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I've been using an acid test kit and some ceramic scratch pads to test gold samples. On larger pieces I'm able to put on a good test "scratch" onto the ceramic then test for 10k, 14k, 18,, and 22k.

I've recently tried some testing on smaller mesh samples. They're too small to scratch on the ceramic plate so I use several of the pieces whole. Maybe about 4-5 times the size of morton salt. Around 20 mesh.

I notice on some that the 22k acid solution will bubble a bit and turn the sample black, is this normal?

And, the 18k acid solution might dull a sample but not turn it black, is this normal?

Here's a picture using a Celestron digital microscope. The solution is an acid testing for 18k. The scratch mark is a known 22k sample.  They're two bits of #20 mesh. Notice that one is getting a little dull but remaining unchanged. The other sample next to it is turning black. 

20 mesh sample test 1 w 18kt.JPG