Yes i saw the videos.
Here are last 3.,a bit different looking on the ceramic ,like a cured stain mark..an less crud ...I am installing NGKs .

Do additives contribute to it also?

Thanks

 

not sure if additives contribute or not..depends on their composition.

Essentially, most of what you are seeing is equivalent to the ash in a fireplace, but superheated to the point that it attaches to the source of heat like solder.