I had this same problem at first. When you combine the Salt and Coal, they disappear and a new object is created. For me, this object appeared in the center of my inventory, and is a barrel topped with black "powder". When you combine this with the Sulfer, both of those objects disappear, and you get a new object ("Explosive Powder" I think). This object is smaller and light-colored, and appeared on the very left-hand side of my inventory. In fact, it was kind of in the area of the icons (for armor, misc., etc.), and really blended in with the border/icons. Search your inventory screen fully, and it should be there.
Hope this helps!
Regards,
Michael
