Is leather tanned via modern methods still considered pure?

Question

(question prompted by the answers on this related question)

There is a sahih hadith wherein the Prophet (Peace be Upon Him) is reported to have said:

When the skin is tanned it becomes purified.

Based on the meaning of this hadith, many schools accept that tanning leather removes najasah, such as when the leather comes from a dead animal (maitah).

However, it is known that with modern advancements in technology, tanning is not necessarily performed in the same manner as it was during the time of the Prophet (Peace be Upon Him). For example, innovations such as chrome tanning were likely unknown to the Arab world in the seventh century.

What ruling does tanning leather fall under? I have been told that it falls under the ruling that tanning chemically alters the leather (i.e., istihalah), meaning that tanned leather is not the same as untanned leather; if this is the case then the resulting leather would be just as pure regardless of the method of tanning, so long as a chemical reaction has taken place.

I am particularly interested in any scholarly opinions on this matter: How was leather tanned during the time of the Prophet (Peace be Upon Him), and is tanning with chromium (or any other modern method) considered the same as tanning via these traditional means?

Answer 1

There are three conditions must be achieved in tanning to consider that the leather is clean:

1. Drying the humidity
2. purifying the najasa and
3. removing the smell.

Any method does this things and makes the leather beneficial and ready for utilization is accepted.
The Encyclopedia of fiqh, vol 20, p. 228

Al-Sherbini (a Shafi'i scholar) says:

tanning is removing the humidity and wetness which rot the leather if not removed, in a way that if tanned leather is put in water the stink wont be back.
Al-Sherbini, Moghni Almohtag, vol 1, p. 238

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Islamic scholars agreed on the previous conditions and they mentioned different ways of tanning, for example:

puttig the leather in some natural substances extracted of trees such as: oak apple or pomegranate peelings, or even using flour or salt or some compounds as alum.

Hanafi scholars say that tanning could be via insolation or ventilation or using soil.

Ibn Abdin, Radd Almohtar, vol 1, p. 203