I am using thicker gauge metal as my metal is flat. This requires more firing. Is this bad news? Do flat pieces always chip or break?
Longer firing times aren’t necessarily bad. If you ask Merri Lee Rae, she prefers lower temp and longer time for some enamels. But it’s always dependant on which enamel, leaded or unleaded, colour, etc. However you may find that the flatness is going to be problematic. I find flat pieces prone to chipping. Especially if larger.
Longer firings aren’t bad. Longer firings+higher temperatures can be problematic for some enamels (reds, and opalescents, in particular). A long firing at a lower temperature (1325-1350 degrees F) is safer.