Rosin is applied to the bow hair of erhu, violin and other musical instruments to increase the friction between the bow hair and the strings.

The refined transparent block rosin is the best, and the natural crystalline rosin secreted and solidified from Pinus tabulaeformis can also be substituted.

The popular folk method of beating rosin is to split a bamboo piece in advance and then put rosin in it. Then, the bamboo pieces are lit with fire, so that the rosin is scalded and dripped on the piano barrel. This method is called "burning incense". Although burning incense is convenient and easy for playing, it hinders the performance of some bow-carrying techniques, and the piano barrel and skin are not clean enough.

Now it is more popular to rub incense, that is, rosin is directly rubbed back and forth on the bow hair repeatedly. This will last at least an hour after rubbing the rosin once. Practice has proved that the piano sound just after "rubbing incense" is not very ideal. It takes two or three minutes for the piano sound to gradually turn to normal. When rubbing incense, the force should not be too heavy to avoid breaking the bow hair.

It will take several hours after the new rosin is used, and the old rosin will have an effect.

Without rosin, the bow will be slippery and unable to make a sound when playing on the strings. With a little rosin, the bow hair will have some viscosity, and when it is pulled away, it will "grab" the strings and make them vibrate to make a sound. Rosin is wrapped in cloth or a small box when it is bought. It is not only sticky but also hard. Generally, it can be used for a long time. The harder the rosin, the easier it is to break.

Many brands of rosin have two colors, but the price is the same, the light color is like honey, and the deep color is like licorice. When playing, rosin will turn into powder, and then its color is the same white.

The hardness of rosin is very high. This brand of rosin may be harder than another brand of rosin. When you press the rosin hard with the bow, you can identify the hardness. Soft rosin will make the strings vibrate better, but because it is too sticky, it is easy to produce noise, especially on narrow strings.

The above is from Baidu Encyclopedia: "Rosin"

I hope it helps you.