I've been wondering about the boil time for a while now but came to the conclusion that when the barrels look black all over, out they come. I was boiling for 20 mins or so when my tank was over a fire but now I've changed to a gas burner for ease of use, I wanted to be economical with the gas . The shorter boil times don't seem to lessen the quality of the finish but as I normally only use french trade brown (good for brown and black, also keeps well and works at room temp in the climate here ) I cannot say the times would suit all solutions but I have read that during boiling most rust is converted to magnetite in the first ten minutes. I tend to give it a bit longer on the very last boil. Don't know if it makes any difference at all or whether it's me just thinking it will.
I've been experimented with different methods and the difference in final results.
I found that for a bright black, shorter, repeated rusting times seem to work. It takes longer as the colour change is slow but the weaker solution and shorter rust time doesn't pickle or etch the surface of the steel too much and a good shiny finish can be obtained .
stronger solutions and double applications can quicken up the rusting but can also very easily cause pitting and uneven colour if applied incorretly or left on for too long. I don't think it's a job that can be done any quicker than how long it takes. I'd like to try steam cupboards but that'll be sometime in the future as I'm rapidly running out of space in my workshop.
As I say , I'm only using one particular solution and have no doubt that others can be applied in different ways. Make sure you neutralise after the last boil if your solution is liable to cause after rust. I don't have this problem with the mix I use unless I add acid to quicken the action or to lengthen the shelf life of the bottle. The flip side is, after rust can be controlled to matt the finish if it is neutralised when the required look is obtained. I'm not an expert, but i hope this makes sense and helps you out.
