A state of flux?

I have a yellow pot of modern rosin flux, whose name I forget, that works perfectly well with lead based solder.
Cliff Coggin.

It depends.
First, "Lead-free flux" can actually mean the flux itself is lead-free - which is not as silly as it sounds, as some fluxes have solder in them.
"Flux for lead-free solder" is usually optimised for lead-free solder, so for instance typically it melts at and activates at a slightly higher temperature than fluxes for leaded solder, and it will withstand higher temperatures.
It's also typically slightly more active, acid and aggressive than flux meant for use with leaded solder.
However fluxes for lead-free solder are usually made from rosin, just like the old fluxes for leaded solder (but not Baker's fluids), and they may be very similar to the old fluxes.
In general they will work with leaded solder, but perhaps not very well - you may well have to get the work a bit hotter, and they may not have the exact properties you desire.
They are also likely to produce a lot more fumes, and nastier.
A whole lot depends on the brand. It's an area where proprietary secrecy is high, and it can be hard to get the formulas of the ingredients, and more important their properties - for instance the properties of rosins vary depending on tree species, tree, age, boiling time, time of year, sun, water, climate etc. etc.
Ever seen a rack of violin rosins? They make hundreds of different types, and a good violinist can tell the difference from the sound.
Some people use violin (or viola) rosins for soldering - if you want chosen and consistent properties in your flux that may well be the best way to go. A medium-brown one is popular.
-- Peter Fairbrother

You can still buy Fluxite from a lot of on-line places...

Yep - that's a zinc chloride-based flux, like Baker's fluids, not a rosin-based one.
Dunno quite why I thought the OP wanted a rosin-based flux, maybe because I thought the "for lead free solder" fluxes are all rosin-based?
-- Peter Fairbrother

Hmm - they must've changed the formulation I reckon. It was always _the_ stuff to use for small electrical work. I have an oldish tin of it around "somewhere" so I can't check at the mo. I do remember it had a very resinous smell though!
The best smelling flux is/was BICC Coraline, used a lot by power cable jointers :-)

What's wrong with tallow?
:-)
Mark Rand RTFM

I'm a vegetarian :-)

I've used Termpler's Telux flux for the past few years and it seems to work very well with normal 60/40 lead solder.
I did a test on brass leaving a lot of flux residue to see if there was any oxidisation or reaction after jointing and there was none after (now) several years - unlike some other fluxes around at the moment.
I got mine in teh local B&Q store, but they have never re-stocked it so I'm not sure who a retail stockist might be today.
Jim.

Thanks to those that responded.
A visit to B&Q revealed the Yellow Pot referred to by Mr.Coggin, plus another pot labelled as suitable for "lead-free and all common soft solders" which I duly purchased.
However, at £7-18p for something the size of a jar of Marmite seemed a but expensive; perhaps I should have tried the Marmite? :-)

You're not supposed to eat it :)
Regards, Tony

Is that where I've been going wrong?
That's probably why my solder joints won't stick... :-)

It's surely a question of horses for courses. If you are using solder to assemble brass models, for example, I'd avoid the rosin type fluxes and use Bakers or similar, because its straightforward to wash Bakers away if you want to paint the work afterwards. Rosin is a bit of a b*****r to clean off, but is fine for electrical or plumbing work and can be wiped on.

I loved aluminium gas welding flux for the reason that all the stuff I used was deliquescent and you could leave it outside for a few hours after welding while doing something else and when you came back the flux had dissolved and ran off the welded pieces. A quick rinse after that and you were done.

Rosin flux is easy to clean off - use meths - takes it straight off.
Regards, Tony