First make sure we have a clear idea of what we mean by the term "Buoyancy". My idea of Buoyancy is the up ward force the liquid exerts on the object placed into the liquid. I assume you are talking about placing say a solid form of the substance. Now if the density of the substance is greater than one the overall buoyancy force will be less than the weight of the object therefore the object will sink, with regards to buoyancy force the weight of the volume of the liquid displaced is equal to the upward buoyancy force, that is why to maximise buoyancy force we change the shape of the solid object so that it pushes aside max volume of liquid.

Hope this make sense. Alan

Hope this make sense. Alan