Coffee Stains

[mrwizard]

Hello Rasdan,

I cleaned this keris with citric acid, so i know what this looks like (a dull grey). Even without citric acid the coffee stained darker. That's the reason it is preferred by knifemakers (who can of course select the steels they are using).
However, I'm not sure what reaction causes the coffee to stain the iron. Main reagent is most likely caffeic acid.
The reaction needs more activation energy than citric acid as it works only when the coffee is hot. The citric acid seems to somehow speed up the reaction. I think when the solution cools down the citric acid becomes the dominant etchant again and slowly degrades the coffee-stain. At least the stain becomes dull grey again after a few hours.

The main reagent of cola is Phosphoric acid. It transforms red iron oxide into black iron phosphate. So it may indeed work. But i am concerned about the sugar. I don't want a sticky keris... ;-)

btw. i'm no chemist either and i guess "keris-professionals" would either use arsenic or have their keris stained professionally

Best Regards,
Thilo