That would make sense Peter-Si in the alloy does funny things during standard metal process chemical treatments-the fact that it anodised at all is a bonus. I found out the hard way about 25 years ago when using the nitric acid trick to remove a broken off tap from one of Des McAnelly's cast FF pans......got rid of the tap OK-but the pan went quite dark and speckled in the process-it too was Si-Al casting alloy-probably very similar to the Doonside cases. Nitric acid is a normal part of the 'desmut' process during anodising (I use 20% v/v HNO3 for this step) so could well have generated the color...
ChrisM
'ffkiwi'
Last edited by ffkiwi; 07-17-2015 at 03:39 PM.