I assume it's yellow. For some reason, the silver percentage of yellow 18K gold is often higher than that of yellow 10K or 14K. From what I've read, it can often be between 10-20%. If so, you may not be able to dissolve it directly in aqua regia and will have to inquart it. You'll need to add 26 grams of silver. The amount to add = (13 x .75 x 4) -13 = 26. The number 13 (appears twice in the formula) is the initial weight. The number .75 is the karat (in this case, 18) divided by 24. The number 4 in the formula is a constant - it is always 4. In this case, the total metal = 13 + 26 = 39 grams. The gold is 13 X .75 = 9.75 grams. The ratio of total metal to gold is therefore 39/9.75 = 4 to 1 - just where it should be. This works for any karat and scrap weight.
