reputation for the high-quality leather they produce. Their transparency in both their sourcing and tanning process is nearly unequalled in the entire industry. They produce some of the highest quality vegetable-tanned leather available on the market.

All of the leathers offered by Siegel Leather use a centuries-old method to produce what is considered by many bookbinders to be among the highest quality vegetable-tanned leather available today. The Sulfur-Free Calf available from Siegel Leather is of such high and pure quality that it is not available from any other leather manufacturer, and has been found to be the best Archival-quality bookbinding leather available world wide by independent laboratory analysis.

Below, please find extensive reference materials, as well as detailed analyses of some of the leather produced by Siegel Leather and offered here at August Bindery & Bespoke Atelier.

Please visit this page at Siegel Leather for:

  • A video on the ‘Siegel Effect’ to test for vegetable-tanned leather.
  • A video on the ‘Ewald Effect’ for oil-tanned leather.
  • ‘Genuine Morocco,’ by Steven Siegel, published in Guild of Book Workers.
  • Affidavits of Archival Native Tanned Sokoto Leather.
  • ‘Native Tanned-Colored Archival Sokoto Leather,’ by Steven Siegel.
  • Plus much more in-depth reference materials.

Sokoto Morocco

This leather is produced from Nigerian skins, among the most desirable and expensive goat skins available world wide. They are not imitations of Nigerian skins. We have developed a new leather – Sokoto Morocco. The crust is archival Sokoto Natural, uncolored.  (We call this archival based on archaeological and literature evidence) It is shipped to the EU for processing and coloring. No pigments are used in finishing. The lab samples were very well received. One evaluator said this was the nicest leather that had been available for 18 years. This leather is made on Nigerian Skins which have a grain pattern suitable for restoration of books prior to the 19th century. Our three products, produced by our partnership, Chemist LLC – Sokoto Morocco, Sokoto Traditional and NTND – are all produced in cooperatives in Sokoto, Nigeria. From every production which we make, funds are additionally set aside and donated to local charities. All of these leathers use natural biologicals in their production to the crust. These are genuine Sokoto skins. There is a confusion in the industry with skins produced from Indian goats where the grain has been “manipulated” (embossed) to produce an imitation. If you are looking for an imitation of Nigerian skins you will have to purchase elsewhere.

The leather was introduced at the SOB show in Bath, UK.  A skin put up for auction to benefit the SOB sold at the highest price for all auctioned leathers according to my source.
-Steven Siegel, owner of Siegel Leather

Smooth Historical Goat

We call this “Historical” because we use techniques in producing this leather that does not involve plating and/or grain manipulation to attain this smooth grain, which is almost calf-like. These production methods require flawless raw stock, as virtually every grain flaw is exaggerated with our production techniques. NO OPAQUE FINISHES are used which hide defects. History has shown that mechanical plating or glazing damages the epidermis of the leather, shortening its lifetime. The grain of the SHG is not enhanced by plating or glazing. Skins are specially picked to be lightweight and very square for excellent cutting.
For some of our products, such as this one, we purchase the skins in the “hair”. We have control over the de-hairing process, tanning, and finishing which all affect the longevity properties of the leather. This goatskin is almost as smooth as calfskin and is very popular with bookbinders.

NTND Traditional Red

Recent Certified Lab Results:  (07/2020)


Sulfur Content:  Below detectable limits


In the early 1800’s, bookbinding leather which was produced after 1800-1830 had a shorter lifespan than leather produced in earlier years.  The time period 1800-1830 coincidentally marks the beginning of the modernization of the tanning industry with the introduction of machinery and new chemical tanning techniques.

The late 1800’s saw the first formal survey of leatherbound books throughout England in rural and urban libraries. The observations from these surveys yielded a plethora of information.

At the turn of the 20th century, a group of British scientists met to create guidelines for producing superior bookbinding leather . They used the observations of these previous surveys and established a guideline of leather tanning/processing criteria to be used for future bookbinding leather.

Several decades later, new library surveys were conducted to ascertain the condition of book bindings and the leather which had been used for their manufacture. There was no significant improvement in longevity.  Manufacturing standards were not recorded, nor was there accountability that the original British standards were followed. As little of the leather produced according to the standards stood up to the test of time, two possibilities were apparent:  Either the leather was not produced according to the new standards OR the new standards were not correct to guarantee longevity.

