how much citric powder /ratio

[Blobber]

I did a bit of reading about it and it seems like the black residue is magnetite and a bit of carbon, apparently this is worse the hotter the citric acid is kept, and that adding the soda leads to less of the magnetite being formed.

Ive been using citric for years and hadn't been able to work out that why the black residue sometimes appeared and other times didn't.

The temperature thing makes sense now you've mentioned it, it's rather chilly at the moment and I was surprised how little dark residue there was on the bits in the above photo