(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?