Positive then negative test

[sayf]

Hello every one , i read alot in the forum and one of the thing i read is testing rocks samples which i followed carefuly , i crushed the small rock very well and took a 50 grams sample of it , then i added aqua regia to the sample and heated the sample for 20 minutes
Then left it to cool and tested it for gold using stannous chloride , the test was positive as the sample color changed from yellow to deep black immediatly , after that i weighted about 1 kg of the same rock to test its yield , crushed it very well to a powder , heated the powder for 2 hours to remove any sulfur in the powder then washed well then added hcl and heated for 2 hours , washed well, then added nitric and heated for 2 hours, washed well then i made the aqua regia , i gave heat for 2 hours also and let the solution cool , then tested with stannous , the test was positive too , i got deep dark color test .
I denoxed nitric using the evaporation method under very low heat without boiling the aqua regia , and made it successfully until no brown fumes appeared when adding hcl to the syrup , i let the solution cool and tested again but i got a negative result , omg i was shocked why that happened, tested again and again but nothing , am not intrested in the gold but intrested why i got negative test , where did the gold go ?!
ill appreciate any help my friends
I attached a pic for the positive test

 

[g_axelsson]

Try to dilute your "positive" sample. Going black isn't a positive test for gold. You are looking for purple.
Whatever you have is too concentrated to show any color.

This is how I do tests, a cotton swab dipped in the solution and then a drop of stannous. If the solution is too concentrated I dip the cotton swab in water first before dipping it in my solution.



You should do a sample solution with some dissolved gold and test it to get a feeling for what color you are looking for in a positive test.

Göran

 

[Lino1406]

If black huing is persistent, this behaviour may indicate tellur or arsenic. Sometimes it is a good sign, they may associate PM's but the concentration is questionable