Gold and silver testing acid

[Yef]

Dear Sir,
Can someone help me for exact formula to make testing acid for gold 18k and 14k.
I tried many times using commercial HNO3 and HCl acids. But in vain.

Thanks in advance.
Yef

 

[nickvc]

I’m fairly sure if you have known pieces you can create your own , I’m fairly certain it’s different strength aqua regia , the lower the oxidizer , nitric , level the more resistance high carat pieces will have, you could even try diluting a standard AR solution to see if that works.