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I don't know if a market exist out there already , but I think Millie13 touch on one key point out the gate , that being scent . Another would be [natural ] colorants fading with time . Castile soap stands up great , I have one from Syria I got as a gift that is 10yrs old that still looks good to me.
I have soap that I made that is older than 10yrs old with vague scents that are not pronounced anymore and natural colorants faded . But the soaps are so mild and joyous { that I only break one out every couple of years }.