When I want to keep cold food cold on a buffet line, I will place a pan of ice water (mostly ice) and sprinkle salt over it. It not only drops the freezing point, but it also seems the ice lasts longer. I will then nestle the serving bowls in the ice/water, to keep the food cold. I never measured the amount of salt. I would just lay down a layer of ice, generously sprinkle some table salt over it, and stir it up a little.