Yak hide

Yak hide

　　The fresh hide peeled from a slaughtered yak; raw hides or salted hides before processing are all called "raw hides" or "raw materials." Uncured and untanned raw materials are called "fur hides," while those that have been depilated and tanned are called "leather."

    Raw yak materials include calf hides (under 1 year old), young cattle hides, and adult yak hides. Calf raw hides can be processed into fur gloves, fur hats, fur coats, and fur shoes, while adult and young hides are made into sole leather, top grain leather, and mechanical leather, used in products such as suitcases, bags, coats, boots, gun cases, artillery covers, belts, and other daily and industrial goods.

    II. Structure of Yak Hide

    (1) Epidermis

    The epidermis is divided into four layers: stratum corneum, stratum granulosum, stratum spinosum, and stratum basale.

    Stratum corneum: A relatively thick layer composed of many layers of flat, scaly, keratinized cells stacked on top of each other.

    Stratum granulosum: A discontinuous cell layer with scale-like or spindle-shaped cells.

    Stratum spinosum: Fewer cell layers, about 5–8 layers, with irregular polygonal cells.

    Stratum basale: Composed of a tightly packed layer of cube-shaped or short columnar cells arranged like a palisade.

    (2) Dermis

    The dermis has an average thickness of 4.52 mm and is divided into the papillary layer and reticular layer. Hair, sebaceous glands, arrector pili muscles, blood vessels, and nerves are distributed within the dermis.

    Papillary layer: Thinner than the reticular layer. Its characteristic is the formation of an especially dense fine fiber mesh at the junction with the epidermis, which is very thick, contains few cellular components, and is mainly composed of coarse collagen fiber bundles.

    (3) Musculocutaneous Layer

    The musculocutaneous layer is the part connecting the dermis and muscle. From an anatomical and histological perspective, this layer does not belong to the skin but is part of the muscle membrane.

    III. Quality and Physical-Chemical Values of Yak Leather

    Yak hides are compared in two categories: 0.5-year-old and adult hides, as follows:

    (1) Quality Comparison between 0.5-Year-Old Raw Hides and Adult Hides

    0.5-year-old calf raw hides have a bright fur color and no gadfly holes or scars. According to measurements and statistics, the fresh hide area is 1.14±0.13 m², the dry hide averages 3.68 kg, and the leather averages 1.09 m², which is 62% of the 1.75 m² of adult hide leather; the grain leather thickness is 1.64 mm, 82.41% of the adult’s 1.99 mm, and is consistently even. Both the yield rate and utilization rate are 100%, indicating that 0.5-year-old calf hides far surpass adult hides and are top-grade raw materials. The 0.5-year-old calf hides specifically refer to those from calves fully nursed before slaughter, the same applies below.

    (2) Quality Comparison between 0.5-Year-Old Calf Leather and Adult Hide Leather

    0.5-year-old calf grain leather has a surface much smoother and finer than adult hide leather, with fewer loose areas, less noticeable wrinkles, consistent fiber density, a full body, and good elasticity. The usable area for shoe uppers can be increased by 6–10%, comparable to the “tight, fine, solid structure and consistent thickness” of yellow calf or young yellow cowhide.

    (3) Physical-Chemical Value Comparison between 0.5-Year-Old Calf Grain Leather and Adult Hide Grain Leather

    Both 0.5-year-old calf grain leather and adult hide grain leather are produced using chrome tanning processes. The main indicators of 0.5-year-old calf leather, such as tensile strength, are not lower than those of adult hides, and other values are quite similar, making its quality no less than that of adult hides, and in some aspects even slightly superior. In addition, 0.5-year-old calf suede leather has more uniform nap than equivalent adult hides; it can be made into high-end, brightly colored products with minimal color variation and a strong sheen.

