What is a door jam? You mean the frame? the frame is 10 inches of thick food built into a plaster wall.
Thick food? Like mashed potatoes?
Obviously I don't know the specifics of your construction, but typically...
When a room is framed out, openings are left for the doors, on either side you have a jack stud an da king stud "sistered" together. When the door it putin place it's frame is shimmed off the stud so it can be perfectly levelled. This creates an air gap that sound and heat and vermin can pass through. Cheaply, this is covered up with moulding, but better (in non sound critical applicaions) when it's filled with insulation, which can typically be a spray foam (expanding or not)... as I'm writing this, the spray foam may help soundproofing as well, but probably not as well as a material meant for the job.
Do you mean 10 inches from inside to outside? I think it'd be pretty unusual to have a frame 10 inches thick to either side of the door...
OK, so your wall is plaster... that required lathing behind the plaster, that lathing has to be mounted on something... studs