Black stain spreading on keris during vinegar cleaning

[apolaki]

Originally Posted by A. G. Maisey

Apolaki, the steel core is not reacting poorly, and it does not need any sort of repair:- it is reacting exactly as it is supposed to react and it is an indication that the blade has been heat treated.

The reason I thought the keris is now damaged is due to the edge. It used to be a straight edge throughout, but now the edges particularly where the black stain is has a serrated/corroded edge look as I have circled in red.

Also the black stain is not removable so it leaves a un-uniform look to the keris.

I am interested to learn more about heat-treated keris blades and how/why vinegar effects it in this manner. Is there any literature you can link me to?

Were the pamor keris I cleaned prior with vinegar all non-heat treated/ did they not have steel?

Also, what is the point of heat treating a keris?

Thanks!