I've come across the iron pot recommendation on many a Middle Eastern or South Asian web site. I'm doing more research to find out what the advantage is of mixing henna in an iron pot. The words that are italicized and in bold have been added (by me) for clarity.
"A specific henna mix is made in an iron utensil (pot) and applied as hair dye. A handful of henna is taken in an iron utensil and to it are added a cup of tea de(con)coction, 2 tablespoons of curd (buttermilk) , and one beaten egg. If the mixture is thick a little water is added to loosen it. The mixture is left overnight and applied in the morning for at least three hours. A henna mix like this makes the hair shiny, prevents hairfall (shedded hair) and cures dandruff."