Additional information on this subject can be found in “Leather for Libraries”, by Hulme, Parker, Jones, Davenport, and Williamson, published for the Sound Leather Committee of the Library Association, 1905. Our vegetable-tanned (native tanned) Sokoto goatskin leather is produced using historical bio-chemicals and no other substances which would question its archival properties. Our “time-enduring” skins are not pickled prior to tanning and no sulfur-containing products are used in any processes, including the unhairing process, such as sodium sulfide or sulfuric acid. The literature indicates that leather in the processes of tanning has a great affinity for sulfur (Betty Haines), which ultimately yields another decomposition route for bookbinding leather.  Our Native Tanned Sokoto and the Native Dyed version of this item contain no sulfur-bearing compounds such as syntans, sulfonated fat liquors, sulfuric acid, sodium sulfide, etc.  They are tanned with a gentle pyrogallol tanning agent, bagaruwa. The “time-enduring” version is never exposed to any metal, e.g. for manipulating the grain, shaving, etc. Skins are sold unshaved at full substance.

Our Native Tanned Native Dyed (NTNDTM) in Traditional Red, dyed with local biologicals, is only produced in Sokoto, Nigeria using the centuries-old formula.

We have affidavits as evidence from reputable institutions in Sokoto, Nigeria that have verified the leather which we offer as Native Tanned Natural and NTNDTM to be of the same grain pattern and most likely the same formula that was used over 200 years ago. To the best of our knowledge, there is no other commercially available vegetable-tanned goatskin which can claim historical/archaeological evidence of longevity.

Our Sokoto goat which is colored in Nigeria is claimed by Bernard Middleton to be the same leather used in the Stonyhurst Gospel book which was taken from the tomb of Saint Cuthbert and which is over one thousand years old. The book is the oldest known surviving intact book in Europe and was bought by the British Library in 2012 for £9 million pounds as part of a fundraising campaign.

Per Bernard Middleton, pg. 117, A History of English Craft Bookbinding Techniques:

“That which covers the Stonyhurst Gospel is dark red and is similar to modern native-dyed Niger”

We are the only purveyor of leather made with these sulfide-free formulations.

“The Trans Saharan Book Trade,” by Lydon, states that the only source for the color “Traditional Red” in NTNDTM can only be produced in Sokoto, Nigeria because of the presence of unique biologicals. This is further evidence that Spanish cordovan leather and perhaps the leather used in the Stonyhurst Gospel are  NTNDTM Sokoto goatskins.

Sulfur-Free Vegetable Calf

About the leather – Our Sulfur-Free 100% Vegetable-Tanned and Sustainable, or SF Calf Leather, is manufactured using a specific and researched tanning and re-tanning process in which mineral tannins and sulfated components are avoided. This calf has no grain manipulation and follows the recommendations of mid-20th-century literature so that it is theoretical Archival. It should last longer than any other vegetable-tanned leather, and we are the first in the marketplace to manufacture it.

Are we the only manufacturer of vegetable calf? As the only producer of Sulfur-Free Vegetable Calf, we are testing other commercially available leathers that are advertised as vegetable-tanned. The leathers which we have tested to date, advertised as vegetable-tanned, are not metal-free. One of the leathers, containing aluminum, decomposes quickly when subjected to hydrolysis. Please view our findings in greater detail below. We are told that our vegetable calf is superior to any bookbinding calf, but what do you expect?

Update on Sulfur-Free Vegetable Calf Project as of the 12th of January, 2021 (a privately funded project for non-commercial purposes) – Information concerning the R&D outline on the SF Calf project and production started since 1 year in Italy. The original idea for this project came from Mr. Steven Siegel, third generation of a proud leather merchant family active since 1917, in the United States. A framework was created how to create such leather and with which specifications and characteristics. Literature study followed and definitions were reestablished. The whole story is based on a simple observation. (Book) leathers from BEFORE 1830, more or less, have a much longer life span than leather produced after 1830. Now it is also the case that around 1830 – 1850 new kinds of leather making and chemicals and modern (fast) techniques were introduced. So, the idea is that these rapid (new) systems do have a negative influence on the longevity of (book) binding leathers.

