After peeling off the skin, it is generally impossible to process it in time because there is a process of collection and concentration. Pretreatment of fresh fish skin is the key to prevent fish skin from rotting and ensure cooked quality.
First of all, the peeled fresh skin should be cleaned up in time, that is, hooves, ears, lips and tails. It should be cut off by hand, cut off the remaining meat and fat on the skin, and then wash away the dirt, feces, blood and other dirt stuck to the skin with clear water. Don't use too much force when scraping the leftover meat, in case you scratch the leather board. In addition, because the plush grows backwards on the leather board, it should be scraped from the head of the leather to the buttocks along the hairy roots to reduce the disability of the leather board.
After cleaning the fresh skin, hang it in an outdoor ventilated place or in weak sunlight to dry it, turn it to 80% dry, fold it, flatten it with boards and materials, and continue to dry it the next day.
Second, the softening and cleaning of dry skin.
Soak the dried skin after primary processing in clean water at room temperature for 16~ 18 hours to soften the fur and restore the fresh state of the skin. Then rinse the dirt with clear water, soak it in 3g/L detergent solution for about 10 minute, rinse it with clear water, take it out and drain it. When soaking in washing powder, listen to the dirt on the hair and remove the connective tissue on the leather board. Fat skin boards can be washed with washing powder in warm water at 35℃ for two or three times, but the action should be fast. Pay attention to stretching the leather board when drying, and the next step can be processed when it is 60% dry.
Third, fur processing
Fur curing, also known as curing or tanning, is the process of transforming raw skin into mature skin. Here are three cooking methods.
1, nitrite noodle cooking method
This is a traditional fur cooking method in China. This method has the advantages of convenient materials, soft products, white fur color and high tensile strength of leather board. However, the product has poor water resistance and heat resistance, and has an unpleasant smell. For families with small output, this method can still be used.
Nitrate solution preparation
Dissolve 10 kg of sodium sulfate (mirabilite) in 100 kg of water, add 25 kg of flour and stir well. The dosage of nitrate solution is about 10 times the weight of fur.
Nitrate system
Soak the softened fur in nitrate solution, stir and soak it evenly, press the organism to immerse it under the liquid surface, and turn it once a day. The curing time depends on the temperature. When the temperature is 15~20℃ in spring and autumn, it is generally soaked for 16~22 days, and when the temperature reaches 30℃ in summer, it can be soaked for about 15 days. The inspection method of the finished product is to fold the leather board in half or squeeze the nitrate solution by hand. If the hide is white and dehydrated well, it means it has been nitrated. Don't touch raw water after the finished product is taken out of the tank, then dry it and shovel it.
2. Alum cooking method
Alum cooking is also an ancient method. The heat resistance of the finished product cooked by this method is slightly higher than that cooked by nitrate powder, but the water resistance is still not ideal, and the leather board will become tanned and hardened when it meets water. The steps are as follows:
(1) acid leaching
Acid leaching is the pretreatment of alum nitration, which aims to prevent leather board from acid swelling and alum hydrolysis and precipitation during nitration, thus losing tanning property.
Acid leaching process: take 3.5bt4 times of water equivalent to fur birthday, add 0. 1 times of sodium chloride, fully deepen it, put it into softened and dried fur and turn it over for 5 minutes, then pick up the fur, add 0.007 times of sulfuric acid (concentration: 66Be) and turn it over, then take out the fur and add 0.006 times of sulfuric acid (adjusted pH = 65438). After acid leaching, PH=3.7 is appropriate. After the acid leaching, take out the fur, squeeze out the acid leaching solution and send it to the next step for treatment. The fur soaked in acid can no longer meet raw water, otherwise the leather board will be swollen by hydrochloric acid.
(2) tanning
Material consumption index: triple water, 8.5% sodium thiosulfate 8bt, 7.5%~8 alum, 15~ 17% sodium sulfate, 0.7~0.8% sulfuric acid, 1.2~ 1.5% bicarbonate. Tanning temperature is 25~28℃ and time is 200~225 hours.
Tanning process: put the above raw materials (except sodium bicarbonate and gypsum powder) into a tank, fully stir to completely dissolve them, put them into fur after acid leaching, stir twice, then turn it 1 time every morning, add gypsum powder 48 hours later, add sodium bicarbonate (dissolved in water in advance) solution three times (2 hours apart) 24 hours before tanning, and stay overnight. Drying, moisture regain and peeling to obtain the finished product.
