Sand in my boots would be the last of my worries during the melting and moulding process.
If you want to see another example of risky (IMO) hot metal use check out YouTube's, "Back yard scientist"
Oh well, as has been said previously by Pete, "Each to their own".
I learn stuff every day on this site, perhaps cos I don't know much to start with. Certainly never done any serious metal melting but I do have a scar on my leg from molten lead as a 12yr old. Ah, memories.