I'm really no expert on this. But the lack of limescale either on the heater or in the boiler in a fifteen year old machine indicates that it was used with very soft water. So the "soot" could be minor oxidation from nickel plating that was corroded over the years by that very soft water.
However, I urge you to post these questions on the alt.coffee newsgroup. Several regulars there restore machines either for a living or as a hobby, and know a heck of a lot more than me.
ianicbrisson Senior Member Joined: 3 Nov 2013 Posts: 2 Location: Montréal, Canada Expertise: Just starting
Posted Sun Nov 3, 2013, 10:11am Subject: Re: Machine Repair: Cleaning Boiler, oops?
I know the thread is very old but I ran into the same issue with a NS Mac 2000 1 group that I'm cleaning up and am curious as to the cause of this black soot. I also concluded that it had run dry but envy body would care to chip in to confirm or explain? I cleaned it up thoroughly but I'm surprised by the complete absence of material deposits aside from the black soot.
calblacksmith Moderator Joined: 25 Nov 2007 Posts: 8,169 Location: Riverside, Ca, U.S.A. Expertise: I live coffee
Espresso: ECM Vene. A1, La Cimbali M32 Grinder: Azkoyen Capriccio, Major Vac Pot: 40s era Silex Drip: Msl. Com. brewers Roaster: gave it a try, decided no
Posted Sun Nov 3, 2013, 11:31am Subject: Re: Machine Repair: Cleaning Boiler, oops?
Jim is pretty much spot on. It is oxidation. You will find it on the inside of brass porta filters too. Unless there was damage I doubt it was ran dry. Many commercial machines have a safety built into them to prevent damage to the heaters if the water is too low. It ls a thermostat probe located in the element bundle. The mixture of colors indicates that the boilr once was lined and now is missing some of the plating, possibly accelerated by your scrubbing, however it likely was thin as it was so easily removed. You may taste some metallic taste until a thin oxidation forms on the surface of the brass. The only danger I can see is if you are overly concerned with lead as bare brass is exposed. I sincerely doubt there will ever be any ill effects at all but in full disclosure I had to say it :-)
In real life, my name is Wayne P. Anything I post is personal opinion and is only worth as much as anyone else's personal opinion. YMMV!
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