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David Lisch once told me his method for fire etching wrought iron. I do not quite remember the details and will check my notebooks, but I think it was heating in the forge until a dull red color, removing and letting it cool. Then wire brushing by hand and repeating until the desired texture appeared. He got this dramatic pitting finish. He then used gun bluing to turn it black.

Gary, I don't know of any specific time length, but how the acid is stored has a lot to do with shelf life. Glass bottles are a killer for acid as exposure to sunlight will weaken the solution. I guess the shelf life probably is directly proportional to the volume and usage rate. What size and type of container do you keep your acid in? I use a black ABS pipe mounted vertically on my bench leg. I also etch using a fish bubbler to agitate the acid for a faster etch.

When you say fish bubblier are you referring to like the little treasure chest in the your kids fish tank? I realize you don't put the toy in there but the air supply to it, and how does that speed up the etching?