Uniform Oxidation


I am making a piece that involves blackening sterling silver with
liver of sulphur for the first time. It is a ss pendant with applied
areas of 22kt gold wire and several stones set into it. I am looking
for a uniform, velvety black color on the silver but so far I am
getting a sort of splotchy dark gray. The instructions on my product
say use about 1 oz los in 1 qt of warm water. Can anyone advise me to
get a better result? Should I warm the solution while the piece is
in? How long should it be in the bath? Does los do any harm to stones
(in this case citrine and carnelian)? Thanks for your help.