I have built a fire pit out of 1/4" steel. 30"x30"x30". I want to attached real stone to the outside of it. I have purchased james hardie backerboard to attach to the metal as a base for my 4" thick stone (12x12x4) which are resting on ground and covering the fire pit. Could anyone let me know if the hardieboard emit any gas upon getting heated up that may be hazardous? what approximate temperature would the steel get? Is there another material I can use to affix the stone to the metal (considering the stone is resting on ground)? thank you in advance.