Flux should be used as a tool, not a crutch. If I'm at a hammer-in or public demo outside in the sun, flux will give an extra couple seconds of 'welding window', or help forgive a shallow or oxidizing coal fire. My shop forge is in a fairly dark corner where I can see what's happening. I try not to use flux if I don't have to. I went to a workshop several years ago, we did the 'water leaf' from the COSIRA book. It involved welding a sheet metal leaf to a length of 3/8" square. We used straight mule borax, and it worked good. The downside is, I found the piece on a shelf last week, and the weld has been precipitating white borax crystals. I need to find out how to pickle out the borax for fancy pieces I want to sell.
I've made 10-15 cablemascus blades, a chainsaw chain letter opener, and half a dozen billets from L-6 saw blade, strap banding, and rasps, all without flux, and wired or hoseclamped together. The trick is a clean deep fire, Knowing where the non-oxidizing layer of the fire is, slow heats, and practice. I turn the radio off so I can hear if the fire is roaring or choked up. This is with a coal or coke fire, I've never been able to stick a weld in the gas forge..
The California BS Assoc. is lucky to have Mark Asprey as a very active demonstrator and instructor. He went through the apprentice system in the U.K., and now lives in central CA. He's brought in British Masters Adrian Legge, Richard Bent, and Mark Constable for workshops and hammer-ins. (See BP0222, split cleft weld). I haven't seen any of these folks use flux with wrought or mild steel.
I'm not a very good welder, and rarely miss an opportunity to mess up a drop tong weld, especially in public. I just felt obliged to say that it's entirely possible to weld without flux, and folks shouldn't tell themselves they can't.