These are silicone carbide shelves. Q) Are they supposed to melt like that?
Well. I thought the answer was a resounding NO, but it turns out that ceramic-bonded silicon carbide is less impervious to the aggressive fluxing action of the soda, and can suffer from "progressive grain boundary corrosion." Son of a bit.
These were bought used, for short money, along with some others that are holding up like champs - those must be nitrite-bonded.
I can't afford new shelves right now, so my option is to grind the living shirt out of these, and the - reluctantly, because I am well aware of the pitfalls - apply a thin coat of kiln wash to the undersides as well as the top.
Oh, what a world, what a world.