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Four identical joints, cleaned and prepaired .Three go great nice flow and joint, one no go try cleaning again
No go.Add a touch of flux and there she flows.

Cheers Richard.

My understanding of a flux (not from soldering but from glass/glaze chemistry) is that it assists flow at the melting point. So, instead of getting progressively more fluid as the solder melts the transition to fluid is almost instantaneous.

The purpose of a flux is to exclude oxygen and to etch any metal oxide layer which may be on the surface. Solder wil not flow over an oxide layer as it is a low energy surface. A clean metal surface is a high energy surface and so the metal will wet the surface and flow.

Apply heat to metal and it will oxadise very quickly. Were the 4 pieces exactly the same metal and where the pieces the same size? Too large a piece, if the soldering iron is not large enough will heat up the parts too slowly, cause oxidation but not heat up the part enough to melt the solder. Personally I use phosphoric acid. Its really good at etching oxide off metal and excludes oxygen real well.

It makes soldering stainless steel real easy. Try soldering stainless steel without a good flux....impossible as it forms an oxide layer too quickly.

In my opinion, the use of the proper flux is just as important to making a good solder joint as having a soldering iron with an adequate amount of heat and a solder that is good. Basically without it, you stand a good chance of having a bad joint.

So, it does not mean you are not good at soldering, just that you, much like everyone else, needs to have the proper flux available to solder.

Just my 2¢ on the subject.

Thanx Guys and Gals. Ýes Doc all the same.Well now I know to always use a good flux.

Cheers Richard.

I remember my first brass chassis....way back last century, sometime in the sixties. I didn't know about flux and just piled solder up on the joints. It looked ugly. When my uncle Sid saw it he quickly pointed me in the right direction....flux paste opened up a whole new world and so did acid/liquid flux.
Brass to brass then the paste is fine but if you are using piano wire then some of the liquid stuff is required.

The right flux combined with the right solder, at the right temperature, is The Secret Sauce. :D

The "liquid stuff" mentioned above by munter, for use with piano wire & other ferrous metals, is an acid flux, rather than one of the rosin(resin) fluxes, which also comes as a liquid.

Thanks Bill, I did not know that.