Liming (leather processing)

Liming is a vital chemical process used in leather and parchment manufacturing. It involves soaking animal hides in an alkali solution to prepare them for tanning. This step is a major part of what makers call beamhouse operations. The process uses either a large drum with paddles or a deep pit. Liming serves several important physical and chemical goals. It removes unwanted proteins and natural fats from the skin. It also changes the physical structure of the skin fibers. These changes allow the leather to absorb dyes and oils much better later on.

One primary goal of liming is the removal of interfibrillary proteins. These proteins sit between the fibers of the skin. When an alkali, specifically sodium sulfide, is present, these proteins are denatured. Denaturing means the proteins change their structure so they become soluble. Once they are soluble, they can be washed away from the hide. This removal often happens during a step called deliming, when the pelt is deswelled. If these proteins are not removed, the final leather will be poor quality. It will become hard, tinny, brittle, and inflexible because the fibers glue together during drying.

Liming also works to remove keratin from the skin. Keratin is the protein found in hair, scales, and the epidermis. When the pH level is greater than 11.5, the alkali performs hydrolysis on the keratin. This process cleaves, or breaks, the disulfide bridges found in the protein. While these bridges can reform, the process eventually leads to hair removal. In traditional leather making, unhairing and liming were done at the same time. Modern methods often separate these steps. For example, sheepskins may have their hair removed before they enter a liming drum. In hair-save technology, hides are unhaired first and then limed for 12 to 18 hours.

A major physical change during this stage is alkaline collagen swelling. This occurs because of the presence of calcium hydroxide in the solution. The calcium hydroxide causes an influx of water into the hide or skin. As water enters, the collagen fibers undergo two specific changes. The diameter of each fiber increases, and the fibers themselves become shorter. This causes the overall thickness of the skin to increase significantly. However, as the skin gets thicker, its total surface area actually decreases. This swelling is a fundamental part of preparing the hide for the next stages.

The swelling of the collagen also leads to collagen fiber bundle splitting. The skin is made of bundles of fibers held together by a sheath. As the individual fibers swell and increase in diameter, they take up more space. Eventually, the fiber bundle sheath cannot contain the thicker fibers. The pressure causes the sheath to burst open. This splitting is very important for the quality of the finished product. It provides much better access to the interior fibers. This allows for more effective tanning, retanning, dyeing, and fatliquoring processes.

Timing and weight changes are significant during the liming operation. For cattle hides, the liming process usually lasts about 18 hours. Because of the massive uptake of water during swelling, the weight of the hide changes. The weight of the skin or hide can actually double during this time. However, makers must account for other changes when measuring weight. Some weight is lost as proteins and hair are removed from the skin. Additionally, the fleshing operation is often performed after the liming is complete. This helps manage the total mass of the material.

In summary, liming is a complex chemical and mechanical preparation. It uses high pH levels to break down proteins and swell fibers. By removing keratin and interfibrillary proteins, it ensures the leather remains flexible. By splitting the fiber bundles, it ensures the leather can be properly treated with dyes and fats. Whether using traditional pits or modern drums, liming transforms a raw hide into a prepared material ready for the tanning stage.