Cobb, What soils and fillers make good Cobb?

Life is too short to make all the mistakes yourself, so you ask others for help

In preparation for next winter we are planning to build mass heaters for under livestock water troughs.

We are planning to use steel pipe, from grain augers for the flue.

What materials are the best for:

making mass heaters?

in what combinations?

Is there any material that should be avoided?

Can portland cement be added for strength?

Can rammed earth techniques be used?

Has anyone ever made a mass heater on skids that can be moved?

Thanks for helping

John

"He who has a question and asks is a fool for five minutes; he who has a question and does not ask remains a fool forever." -Chinese Proverb