Hi Patrick,
the word for "killing" (or calcination) is usually māraṇa, not jāraṇa. I have not come across sam-śam as a technical term for killing (or quenching) in alchemical literature, and, just looking it up in the Digital Corpus of Sanskrit, it does not seem to occur in the alchemical works listed there. It is also not featured in Hellwig's Dictionary of Alchemy (Woerterbuch der Mittelalterlichen Indischen Alchemie).
I would understand it as cooling down in your context. Usually, the causative of nir-vāp is used for "quenching". Heating metals and then quenching them is normally done in alchemy to break down the metal so that it can then be powdered (and used in a medicine or elixir). I think what is referenced here is tempering, which is meant to reduce brittleness. It involves reheating the
metal to a specific, controlled temperature below its critical
temperature and then slowly cooling it. So heating the gold too much and then cooling it down too suddenly would have the effect of making it brittle: a desirable outcome in alchemy and medicine, an undesirable one in metallurgy. So, I think it's a metallurgical reference rather than an alchemical one.
All the best,
Dagmar