After watching molten iron being poured, cast and cooled, visitors to Dotson Iron Castings got to make their own molded metal souvenirs Thursday. They cast tin instead of iron and it was only heated to 350 degrees instead of 2,800 degrees, but the process visitors used to make pumpkin-shaped trinkets were otherwise a small-scale example of the 3 million pieces of metal the 135-year-old foundry makes each…