OK here's a puzzle

Hopefully its easy enough for an idiot to solve, and hard enough that they'll feel smart afterward.
Theres a room with a shelf full of various specimens in jars. There's a table with a crucible and a retort, in the retort is quicksilver. There's some other distracting stuff in the room - a skeleton in a cage over a vat of acid and a chest full of bottles of 'Emotional Ooze' monsters. On the shelf is a book . Were gonna make it real obvious, the book has a key on the binding. No, it'll be an HP Lovecraft reference. The book is titled 'The Silver Key'. The book, when you open it, is actually a paper mold for a key. You obviously can't use melted gold because its paper. There's a jar on the shelf full of metal a amalgam we're going for the lowest common denominator so we'll put a label on the jar denoting that it is in fact, amalgam. But we'll distract from it by putting a bunch of other jars with alchemical sounding labels... 'Fire Salts' 'Bone Meal'... 'Violet Fungus Ichor'... etc. So. You put the mercury in the book through the hold in the top after mixing it with the amalgam and bada big, you've got your key. We'll prevent the players from using the mold multiple times by saying it works on whatever door they decide to try it on, and no other. I had the mold idea but my celly helped me with the mercury and amalgam bit.