Leather outsoles, not to be overlooked
Innovations in footwear might suggest that comfort, performance, and wearability are in a constant state of evolution.
While fashion-driven changes certainly influence the industry, some components of shoes have remained remarkably consistent over time. One such example is the leather outsole. Until the 1940s, leather was the dominant material used for outsoles in most footwear. The development of the vulcanisation process revolutionised natural rubber, making it more durable and elastic, while advances in plastics and polymers have since introduced new soling options. Despite these innovations, vegetable-tanned leather soles have retained their prominence in traditional footwear.
Leather outsoles are valued for their ability to last for decades, often outliving other materials as a result of their natural durability. Their longevity is a key factor in the sustainability of footwear, as they can be easily repaired or resoled, significantly extending the life of the shoe. Additionally, vegetable tanning, a traditional method that uses plant-based tannins, can be seen as a more environmentally friendly process compared to modern chemical tanning methods. The combination of sustainability, long-lasting wear, and the potential for repair makes vegetable-tanned leather a preferred choice in high-quality, traditional footwear.
As the name suggests, vegetable-tanned leather is produced using naturally occurring tannins found in plant-based materials, such as leaves, tree wood, bark, nuts, and galls, to convert hides and skins into leather. This practice has evolved, though the traditional methods of vegetable tanning, especially for sole leather, remain largely unchanged.
The selection of the raw material is crucial. Sole leather is characterised by its thick, firm texture, and to achieve this, only heavy hides weighing over 36kg are deemed suitable. From the tanner's perspective, a heavy hide signifies one of substantial thickness. While the thickness of the resulting leather can be adjusted during processing, the focus is placed on selecting hides with an appropriate weight and thickness, as increasing thickness is extremely challenging, if not impossible.
Chemical and physical
The tanning process typically follows one of two main methods, with the counter-current system being the most widely used. Oak bark tanning is considered the absolute best for sole leather and while it shares certain similarities with other methods, these distinctions will be expanded on later.
Vegetable tanning is both a chemical and a physical process. While the preservation or tanning of the hide is driven by the chemical interactions between tannins and collagen, the main protein that constitutes animal hides, there are also physical interactions at play where “non-tans” physically fill voids and spaces within the fibrous structure of the hide. As a result, vegetable-tanned leather tends to be fuller and heavier.
For effective tannin fixation, the tannins must first penetrate the hide, which is only possible when the tannin solution has a low concentration. The penetration of the tanning liquid through the full thickness of the hide occurs over several months, as opposed to the days required in modern tanning methods. Accelerating this process by using stronger tannin solutions is not advisable, as it can lead to “case hardening”. In this condition, the tannins will fix on the outer surface of the hide too quickly, preventing further penetration.
The counter-current system addresses this issue by beginning with the least concentrated tannin solution. The hides are then gradually moved through a series of pits with progressively stronger tannin concentrations. This ensures that the tannins penetrate and fix into the hide over time, resulting in fully tanned leather after several months. The term ‘counter-current’ refers to the direction of movement in the system: while the hides move through the pits, the tannin concentration flows in the opposite direction.
Bark blends
Until the mid-19th century, oak bark was the predominant tannin of choice, due to its ability to produce firm yet flexible soles with a degree of water resistance. However, oak bark is naturally low in tannin content, making the tanning process slow and often cited as taking “a year and a day” for effective tanning. While oak bark tannage is still in use today, most tanneries now favour blends of vegetable tannins. These blends offer improved penetration within a more reasonable time frame, while still achieving the desired texture and colour, similar to that of oak. Typically, these tannin blends (liquors) include chestnut, derived from the wood of the chestnut tree, which imparts firmness and a yellowish hue. Additionally, mimosa, extracted from the black wattle (an acacia species), is often added to accelerate the penetration process, striking a balance between time efficiency and the desired quality of the leather.
Oak bark tanned sole leather is a premium material, renowned for its durability, aesthetic appeal, and environmentally friendly production. Unlike the slightly faster full counter-current system, oak bark tanning begins with a slow pit tanning phase. During this stage, the hides are immersed for several months to gradually start the tanning process and “set” the grain. This is followed by a layering stage where the partially tanned hides are placed in pits filled with water or oak tannin liquor and interspersed with oak bark chippings, creating what can best be described as ‘bark and hide lasagna’.
This method, relying on the tannins extracted from the oak bark to impart unique properties to the leather, results in a dense, firm material with a close grain, making it ideal for applications requiring structural integrity, such as shoe soles, essentially requiring a slow wearing out due to abrasion. The leather’s rich, natural colour typically ranges from golden to brown, with subtle variations depending on the specific tanning process used and subsequent processing that involves bleaching, oiling, rolling and slow drying.
One distinctive feature of sole leather is that it is traditionally sold by weight rather than by area, as is common with most other types of leather. Historically, this practice led tanners to incorporate fillers such as sugar (glucose/dextrose), China clay (kaolin), or Epsom salts (magnesium sulfate) to increase weight at minimal cost. While the controlled use of fillers during the final oiling and drying process can enhance the leather’s appearance and properties, excessive use can result in issues such as discolouration. In modern practice, the inclusion of mineral ash content as a stipulated test criterion ensures that such adulterants are not overused.
Another notable benefit is its ability to adapt to the wearer. Over time, it moulds to the shape of the foot, providing a customised fit and enhanced comfort. Its breathable nature helps to reduce moisture buildup, promoting healthier feet. This can be evidenced by a study conducted by the University of Milan that revealed that tanning leather with vegetable tannins produces materials with remarkable antibacterial properties, particularly against gram-positive bacteria. (See World Footwear issue 3 2020.) Leathers treated with these vegetable tannins function as non-leaching antimicrobial materials, making them safe for direct contact with human skin. Consequently, leather tanned with vegetable tannins presents a promising solution to reduce or prevent the development of unpleasant odours caused by the microbial fermentation of body secretions, such as those occurring in feet.
As a sustainable choice, this leather resonates with environmentally conscious consumers due to its natural tanning process, using raw materials from coppiced woodlands, and its extended lifespan. Moreover, items made from oak bark tanned leather are repairable and resoleable, allowing them to remain functional and beautiful for years.
Leather soled footwear and its ability to be repaired and resoled gives longevity and timeless appeal, making it a sustainable and stylish choice for those who value quality and craftsmanship.
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