All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
The seat or the bed that gets stained with impurity does not become pure by the impurity drying out. Rather, it becomes pure by washing it.
Shaykh Ibn 'Uthaymeen clarified the method of purifying a bed that gets stained with impurity, saying:
"If a boy urinates on the bed, then we hasten to wash it. But how can we wash it if it is something that is glued to the ground such as big carpets?" We say: First, we bring a sponge and absorb the urine until it dries. After that, we pour water on the urine twice or three times, and by doing so, it becomes pure.” [End of quote]
If the impurity becomes dry on the chair or bed before washing it, then it does not transfer to whatever it comes in contact with if the latter is also dry. Therefore, if someone sits on a chair that was stained with impurity, and its impurity had dried, then his clothes do not become impure just by getting in contact with the place of impurity.
For more benefit, please refer to Fatwa 90907.
Allah knows best.