I had a lab prep insight that may be helpful to someone in the community. I prep PYCa Top Agar in 100 mL screw cap, borosilicate reagent bottles that are nice reusable, student safe containers (like these: https://www.fishersci.com/shop/products/clear-glass-graduated-reagent-bottles-screw-caps-drip-free-pouring-rings-4/S139271#?keyword=reagent%20bottle). I mix the peptone, yeast extract, and agar in batch, boil to mix, then aliquot into the bottles, autoclave, place into the 55 C water bath to cool, and finally add CaCl and dextrose. About halfway through the semester, I made a batch where the next day the CaCl had clearly precipitated (fine white dust settled on bottom). I thought maybe I hadn't let the bottles cool sufficiently, but then the same thing happened to the next batch. After inspecting the bottles, I noticed they had accumulated a thin layer of calcium deposit on the bottom which did not clean with soap and water. I cleaned the bottles with 1 M HCl, which easily removed the Ca scum, and did not experience any CaCl precipitating for the rest of the term. I am hypothesizing that the Ca scum was accumulating after multiple rounds of use and that it was nucleating precipitation for fresh PYCa. This is all conjecture, but I have heard other faculty mention they have experienced Ca precipitation without a clear explanation, so maybe this is something to think about.