Quenching, or quick cooling, copper, brass, and silver is done by immersing a piece in water after heating it so it can be dried and then worked with again quickly. Because metals cool at different rates, how do you know when they are ready to quench? Generally, when the red glow is gone from a piece of metal, around 10 seconds or so, it is ready to be quenched.
NEVER quench in a pickling solution. Even though you may be using the least caustic pickling solution, it can still cause problems if the droplets from the solution get into the air, into your lungs, on you, your clothing, or work surface and tools. Stick with water for quenching.
Argentium is my precious metal of choice and I use it in a lot of my mixed metal pieces as well. These are some things I have found through research and a lot of trial and error.
Argentium, a silver alloy, needs to be treated differently than sterling silver. It should not be moved and quenched immediately after heating and/or soldering as it can crack apart and/or warp beyond saving. Many Argentium pieces could be ruined learning when to quench so I found it is easier (and cheaper, too, because there are no mistakes made in haste to quench) to let the piece cool naturally for 30 seconds or so before quenching. What is a few seconds before quenching or moving the piece to a bench block to cool compared to losing all your hard work?