rock or steel or urbanite in mass ?

Hi All ; Still working on my rmh, and i'm looking ahead to my mass. I understand encasing my flue pipes in solid cobb , to form a natural tunnel in the cob. I had been planning on using local rock and some urbanite from the landfill as (filler) in my mass. Today I noticed my scrap metal pile... Is it better ? as good ? or a bad idea to bury brake drums , rotors , cly heads, some are alluminum ? in my mass ? I have many years worth of scrap metal i could bury and i also have large amounts of shale that i could use and the local landfill has vast piles of concreat. what one would make the best filler ? I'm still working on my horizontal transition area but thinking ahead. Thanks for any advice. tom

T.R. : Mere cob,which is what you want in close contact with your - everything! weighs 1/2 of most rocks, the more they weigh, the faster heat travels through them, a big hunk of
very dense material close to your horizontal pipe and the top of your bench can and will make a hot spot, this said don't forget to apply a thin layer of Clay Slip between each and
every layer to achieve good contact and bonding between layers. (use a paint brush ! )

I would be very tempted to put my mass as low as Icould get it, but if that moved the horizontal pipe higher in the Thermal Mass Then I will get prompt heat Quickly but less long
term storage, More dense material will hold more heat, the only tradeoffI can see is, will I ever want to tear into it to make ?Repairs ? Have fun with this Big AL !

I just posted a blog about thermal mass with a chart comparing several types of materials. It might help you think through your selection.

http://blog.dragonheaters.com/category/reference/thermal-mass/