In recent years, there has been an increase in the decoration of bags, shoes, clothes, and other leather goods with unique and artistic graphics or personalized texts. This trend of design diversification and personalization is spreading from luxury brand products such as Gucci and Louis Vuitton to smaller boutiques and is attracting a lot of attention. One of the factors that has made the decoration of leather more accessible is the evolution of digital printing using UV inkjet technology. In this article, I would like to introduce printing methods on leather, including digital printing.

Before getting into the main topic, I would like to explain how leather products are made from raw animal hides and skins, and the tanning process made famous in Italy.

The leather manufacturing process

Italy is famous for the production of high-quality tanned leather and is known as a world leader in this industry, with about 1,200 tanneries and more than 17,000 employees in the regions of Veneto, Tuscany, Campania and Lombardy^*1. The sometimes lengthy leather manufacturing process originated from ancient craftsmanship and offers unique products to the world of leather goods, shoes, clothing, furniture and even the automotive sector. Chemical methods are used in the manufacturing process, and the way they are done depends on what type of skin is used and what is being made. Even in industrial production, the process is purely artisanal, with skills gained from long experience. Each tannery has its own recipe that has been passed down from generation to generation, making the process unique and unparalleled.

*1 Source: UNIC, ECONOMIC PERFORMANCE ITALIAN TANNING INDUSTRY, 2019

Preparation

The first step is conservation. After the hides are separated, they need to be salted to prevent them from rotting. The next step is soaking, which is to moisten the hides to bring them back to their original humidity. Depilation is the process to remove hairs from the skin, which is done by chemically dissolving and separating the epidermis from the hairs. Then there is the Liming, which enables swelling of the skin in preparation for receiving the tanning substance caused by the swelling of the skin. For thicker hides, an optional process called splitting, which is used to even out the thickness of the entire hide, is conducted.

Next, there are the important processes of deliming, maceration and degreasing. Deliming removes the lime residue from the leather fibers, while maceration stretches the leather fibers to make them more absorbent. The final degreasing process reduces or eliminates the natural fat content of the hides, thereby facilitating the absorption of chemical products in the subsequent tanning process. There is a process called pickling that lowers the pH of the hides from 7 to about 4 to allow for long term preservation. This is also necessary to stop the maceration process.

Tanning

The main processing of leather is tanning, which provides mechanical resistance to humidity, temperature, and chemicals, and requires all the knowledge of the artisan who uses tanning substances such as chromium, vegetable tannin, aluminum, zirconium, and aldehydes.

Post-tanning finishing

After tanning, the leather is pressed into a cylindrical shape to even out and reduce the moisture absorbed by the leather. As an optional process, skiving or splitting may be done to make the overall thickness of the leather uniform, in this case wet. The skiving process is very important to the printing process because it ensures flatness of the leather product to be placed on the flatbed printer.

In addition, the re-tanning process gives the leather the softness, resistance to sweat, and structure it needs. The re-tanning takes place in a drum, into which additional tanning substances and fillers are added.

Then there is the opportunity to color the hides through the processing of dyeing that uses a diversity of natural or artificial coloring materials. This stage also involves a process called fat liquoring, which gives the leather softness and hydrophobicity, but can be problematic during the printing stage because the synthetic vegetable or animal oils used may be incompatible with the printing technique.

Since the skin is still soaked with water, it needs a further process called drying. In this process, the finished leather is placed on a vacuum heating plate or hung from an aerial chain.

The leather treatment cycle is not yet complete, as finishing involves a series of operations that characterize the product in terms of shine, firmness, elasticity, and resistance to dirt, water, and scratches. At this stage, various chemical and mechanical treatments are carried out, including spraying, varnishing, and padding. Once these processes are completed, the leather goes on to trimming, measuring, and packing.

Types of leather finishes

Shrink leather: leather with grain texture made more obvious by the effect of the rotation in a dry drum.

Nappa: soft full grain leathers.

Laminated leather: skin on the surface of which sheets have been applied to give a metallic appearance by films or foils.

Nuback: buffed leather on the grain side to give it a velvety surface. The grain layer is still present and visible.

Suede, or velour: buffed leather on the flesh side. Generally, it has a velvety surface, like nubuck, but being the flesh side is made up of longer and coarser fibers and has greater graininess.

Buffered: hand-finished leather with a tampon soaked with the finishing mixture, usually to obtain staining, two-tone or different shades of gloss effects.

Patent leather: leather finished with lacquer coating to give a shiny look.

Printed: leather whose surface has been printed with different techniques, including embossing (called relief printing), screen printing, and digital printing.