    (4) Surface Comparison between 0.5-Year-Old Calf Grain Leather and Adult Hide Grain Leather

    Grain leather, corrected grain leather, and suede leather made from 0.5-year-old calf hides, when compared to equivalent adult hide types, can generally improve by one grade on average, with no substandard grades and higher proportions of second and third grades. Grade 2 grain leather accounts for 20%, 15–17% higher than adult hide leather, and grade 1 suede leather makes up 2.3%, 2% higher than adult hide suede. If properly stored to avoid mold and undamaged during transport, there would be no suede issues, and the proportion of first and second grade corrected grain leathers could exceed 28% (based on data from promptly processed fresh hides).

    IV. Defects in Yak Hide

    Any factor that reduces the quality of a hide is generally referred to as a defect. Defects fall into two categories: one is life defects, such as gadfly damage, ringworm, lice bites, whip marks, saddle sores, bites, horn punctures, stone impacts, and hair-pulling injuries; the other is post-slaughter defects caused during skinning, such as skinning injuries, improper preservation, or curing flaws.

    (1) Gadfly Damage

    Horn fly damage refers to harm caused by the cattle grub. These pests mostly infest the back, waist, and rump of cattle—essentially the middle part of the hide—which is the most serious defect in yak hides, severely affecting leather quality and utilization.

    (2) Knife Marks

    Knife marks are defects caused by poor slaughtering and skinning techniques. Training or promoting better operational methods for skinning is necessary.

    (3) Rot

    Fold rot: Occurs when fresh hides are not laid flat during drying. The folded areas become damp and moldy due to trapped moisture.

    Core rot: Results from rapid dehydration caused by excessive sun exposure or extremely dry weather, where the edges of the hide dry out while the middle remains moist, leading to rot.

    Rain rot: Happens when fresh hides are not transported to processing facilities promptly during rainy weather and no preservation methods such as salting or spreading are used.

    Thaw rot: Caused by the freeze-thaw cycle of hides that are sporadically slaughtered and folded-dried during cold seasons.

    Stewed rot: Occurs when fats and muscles still attached to the fresh hide are not removed promptly after skinning. Uneven drying or moisture trapped in folded areas leads to rot over time.

    (4) Wrinkling and Cracking

    In many production areas, instead of laying fresh hides flat to dry, they are folded and turned during sun drying, resulting in "folded hides" or "multi-folded hides." In worse cases, hides are simply discarded to dry naturally, forming "lumpy hides," all of which severely degrade leather quality.

    (5) Scorching

    Scorching is not burning, but rather damage caused by intense sunlight during drying. Placing fresh hides on hot stones, slabs, cement floors, or sandy ground causes the hide to shrink, deteriorate, and become brittle, resulting in "stone-scorched," "oily-stained," or "oil-leached" leather. This reduces leather quality and, in severe cases, renders the hide useless.

    (6) Withered and Lean

    At the end of the cold season, yaks that die due to lack of grass on the pasture or disease produce what is called "withered and lean hide," which has very low utility value.

    V. Preservation of Yak Hides

    (1) Natural Drying

    This involves air-drying fresh hides until their moisture content drops below 15%, inhibiting microbial growth to achieve preservation. Dried yak hides offer advantages such as simplicity, low cost, and ease of transport, making this a common preservation method in yak-producing regions. When naturally drying hides, the following points should be noted:

    1. Remove any residual fat, muscle, or coagulated blood from the freshly skinned hide.

    2. Spread the hide flat on a smooth, moisture-proof surface for drying. The drying temperature should not exceed 35°C.

    3. If drying on stones, slabs, cement, or sand, ensure the ground is cool—preferably in the early morning before the sun heats it up.

    4. During drying, the hair side should face the sun, while the flesh side should face upward when it’s overcast.

    (2) Freeze-Drying

    This is also a commonly used drying method in yak-producing areas. Instead of sun drying, freshly skinned hides are preserved or freeze-dried by taking advantage of the naturally low temperatures after slaughter. This method saves effort compared to sun drying but can lead to thaw rot if not processed promptly.

    (3) Salting Method

    The salting method is a more scientific approach that uses salt or brine to treat fresh hides. Hides treated this way do not rot, retain the characteristics of fresh hides, and more importantly, prevent loose grain often seen in air-dried raw hides, avoiding negative impacts during processing. According to the Gansu Provincial Light Industry Research Institute, air-dried yak hides have a 100% incidence of loose grain, whereas salted hides have a maximum of 25%. Additionally, salted hides can shorten the production cycle and eliminate the need for softening. Salt is abundant in yak-producing regions, making this method easy to promote.

