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I used to do this too, but I've found many styro meat trays these days have a built-in sort of blood sponge which is impossible to rinse out properly. You can tell if there is no absorbent bit under the meat and the tray has a grid of what looks like drain holes in the top side.
A handy qualitative test for how much carbon is in a bit of steel is the grinder spark test. The sparks from grinding mild steel are visually different from the sparks given off by tool steel. There are heaps of videos demonstrating this. Here' is just one:It's easier to just get known samples of steel, though, and just do the test yourself since video can't really capture all the detail you can easily see for yourself. !שלום, חבר