black or white flux?

[JohnHudak]

I've been trying to Silver Solder some 1/8" 1018 CRS together, and am not having much success.. Once it gets hot enough, it seems as if all the white flux has burned away, leaving the steel unprotected, and allowing the impurities to contaminate the steel.. Needless to say, that the solder didn't flow at all.. I'm using a Sievert torch, and I'm sure I got the steel hot enough.. Will the black flux possibly work better?
Thanks; John

[GregE]

Hi John,
I've been successfully using black for all of the K-27 soldered steel parts.
For the smaller bits on the A3, white was fine, but I've had better luck with black on larger/steel.
If I have a problem, it's been not getting it hot fast enough or not balancing heat between unequal size components.

[JohnHudak]

Thanks Greg, I'll order a can of the black and try it again..

[jcbrock]

I second Greg's opinion, I have had much better luck with the black (or dark brown) flux. You should be able to pick some up at any welding supply.

[Greg_Lewis]

I use black exclusively. It withstands slight overheating much better. and cleans of easier. Also, I heat parts from the underside so the heat soaks through, melting the solder. I find a direct flame can burn off the flux or melt the solder before the part is hot enough and a weak joint results. Small parts are set on a piece of angle iron held in the bench vise; the angle is heated from below. Solder is applied by setting small bits on the joints; I don't feed solder like one would do for electrical work. Finally, heating slowly also helps the parts get to temp without overheating.

[Builder01]

Black flux for me. And as Greg has said, always, if possible, heat from the other side. When the metal comes up to temperature, the solder will flow. You can also get black flux off of EBay.

David

[JohnHudak]

Well that settles it.... Black flux from now on...

[Greg_Lewis]

One other thought: I use the cadmium alloy solder which I get from McMaster. The cad free stuff just doesn't flow as well. In the amounts we use, I can't imagine the cad being a health problem. Use common sense there.

[JohnHudak]

One other thought: I use the cadmium alloy solder which I get from McMaster. The cad free stuff just doesn't flow as well. In the amounts we use, I can't imagine the cad being a health problem. Use common sense there.

Yeah, I suppose, I always solder outside anyway..

[Rich_Carlstedt]

Using Black flux DEMANDS it be done outside
The vapor can be harmful
Rich

[Steggy]

Ditto on the black flux. Also, heed the warning about noxious vapors!

[ccvstmr]

John,
I'll cast my vote for black/brown flux. Think between the two fluses (brown or white), white has a higher "melting" temperature. May be more likely to burn during the heating process. Have had my best luck with brown flux. If it dries out, add some water and turn back into a thin paste.

While it's a good idea to ventilate anytime you're working with open flames, as long as you're not silver soldering for long periods of time, you're probably okay. After all, you prepare your metal pieces, apply flux, heat, apply solder and you're done....flame off (this is for small parts, not for boiler work). The by-products of combustion may be more of a problem then the flux fumes.

Best to heat the metals...NOT heat the flux. As such, keep the flame moving. Use the tip of the flame (the blue part) to heat the surrounding metal. When the metals are at the proper temperature, most of the flux will have evaporated leaving clean metals surfaces ready for solder application. That's the time to apply or "wipe" the solder on the joint. Some people cut small pieces of solder. If the flux bubbles, it might move the solder away from the joint. Think you'll find it takes longer to bring the metals up to temperature than the actual soldering act. If you burn the flux...the solder won't flow. It'll bead up. At that point, clean off the ash and start over again. Another tidbit...solder flows TOWARDS the source of heat.

For my soldering, I use silver wire. Might be a little more expensive than silver solders, but the material has never failed me. A good source for info or materials are jewelry suppliers. But, if you have problems or are unsure of your abilities...experiment on a couple pieces of scrap metal.
Hope that helps. Carl B.