I am a high school teacher about to build a cob oven with students in Homestead PA (outside Pittsburgh). We want to use materials form our site, but I have concerns. WE are within sight of the former Homestead Works - once the largest steel mill in the world. For about a century it pumped all kinds of nasty stuff into the environment, and I worry about our clay. Local soils are notoriously contaminated. Our site tested positive for arsenic. Is it safe to use clay form our site in our cob oven?