No, this is not another Black Friday/Black Week/Cyber Something ad. (The concept of these "deep discount days", by the way, is something that I find weird and a little irritating, but that's an entirely different story. If you've been following the blog for a while, you'll probably know my stance on pricing and fair pay for work, and fair pricing like that means that there's no reason for special deals with deep discounts, and that they are also not possible.)
No, this is the clickbait title for two articles on medieval Zwischgold, that metal leaf that is mostly silver with a very, very, VERY thin layer of gold on top, making it look like gold. At least for a while, until the silver corrodes and the black silver oxide finds its way through the layer of gold on top and makes everything look black.
There's been research on (mostly late-medieval) use of this material in artworks, with analyses of the metal and the changes in its structure over time. The original article has been published here and is freely available. If you read German, there's also a German article about this project, with fewer numbers and formulae and a bit more general information about the research and the material; you can find it here.
The German article also inspired the clickbaity title: Regular gold leaf was about 140 nanometres in thickness, but the Zwischgold made in the same region and the same timespan only had a gold layer of about 30 nanometres. And if this does not count as big savings, I don't know what will do!