It is due to fiber aging and impurity deposition.

You can add boiling water and washing powder and boil it for 20 minutes, then wash it and it will be as soft as new.

Extended information:

Quality identification of socks

1. Selection of types: The main types of socks sold on the market are chemical fiber socks (nylon, card silk, thin elastic, etc.) , cotton socks and blended, interwoven, wool, silk socks, etc. Depending on the season and foot characteristics, generally choose nylon socks and towel socks in winter; choose cotton or blended or interwoven socks for sweaty or cracked feet; wear elastic cardstockings, silk stockings, etc. in summer; thin elastic, mesh socks, etc. should be worn in spring and autumn. Women should wear stockings when wearing skirts.

2. Size selection: The size specifications of socks are based on the size of the bottom of the socks (referring to the area from the heel to the tip of the socks). General dimensions are indicated on the label. It is better to choose the same or slightly larger size according to the length of the foot. It is not advisable to choose the small size.

3. Quality selection: According to the intrinsic quality and appearance quality, socks are divided into first-class products, second-class products, third-class products (all qualified products) and non-class products. Generally, first-class products are used, but second-class and third-class products can also be used when the requirements are not high.

4. Selection of key parts:

I) The socks you choose should have large heels, be bag-shaped, and be as close to the human foot shape as possible. The size of the sock heel will cause the sock tube to sag and the sock heel to slide to the bottom of the sock after wearing it. You cannot try them on when purchasing. Just fold the sock top and sock sole in half from the center line. Generally, the ratio of sock top to sock heel is 2:3.

II) Inspection of the density and elasticity of the sock opening: the density of the sock opening should be high, stretched horizontally to double the width, and have good recovery properties. Its elasticity is small and it is difficult to reset when pulled horizontally, which is also one of the reasons for the slippage of socks.

III). Check whether the seam interface is out of needle. Generally, sewing the ends of socks is another process. If the stitches come off during sewing, the mouth will open when you wear them. When selecting, look closely at the seam head to see if the needle comes off smoothly.

IV), check whether there are holes or broken wires. Because socks are knitted and have a certain degree of extensibility and elasticity, broken threads and small holes are generally difficult to detect. Depending on the process conditions, the shaping plate may easily collide with other objects and cause broken wires or holes. Therefore, when purchasing, check the shaping edges of the sock soles and sock tops, and gently pull them horizontally to check.

V), check the length of socks. Because each pair of socks is optional, it is easy to have uneven lengths. Generally, the distance between each pair of first-class products should be no more than 0.5cm.

5. Identification of regular products and inferior products: Large-scale sock factories have advanced equipment, stable processes, and good selection of raw materials. Through the control of each process, the quality is stable, which is reflected in the appearance, uniform fabric density, Thick, pure color, good shape and formal trademark. Miscellaneous and inferior products are mostly due to simple equipment, manual operation, poor selection of raw materials, thin and uneven fabrics, low density, low luster, many defects, poor molding, and no formal trademarks.

Reference: Baidu Encyclopedia--Socks