Chrome tanning produces leathers of good stability, permeability and colour absorption, resulting in a product that is ideal for leatherware, clothing and upholstery, among others.

Titanium tanning is used for hides or skins in a similar way as chromium tanning, but without causing the characteristic bluish colouring (wet-blue) in the leather. It allows a wide range of colours that are difficult to obtain with chromium tanning.

Aluminium tanning is one of the oldest chemical tanning processes known. This is the most suitable method for bleaching hides and skins and giving them a softness that is ideal for clothing or gloves. Hides tanned with aluminium salts have an opaque white colour and soft texture but can easily become untanned by just washing them. In spite of this drawback aluminium salts have the advantage of being colourless and eco-friendly and are still used today in fur production. However, given their lack of sufficient stability, they are applied in combination with vegetable extracts, chromium salts, aldehydes, etc.

Mixed vegetable-aluminium tanning is used for manufacturing vegetable tanned insoles as it achieves greater perspiration fastness and deformation stability. Between 2.5 – 3% aluminium oxide, calculated on a dry weight basis, in the form of salts, is incorporated to hides that have first been vegetable tanned. This reduces the amount of washable matter in the leather and forms lacquers with the tannins. The leather obtained has a shrinkage temperature of around 107ºC and better resistance to wear.

Aluminium salts are also added in chrome tanning to achieve increased firmness of the leather and to facilitate buffing. In addition, this kind of mixed tanning improves chrome bath exhaustion and reduces the environmental impact of this type of tanning.

Advantages of aluminium tanning:

1. Enhances solution hydrolysis for washes with chromium salts.
2. Improves salt deposition.
3. Increases the formation of the surface bonds of the leather fibres in combination with chrome tanning agents.
4. Reduces the shrinkage temperature of leathers (80-90ºC) in comparison to that of chrome-tanned leathers.
5. In combination with chrome tanning it improves the degree of chrome exhaustion in the bath.

The SUFAL^®, aluminium sulfate family of products is widely used in the hide and skin tanning industry, specifically, in the aluminium tanning sector. This process is based on the hide or skin being subjected to an aluminium salt bath at an acid pH where the mineral tanning agent (aluminium) spreads easily inside the hide or skin resulting in colourless tanning and providing the leather with good firmness.

The FORAL 25 product is a mix of organic and inorganic aluminium salts. It is used for all types of leather hides and skins thanks to its ease of handling and eco-friendliness. It is readily soluble in hot water and weak acids and its application results in white, dry, soft and silky leather. Its fields of application are:

• White leather production
• Tanning process and finishing of all types of fine leather
• Combined application in chrome tanning
• Fixing waterproofing agents
• Fixing dyes and increasing dyeing quality
• Improving sandability in nobuck and split leathers

ZEOLITA 4A is a crystalline aluminium silicate product of a hydrophilic nature, with high ion exchange and adsorption capacity. It is used in aluminium-based tanning processes as although in pH of more than 7.0 it is insoluble, in pH of less than 5.0 it hydrolyses forming aluminium salts and silicic acids. Its applications are:

• Pre-tanning: in conjunction with glutaric dialdehyde and carbonic acid. Finishing with vegetable tanning to obtain chrome-free leathers.
• Depickling: pre-treatment with Zeolite 4A.
• Degreasing: increases the shrinkage temperature and allows solvent-free degreasing at 40-45ºC.
• Basification of chrome tanning. After basification with magnesium oxide and carbonic acid resulting in pH of 4.2, good chrome exhaustion is obtained as well as fullness and resistance of the grain leather.
• Neutralization: improves the fullness and resistance of grain leather and facilitates buffing.
• Solid ion exchangers: it complexes metal and calcium ions. Hide or skin tanning is probably one of the oldest trades in the world, arising from our predecessors’ need to protect themselves from inclement weather conditions.