The requirements for the PH value of tanning liquid are: PH = 2.5 at the beginning of tanning; Ph = 3； before adding sodium bicarbonate; When tanning is completed, the pH value is 4.7 ~ 4.8, and the tanning temperature is finally required to reach 38℃. If the temperature is low, the time will be prolonged.
3. Chrome tanning method
Nitrate tanning and alum tanning of fur belong to false tanning. When it comes into contact with water, leather becomes hard and hairy. Chrome salt tanning can make the finished product have water resistance, temperature resistance, sweat resistance and aging resistance.
(1) acid leaching
The composition of acid leaching solution is 5-6g/L sulfuric acid (pH =1.015); Sodium sulfate 30-50g/L; Sodium chloride 30 ~ 40g/L, temperature 36~38℃, time 36~38 hours, and water consumption per skin is about 14 liter, which increases or decreases according to the size of the skin.
Acid leaching operation is to dissolve and clarify salt and sulfuric acid, pour it into the trench, add water to make it reach the required concentration and water consumption, and then add acid to heat it to the specified requirements. Stir the paddle evenly, throw in the fur and continue paddling 1~2 minutes. After that, heat paddling 1 time every day. Soak for 36 hours to check the quality; Make the back extend horizontally, the fiber is loose, the folding extrusion dehydration is good, and it is white and tough. When examined with tetra basic red reagent, the incision is red.
(2) tanning
Tanning liquid level is basic chromium sulfate 0.2g/L (calculated as chromium trioxide), alum 10g/L, sodium chloride 30g/L, sodium sulfate 30g/L, sodium thiosulfate 2g/L, JFC wetting agent 0.3g/L and proper amount of sodium bicarbonate. Water consumption 10~ 12 liter/long skin, PH value 1.2~2 (adjusted by sulfuric acid), time 4~4.5, temperature 40~45℃, time 72 hours.
Operation process: Dissolve and clarify salt, sodium sulfate and alum, then pour into the tank, add water and heat to the specified requirements. Then other raw materials (except sodium carbonate, sodium thiosulfate and JFC) are dissolved in the tank and stirred evenly. Soak the cured fur again and paddle for 1 ~ 2 minutes, then paddle every 1 hour for 1 ~ 2 minutes, and control the temperature at 40℃ on the first 1 day and 45℃ on the second and third days. Take out the fur and control the water the next day. The shrinkage temperature of the leather board before it is taken out of the barrel should reach above 70℃; PH=4~4.5。 After tanning for 72 hours, take it out and put it on a wooden bench for 1~2 days to make the tanning agent firmly combine.
The used tanning liquor can be used continuously in 15 batches, and each batch is added with 0. 12 ~ 0. 15g/L (calculated by chromium trioxide), 3 ~ 5g/L of salt, 3 ~ 5g/L of sodium sulfate and 3.5g/l of sulfuric acid (pH = 655)
March wool can be finished after tanning, and the steps are as follows: washing, spin-drying, drying, moisture regain, peeling and inspection.
When washing with water, the amount of water in the cylinder or in the cylinder should be large, and the number of times when water cannot be changed should be more, and impurities such as nitrate should be removed.
Spinning is to remove moisture from fur by centrifuge or hand.
Fatliquoring is to make the leather board softer. The formula of fat is 890g of warm water at 50℃, soft white oil or synthetic fatliquor 100g, JFC wetting agent 10g. Brush the board with a brush dipped in grease emulsion at 40~50℃, and the dosage of each layer is about 300 ml.
Drying is sun drying or air drying. When drying, the temperature should be 40℃. When drying, dry the leather board first, and then dry the quilt until the leather board and hair are completely dry. Rub gypsum powder evenly. The second time, shovel the floating meat with a knife, and then sprinkle gypsum powder for the third time, paying attention to the softening of the shovel edge. After shoveling hard, clean your coat with a dry cloth. Then let the fur dry the fur on the fur. After drying, pile it overnight and shovel it again to get the finished product.
Drying, moisture regain and peeling are also suitable for the processing of fur tanned by nitrate cooking and alum separation.