User blog comment:James of the iron hills/Snoke the revived Darth Vader? Contains Star Wars 7 spoilers/@comment-26298013-20160110131949

The problem is that human remains are combustible, they do not melt in a pyre. Instead, they will burn to ash and, sometimes, bone, though a hot enough fire will also destroy bone material. A funeral pyre will completely incinerate a human body, just as a long and hot fire will reduce logs to ashes completely.

Snoke physically cannot be Sidious (Who exploded...twice. Once with the purple flash of Dark Side energy and the second time when the Death Star 2.0's reactor went off) or Vader (Whose body was completely burned to ash.)

And about the ash tray....maybe it's Vader's ashes? Ren does seem unhealthily fixated on the fact that Vader was a Sith...and not that he died to save others, redeeming himself and bringing balance to the Force, etc.