Goal: – To produce a leather that meets the Archival criteria in terms of appearance, physical and chemical properties.

– Be sulfur free (SF), using a beamhouse system based on hair removal and not destruction.

– 100% vegetable (preferable pyrogallic tannins) tanned without usage of any mineral and synthetic tanning agents, and no sulphited, sulphated or sulfonated oils.

– Meet the organo-leptic characteristics, including gold lettering with albumen.

– Pass the ageing and various physical tests.

– Be Archival.

– Be as sustainable as possible.

Archival, or a better term, long-term durable, defines materials intended for restoration of cultural heritage in public and private archives, libraries and museums. This requires that materials used for restoration should be as close as possible to the original material so that the restoration does not interfere with the physical and visual expression of the original object. In addition, the restoration material should have as long a life as possible and should not accelerate the degradation of the original object. Publications from renowned people like Betty Haines, Roy Thomson and Dr. René Larsen in relation with the STEP, CRAFT and ENVIRONMENT projects were consulted to check if an unanimous testing regime and goal was agreed upon. Unfortunately no clear agreement or standardization can be found. This indicates it had to be created.

From the literature, one can understand that something important happened in leather making between 1830 – 1850 which resulted in a dramatic reduction in the lifespan of binding leathers. Introduction of modern tanning methods and Na2S and sulfuric acid and the atmospheric conditions (SO2) degenerated leathers at an alarming speed. This in contrast of very much older leathers which still today exist and are not deteriorated (too much). 

What has been done so far? After developing the SF Calf, a testing regime was set up and four other leathers were collected on purpose, since they represent the benchmark in the binding community.

Quantitative analysis for Organic Sulfur – After solvent extraction on dried leather, sample was concentrated to dryness and then recovered precipitate was oxidized. The result, expressed as sulfur, is obtained by ion chromatography analysis of total sulfates- (Heavy) Metal analysis – ISO 17072-2:2019 and UNI EN 15987- Qualitative FT-IR analysis of fatty substance –UNI EN ISO 4048:2008 + “Internal method GC-MS”- Measurement of tensile strength and percentage elongation – IUP6- Measurement of tear load – IUP8- Measurement of distension and strength of grain by the ball burst test – IUP9- Determination of flex resistance – IUP20- Tropical test (internal) at 50°C and 90% RH for 20 days. No light source and no gasses (NO2 and SO2)- Oxidation test at 120°C in a furnace – sample taken after 24, 48 and 96 hours to determine oxidation.

What has been achieved for results so far? (some tests still continuing)- Quantitative analysis for Organic Sulfur: Done on SF Calf and 4 other supposedly full vegetable-tanned leathers promoted as Archival and suitable for restoration and binding.

Sample 1: 0 ppm. SF Calf fresh (Siegel leather). Calf skin of about 13 – 15 square feet. Sample 2: 78.1 ppm (other leather). Sumac-retanned cow calf crust. Size of about 20 square feet – full hide. Sample 3: 108.1 ppm (other leather). Cow side crust. Size of about 20 square feet. Sample 4: < 50 ppm (other leather). Benchmark full vegetable calf of about 10 square feet. Sample 5: 60.0 ppm (other leather). Baby Calf (Small). Size about 5-6 square feet.

We also tested on the presence of (heavy) metals in the leather samples 1 (ours), 3 (indicated as full vegetable-tanned calf leather, but made on cow or bull sides) and 4 (indicated as the benchmark full vegetable-tanned calf leather). Sample 1 is officially indicated as being Metal-Free under UNI EN 15987, as the sum of tanned metals is less or equal to 0.1% (metal mass/total dry weight of leather). Sample 3 contains 2,580 ppm Cr, which is not a contamination. So, it is not considered Metal-Free, as the sum of tanned metals is higher or equal to 0.1% (metal mass/total dry weight of leather). Sample 4 contains 9,390 ppm Al, which is not a contamination. So, it is not considered Metal-Free, as the sum of tanned metals is higher or equal to 0.1% (metal mass/total dry weight of leather). These results are somewhat worrying because indicated leathers (3 and 4) are considered – and marketed as – being THE benchmark products to go for. The information that the leathers are fully vegetable tanned is incorrect. The physical tests are available in an Exel spreadsheet on request.

Courtesy of Steven Siegel, Siegel Leather.

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