We have had this discussion before and there are a number of websites including my which describe this process. http://stampsmarter.com/learning/Ho...idation.html
I have stopped doing this procedure and enjoy the stamp with whatever patina a stamp might have when I become the temporary caretaker.
First, I have noticed that stamps which have sulphurated have a tendency to re-sulphurated and I found myself repeating the process on the same stamp over the years. This in turn lead me to ask myself the following questions;
1. How many times has this stamp been treated?
2. Should I be documenting the chemical changes that I make to a stamp?
3. If I sell the stamp in the future do I have an ethical obligation to disclose what I have done to it?
4. How many times can a stamp be treated before it makes permanent chemical changes or is detrimental to the stamp?
5. Is this process conserving stamps or restoring stamps?
Obviously folks can do what they desire with their own stamps but for me I have decided to not do this anymore.