If the wall ash is not dry before the check-in, the wallpaper will be chemically reacted with the alkaline material in the wall ash. The surface of the wallpaper turns yellow. This is generally difficult to handle. If the wallpaper is yellowed after a few years of stay, most of the time because the wallpaper is exposed to air or sunlight, it can be wiped with a towel and rubbed with a safe bleach. Wipe the side of the water, you can first test in the hidden place of the wall, observe the effect, and then carry out large-scale treatment after the effect is ideal; if the wallpaper is smoked and yellowed, you can directly use the diluted caustic soda to wipe.