The hot solution of bicarbonate and some carbonate does nothing to the scale.
I put aluminium foil in the solution, it fizzled without any nickel silver. A small tube of aluminium foil was to release more concentrated gas. With a lighter, no explosion. Maybe the gas burns faintly with a purple flame resembling hydrogen, but the amount is too small for clear observation, and the lighter's propane could also have reignited further away. Near to the solution, in the vicinity of aluminium, something extinguishes the lighter, while this doesn't happpen clearly away from the fizzle. Vapour? Dioxide?
Heat is paramount to obtain bubbles, and the aluminium foil is inactivated once corroded. A more corrodable alloy could improve.
I let an aluminium foil react for some time in the solution, then I stirred and removed the foil. The new solution did nothing or very little to the scale.
I put a new foil at one side of the solution, and the scaled nickel silver part at the other side, and nothing happened, or very little.
The I put the foil at the bath's bottom. In the air over the bubbles, nothing happened to the scale. But the bubbles in the solution descale nickel silver instantly and very efficiently. Contact with the aluminium changes nothing, neither for the removal of scale, nor at the amount of bubbles.
The composition of the magic bubbles remains obscure to me. Aluminium may react with the bicarbonate, carbonate, or hydroxide ions, and with water. The bubbles could contain hydrogen, carbon dioxide, monoxide, or some reaction product with water, or something else. Suggestions?
Marc Schaefer, aka Enthalpy