What would happend...

....if I would take my 14K gold jewellery, together with my 24K gold coins to
jewellers, and say that I'd like him to mix those two to make a single piece
of jewellery? Mathematically, from mixing 14 and 24K in the right ratio, you
can (for example) get 18K gold. But, how smart is to ask jeweller to make an
alloy for you, instead of getting it from a refinery? What's the risk of
alloy not being done well? Hope I'm not asking too much! Thank you!