If there is no water available to perform wudu' or ghusl or if, though there is water available, it is not possible to use it, one can do tayammum with any sort of clean earthen thing, such as clean soil, sand, lime, and stone. According to the Hanafi Madhhab, tayammum can be done before the time for a fard salat arrives. However, according to the other three madhhabs, it is not permissible to do it before the time for a fard salat has come. Question: If using water for ghusl causes worsening of a disease and a delay in recovery, is it permissible to do tayammum instead of it? Know when tayammum is allowed. Tayammum is permitted in the following instances [5] X Research source :

According to the madhhabs of Shafi'i, Hanbali, and Maliki, tayammum is done after the time for a fard salat has come, and only one fard salat is offered with a single tayammum. In Wasa'il al-Shi'a and Mustadrak al-wasa'il, there are more than 220 hadiths about the manner and conditions of tayammum. A junub person who does not have water sufficient for washing his entire body once does one tayammum for wudu' and ghusl. However, if his wudu' breaks after tayammum, he must make only wudu' with that water. (Se'adet-i Ebediyye) First the person needs to put the palms of hands on something which is alright for tayammum having the intention of performing God's command (some marja's also require a small hit of hands, not just slowly putting the hands on the soil). [9] If the available time for Salah in too short that the Salah would become Qada (i.e. its stipulated time would elapse) in case of making Wudu or Ghusl, perform Tayammum and offer Salah. Then repetition of that Salah in must having performed Wudu / Ghusl. (Fatawa Radawiyyah ( Jad id), pp. 307, vol. 3)If there is the danger that a person, when he takes ghusl, may die or become ill because of cold, he does tayammum for ghusl and wudu' with water. If he, though he is in a city, does not have enough money to go to a bath and has no other alternative, he does tayammum instead of ghusl and performs wudu' with water. If a prisoner is not allowed to perform Wudu, he should perform Tayammum and pray his Salah but repeat this Salah later. If jailers or enemies do not let the prisoner offer prayer, then he has to offer Salah by gestures, and repeat this Salah later on. (ibid, pp. 349) What is going to perform tayammum on needs to be pure and not illegally obtained. [24] Other Rulings Those scholars who say that tayammum has two obligatory elements explain the second and the third acts under the same item.

Going through the booklets ‘Method of Wudu’ and ‘Method of Salah’ will be useful in learning the rulings concerning Wudu and Salah respectively. The dust, stone etc, that is being used for Tayammum must be pure i.e. there should be no traces of impurity on it. Further, even if the impression of impurity had disappeared due to evaporation, it is not allowable for Tayammum. (ibid, pp. 357) If a peice of land, a wall, or mud on the ground becomes stained with impurity but the stain then dreis and vanishes due to the blowing of the wind or the heat of the sun, they are considered pure for Salah, but cannot be used for Tayammum. One can do tayammum with any sort of clean thing whose origin is from earth, such as sand, lime, and stone, so it can be done with a tile or unglazed marble, too. If they need to perform Tayammum, they have to perform it with clean earth. Allah (Exalted be He) says (what means):

Question: Is it permissible to do tayammum with dust that gathers on household items (a cushion, for example)?