    After testing over 5,000 hides, the Qinghai Leather Factory found that salted hides are not only fuller, more elastic, with finer grain and clearer pores, but also show an average 38% increase in tensile strength, a 42% increase in bursting strength, a 2.6% rise in first-grade rate, and a 52% reduction in loose grain rate. For all preservation methods mentioned above, it is essential to remove fats, meat, blood, feces, and other substances from the hide, and never wash it to clean, to avoid creating “water-soaked hides.”

    VI. Storage and Transportation of Yak Hides

    Although properly preserved yak fresh hides are relatively resistant to storage, any preservation method will gradually deteriorate over time and with changing storage conditions. A high-quality hide can turn into a substandard one if improperly stored—affected by moisture, mold, pests, rodents, or pressure that causes cracking. Therefore, proper storage and transportation of hides are crucial.

    Hides are sensitive to heat, moisture, pests/rodents, and water. As such, they should never be piled outdoors but stored in dedicated warehouses. The ideal warehouse temperature is around 10°C, not exceeding 25°C, with optimal humidity at approximately 50% to maintain the raw hide’s moisture content between 12–20%. This prevents both drying cracks and moisture-induced mold. Warehouses should be built in high, dry locations, kept dry, ventilated (air conditioning is preferable), cool, and free of rodents or pests. Air-dried hides should be stacked on wooden pallets 17–33 cm above the ground and covered with mats.

    Fur hides should be stacked by type and grade. Freshly dried hides should be stacked after sunset when temperatures have dropped; do not stack them directly against walls, and insect repellent or camphor should be applied or placed in the middle of the stack at regular intervals. When bundling for transport, the hides should be arranged neatly without folding.

    VII. Tanning of Yak Hide

    (I) Methods for Tanning Yak Fur Hides

    Like other fur hides, there are many methods for tanning yak fur hides, mainly including chrome tanning, alum tanning, formaldehyde tanning, mixed tanning, and traditional acid tanning. Among these, alum tanning and mixed tanning are relatively simple and practical. However, regardless of the tanning method used, the entire process is divided into three stages: preparation, tanning, and finishing.

    1. Preparation Stage

    Soaking in clean water: Soak air-dried hides in water to soften them and restore them to a fresh hide state (salt-cured hides must be desalted).

    Fleshing: Place the rehydrated wet hides on a fleshing beam or in a fleshing machine to remove residual flesh, fat, and other materials, while further softening the hide.

    Degreasing: Prepare a degreasing solution with three parts soap, one part sodium carbonate, and ten parts water. Then, in a container, add warm water (38–40°C) at four to five times the weight of the wet hides, followed by 15% of the degreasing solution. Stir the fleshed hides thoroughly for 5–10 minutes, then replace the washing solution and stir carefully until no greasy odor remains and the soap foam in the degreasing solution no longer disappears. If hair loss is observed on the belly or udder areas, remove the hides immediately and rinse them.

    Rinsing: Immediately rinse the degreased hides with clean water to remove all soap residue from the fur. After rinsing, drain the water and rinse the hides again with clean water.

    2. Tanning Stage

    (1) Alum Tanning Method

    Preparing the tanning solution: Mix four to five parts alum with three to five parts salt and 100 parts water. First dissolve the alum in warm water, then add the remaining water and salt and mix well.

    Tanning process: Pour four to five times the weight of the wet hides of the tanning solution into a vat (or tank), add the rinsed and drained hides, and stir thoroughly. After soaking overnight, stir once in the morning and once in the evening for about 30 minutes each day, for a total soaking time of 7–10 hours. Once tanning is complete, fold the hides with the flesh side out into quarters, pressing firmly at the corners to squeeze out as much moisture as possible. The folded areas should appear opaque white, resembling tissue paper. During tanning, maintain a temperature of around 30°C. After tanning, do not rinse the flesh side; rinse the fur side lightly with water.

