after a few years of chemical hair dyes I am considering whether I should go back to henna. However, once in a while my hobby requires some hairdressing and the hairdresser sometimes use a curling iron or hair straightener. I heard and read that curling iron makes pure copper henna brown or even burgundy which is something I would like to avoid. Does that happen 100% of times? I remember I was testing it 2 years ago on henna and cassia mix on hair from my comb and nothing happened.

Thank you!