Lava Rocks for steam, how to use

Not an expert so I would encourage you to track down detailed guidance.  

However, just some thoughts that may help your search.  Probably best to ensure there is nothing unwanted in them if you are going to use them for baking.  You may also want to find details on a good initial heat cycle (At a minimum you would probably need to go well above the cooking temperature of your oven to ensure you burn of anything volatile at your baking temperature).  

I would also beware if there is any sulphurous odour when heated or steamed- even a small amount may transfer to your food and the steam may only enhance that.

When making steam, I find ice is easier to work with than water.

P.S. Personally I would only get something like this from a cooking supplier who warrants their stone for food.  Industrial processes can leave lots of unwanted traces - hence why food grade comes at a premium.

I use the bottom of the broiler pan that came with the oven and some bbq lava rocks (food safe) I got from a home improvement store on the floor of the oven.

I rinsed the lava rocks 2-3 times to get rid of the dust and grit.

My gas stove vents pretty aggressively so I pour about 500ml/ pint of boiling water into the rock filled tray after loading the oven and final closing of the door. This works reasonably well considering the venting issue.

Good luck

Tony