Is white alum pure (tahir)?
Question

Is white alum pure (tahir)?
Praise be to Allah.Alum is a strongly astringent substance, which is composed of crystals of aqueous potassium sulfate and aluminium sulfates. It appears in the form of transparent, colourless crystals which are solid.
White alum is used for many purposes, and is used in cosmetics, deodorants and so on. It is also used for medical purposes, such as sterilization of wounds and relieving pain and infections in the gum, and so on.
Alum is a pure (tahir) substance which it is permissible to use. It is like any other substance for which there is no evidence (in the religious texts) that it is impure or prohibited, because the basic principle is that things are pure and permissible, and cannot be deemed otherwise except on the basis of textual evidence.
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: You should understand that in principle all things, regardless their types and characteristics, are permissible for humans and are pure, and it is not prohibited to handle them and touch them. This statement is very general in application and applies to all substances; these are very decisive and clear-cut words of great benefit and immense blessing, and the scholars refer to this principle in many cases and situations.
There is a great deal of evidence for that – from what I can call to mind of the religious texts – namely the Book of Allah, the Sunnah of His Messenger, and following the way of the believers which is referred to in the verses (interpretation of the meaning):
{O you who have believed, obey Allah and obey the Messenger and those in authority among you} [An-Nisa’ 4:59]
{Your ally is none but Allah and [therefore] His Messenger and those who have believed} [Al-Ma’idah 5:55].
Then come the methods of analogy, rational thought, and the right way of thinking and understanding."(Majmu‘ al-Fatawa  21/535).
We have previously discussed this issue in the answer to question no. 231261 .
The jurists have stated that the Muslim can buy and sell it and use it for dyeing and the like. All of that is based on the basic ruling that it is pure.
It says in al-Muhit al-Burhani (7/83), which is one of the books of the Hanafis: There is nothing wrong with paying in advance [salam transaction] for cotton, linen, copper, iron, lead, brass, alum and so on.
And Allah knows best.