The Production Process

Intro

Intro

The tanning production process is highly complex, consisting of an alternating sequence of chemical and mechanical operations.
The chemical phases, up to the finishing stage, are carried out using water in the tannery’s typical machine known as the drum — essentially a rotating cylinder in which hides/skins, water and the required chemical agents are loaded and processed.
Preparation to Tanning

Soaking

It consists of treating the hides/skins with water and, in some cases, small amounts of chemical agents (such as surfactants, basic salts, or enzymes). During the soaking process, the normal moisture content of the hide/skin is restored, while salt (in the case of salted hides) and other substances not useful for the tanning process are removed.
Preparation to Tanning

Liming – Unhairing

Often referred to simply as the “liming stage”, it consists of treating the hides/skins with lime and sodium sulphide to remove the hair and epidermis — except, of course, when producing hair-on leathers.
Preparation to Tanning

Fleshing

It is the mechanical operation through which fat and residual tissues still adhering to the hide/skin after flaying are removed.
Preparation to Tanning

Splitting

Hides with greater thickness (especially those from adult cattle) are often subjected to splitting. This is a mechanical operation that divides the hide into two or more layers to reduce its thickness to the desired level. The lower layer obtained from this process is referred to as the “flesh split”.
Preparation to Tanning

Deliming

It is a chemical operation, carried out in the drum, aimed at removing the lime used during the liming stage and reducing the swelling and turgidity of the hide/skin.
Preparation to Tanning

Bating

When the goal is to produce a soft leather, this operation is carried out using enzymes.
Preparation to Tanning

Degreasing

When hides/skins are naturally rich in fats (as in the case of sheepskins), degreasing becomes necessary. This operation, usually performed with surfactants, serves to remove most of the fat and to distribute the remaining portion evenly throughout the hide/skin.
Actual Tanning

Tanning

Tanning consists of treating the hide/skin with specific substances — the tanning agents — capable of chemically bonding with the collagen fibres, making them non-putrescible. During this process, the hide/skin’s original fibrous structure, which gives it its extraordinary and unique properties, is preserved.

After the tanning stage, hides/skins are referred to as “leather”.
Actual Tanning

Vegetable Tanning

It is one of the oldest tanning methods, still mainly used today for leathers intended for the production of soles, welts, technical articles, linings, saddlery, upholstery and other types of articles.

The delimed hides/skins are treated with vegetable tannins — complex organic substances found in all plants, which are extracted and then concentrated or reduced to powder for use in the tanning process.
Actual Tanning

Mineral Tanning

The vast majority of hides/skins are processed using mineral tanning, which is preceded by pickling — a treatment involving water, salt and acid (usually a mixture of sulphuric and formic acids). The most common method by far is chrome tanning, although aluminium, zirconium and other mineral-based tanning agents are also used. To promote the chemical tanning reaction, a basification step is carried out by adding slightly alkaline compounds such as sodium bicarbonate, acetate or formate.
The leather takes on the colour of the tanning agent: immediately after chrome tanning, the semi-processed material is known as “wet-blue”, whereas after other tanning systems — which give the leather a whitish hue — the semi-processed material is referred to as “wet-white”.
Post-Tanning Treatments

Splitting and Shaving

In the tanned state, leather may undergo additional mechanical operations — for example, splitting (if not previously performed after liming) and shaving, which serves to even out the thickness, bringing it to the desired value.
Post-Tanning Treatments

Retanning

Through the chemical process known as retanning, the chemical and commercial properties obtained during the main tanning phase are modified and refined.
Post-Tanning Treatments

Dyeing

Drum dyeing gives the leather the desired colour. The depth of dye penetration into the leather can be controlled by adjusting the pH of the dye bath. When the whole leather is dyed — including its inner structure — the process is referred to as “through-dyeing”.
Post-Tanning Treatments

Fatliquoring

Fatliquoring introduces into the leather structure specific substances, known as fatliquoring agents, which act as lubricants, allowing the collagen fibres to remain well separated and to slide smoothly over one another.
This treatment gives the leather flexibility and softness, even after drying. To achieve this, natural or synthetic fats are used — carefully treated or partially modified to make them water-emulsifiable and capable of bonding with the leather fibres.
At this stage, the leather is ready to be dried.
Post-Tanning Treatments

Drying

Drying can be carried out using different methods, depending on the type of leather and its intended use — for example: suspension drying, toggling, pasting or vacuum drying.
Finishing

Finishing

It includes all the operations performed on the surface of dried leathers, aimed at either improving performance or simply giving the leather the desired appearance and characteristics — such as surface colour, gloss level, grain fineness and touch.
These treatments can be mechanical, chemical or combined.
A mechanical operation is buffing, which produces a velvety surface: when carried out on the flesh side, it results in “suede”, while on the grain side it produces “nubuck”. Through dry milling, the leather is tumbled rapidly in a drum under dry conditions to soften it and make the natural grain pattern more evident.
In most cases, the surface of the leather is treated with a mixture of chemical substances that forms a finishing film — more or less thin — and transparent or pigmented. This film provides various effects, including the desired colour tone, protection from external agents, degree of gloss, touch sensation and even decorative patterns. In some cases, preformed foils are applied to the surface to create special effects.
Measurement

Measurement

Finished leathers are sold based on their surface area, expressed in square metres, although the traditional unit of square feet (sq ft) is still commonly used.
Sole leather, on the other hand, is sold by weight, measured in kilograms.