    (2) Mixed Chrome-Alum and Sodium Carbonate Tanning Method

    Preparing the tanning solution: Use 280g chrome alum, 56g sodium carbonate, 410g salt, and 10kg water. Weigh 1.5kg of water, add the chrome alum, and heat to dissolve. Separately, weigh 0.5kg of water to dissolve the sodium carbonate, then slowly add this to the chrome alum solution while stirring to mix well.

    Tanning process: Pour the remaining 8kg of water into a container, dissolve the salt, then add two-thirds of the prepared tanning solution to create the final tanning mixture. Submerge the acid-pickled hides (i.e., hides soaked for 2–3 hours in an acid solution made by dissolving 500g salt and 20ml hydrochloric acid in 100g water, then drained—initial stirring should be continuous for the first 20 minutes) into the mixture and stir continuously to allow the hides to evenly absorb the tanning solution. Maintain the tanning solution temperature at around 35°C. The next day, add the remaining one-third of the tanning solution and stir again. The total liquid volume used during tanning should be about three to four times the weight of the wet hides. The inspection method after tanning is the same as with alum tanning.

    (3) Neutralization

    The purpose of neutralization is to prevent the finished product from hardening, which could affect quality. Thoroughly rinse the chrome-tanned hides to remove excess tanning solution. Then immerse them in a 2% borax solution and stir for one hour. Test a small piece of the hide with litmus paper; once it shows slight acidity, remove the hides, rinse, and dry them.

    3. Finishing Stage

    (1) Fatliquoring: Emulsify 10 parts castor oil in 10 parts soap solution to create a fat liquor (with 100 parts water). Evenly apply the fat liquor to the semi-dry inner side of the hides. After application, fold the flesh sides together and let them sit overnight, then continue drying.

    (2) Conditioning: Lightly spray moisture onto the flesh side of the fatliquored hides to condition them. After spraying, overlap the flesh sides, wrap the hides in plastic bags or cloth, and press them with stones or weights to allow thorough moisture absorption. Conditioning is complete after 24 hours.

    (3) Softening: Place the conditioned hides in a softening machine or on a flat wooden board and lightly scrape the flesh side with a blunt knife to whiten it.

    (4) Shaping and Grooming: With the softened hides’ fur side down, nail them onto a wooden board to stretch and air-dry them. Avoid direct sunlight. Once fully dried, use pumice or sandpaper to smooth the flesh side, then remove and trim the hides. Finally, comb and trim the fur. In factories, shaping and grooming are done mechanically. Tanning of yak fur hides is limited to calves under one year old.

    (II) Yak Hide Leather Production Methods

    The process involves drum and vat combined soaking, lime and lime sulfur hair removal, one-bath chrome tanning (or other tanning methods), and retanning/filling.

    (III) Yak Hide Suede Production Methods

    The process involves drum and vat combined soaking, lime and lime sulfur hair removal, and chrome-lime combined tanning.

    VIII. Development Outlook for Yak Hide

    China has approximately 13 million yaks, and it is projected that this number may exceed 15 million by the end of this century. In Xinjiang alone, utilizing 110 million mu of alpine grasslands in the Tianshan Mountains could increase the yak population from the current 180,000 to over 1 million. With gradual progress in economic crossbreeding and selective breeding of native yak breeds, while resolving competition between humans and calves for milk, the turnover rate of yak herds will accelerate, easing supply-demand imbalances for meat, milk, cashmere (wool), hides, and related products. By the end of this century, assuming a baseline of 15 million yaks—including 50% female breeding stock, 60% annual survival rate of calves, and 2 million fully weaned male calves slaughtered annually (half male, half female, with 250,000 kept as reserves) along with 2.05 million other yaks—totaling 4.05 million yaks (27% slaughtered), this would result in 3 million more yaks slaughtered annually compared to previous levels (historically around 7% were slaughtered annually), equating to 3 million additional raw hides and 210,000 tons more beef production. Of the 3 million hides, 2 million will be high-quality calf hides, along with 200,000kg of cashmere (averaging 1kg per yak).