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Barrier Flux vs Flow Flux

I read an article that Noel Yovovich wrote on this forum in 2009, in which he discussed an experiment he did using different brands of flux. He reported that the results he received by using Magic Flame Flux were outstanding, and far above the other tested brands. I have found a source for that flux, but then read another article that said two types of flux must be used to best results (I work with sterling silver), barrier flux and flow flux. Can someone tell me which type the Magic Flame Flux is and what brand I might consider for the other type? And is it true that using both types gives the best result? Thanks!

It isn’t an either situation, many times it is both. Barrier flux keeps the entire piece from being covered with fire scale, but it doesn’t help the solder flow very well. My barrier flux is just a solution of boric acid and denatured alcohol. I keep it in a mason jar with a top that can be easily sealed. Once the piece to be soldered is clean and the joint prepared, I shake up the mason jar and dip the pieces to be soldered into the solution. Set them on a soldering pad and hit it with your torch just to light the alcohol, then let it burn off. Caution…make sure you close your mason jar first so that you don’t set the whole thing on fire accidentally. After the alcohol burns off, the piece should be covered with a nice white powdery layer of boric acid. Then use your flow flux, as you call it, on the joint to prevent O2 from reacting with the copper in the metal creating an oxide layer that will prevent the solder from flowing. Your flow flux can be as simple as a thick pasty solution of borax and water. As you heat up the entire piece(s) to be soldered, you are trying to get everything up to flow temperature as quickly and evenly as possible. The flame doesn’t melt the solder, the heated metal does, so the solder needs to touch both pieces being joined. Remember that the solder will flow towards the hottest area. You can control your torch position to take advantage of this fact. Your flow flux will become liquid and then your solder will liquefy and flow around your joint. It’s a beautiful thing to see when it is all working. There is a lot in the archives about this topic and the chemists among us can tell why it all works… I just know what works for me. Good luck…Rob

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Thank you very much for the reply! It was quite helpful.

Research Prips flux too. It is similar to the barrier flux that I described in my first post. You don’t have to spend a lot of money to get fluxes that work. The trick is planning what you are making so that the bare metal doesn’t see any direct flame possibly causing fire scale. This is especially true if you are soldering pieces that have been pre-finished like a bezel to a backing.That being said, I make a lot of pieces that will be forged, ground and sanded before they are polished. I don’t usually worry about a barrier flux because I will remove any fire scale in the process of finishing them. I have found that adding a barrier flux helps the first solder joint go more smoothly than just using flow flux and is a better looking joint. Good luck…Rob

Kim- Here is a paper on soldering and annealing by Ann Cahoon. She really knows her stuff. She covers liquid, paste and and barrier fluxes as well as which surfaces produce the least amount of oxidation. I use both a barrier flux, (powdered boric acid in alcohol) and a paste flux on most metals. I use the barrier flux to cover the entire metal area and a paste flux on the solder area.
Cahoon_Ann.pdf (356.6 KB)

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Thank you for the paper reference. I will look forward to reading it! I have only ever used paste solder, and I would like to try using wire or sheet solder. I am learning so much! I see that most people recommend using boric acid in alcohol, but I have also read about a product that Pepe Tools makes called Smart Flux. It is a barrier flux, from what I understand. I am hoping someone here can recommend a brand of flow flux that they have had a great experience using. I want to have the right tools before I begin practicing, for the best chance at success.

I have soldered a number of projects using just Boric Acid and Alcohol. I have done so as well with just Borax in a distilled water suspension. And I have used Battern’s or Green Flux as well. All worked tho one was easier all the way around. Boric Acid and Alcohol as a coat and Battern’s at the joints gave great results. One procedure worked better still but it is such a pain in the ass I rarely use it. I would heat the project untill it was just becoming straw colored, dip it in the Borax and Water and let it dry in place and then drop Battern’s through the joints before I soldered. The heat you use and the flame is important. The one consistent step in all of these is being scrupulously clean with your silver, solder, and joints. No technique can defeat a poorly fit, dirty joint.

All that said, Rob and I have both remarked to each other we seem to need to learn how to solder all over again at least once a year.

Don Meixner

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I owe a debt of gratitude to whoever posted the YouTube video about mixing borax with boric acid. First time I tried it I used too much Borax - the next day my barrier flux was set up like concrete. However my initial experiment was encouraging enough to pursue it. I use a small 1/2 pint Mason Jar really impossible to knock over! I use 3 Tablespoons boric acid and 1/2 teaspoon of Borax. The jar has a 125ml mark that I fill to with the alcohol. The Borax doesn’t dissolve in denatured alcohol but the solution holds it in suspension. It has a higher melting temperature than the boric acid so as the acid is losing it’s protection ability the Borax takes over. Nothing scientific, just my observations. I believe that it’s better than straight boric acid. I do use Handy Flux for flow flux. BTW - I do heavy buckles - big flame and lots of heat and it works!

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hi,

the recipe for prips flux is located here in the archives.
borax
boric acid
tsp
hot water

i love prips flux as a barrier

julie

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Per Tim McCreight…75 ml borax, 75 ml TSP, 90 ml boric acid, 2 quarts of water. I usually reduce the amounts in these proportions and store the solution in a jar for when I need to use it. In New York where I live, you can’t buy real TSP, so I buy it via Amazon. Nancy Hamilton also has a good description of how to make and apply Prips…Rob

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Thank you, again!

Good info! Thanks!!

Hello Kim,
I have a book on soldering by Joe Silvera and he states that there are three fluxes which function as both barrier and flow fluxes, so you need only one product. One is the Magic Flame Flux to which you refer and the two others are Cupronil and Firescoff. The latter two are spray on and I understand that the MFF is a dip in. Firescoff is very effective, but expensive, Cupronil less expensive and almost as effective. IDK anything about MFF. You can look at the MSDS for Cupronil and make your own. I posted on that a long time ago, but there were no takers. I made some, but haven’t used it yet. Hope some of this is helpful. Many ways to solder, many fluxes, they all work, esp. if you are good and don’t overheat your work, but you probably know that!

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Fabulous info! I believe I will start by trying the Magic Flame Flux, which is available at Stuller. Thank you so very much! I really appreciate all the assistance this community has provided. So pleased that I purchased the subscription!

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how do you keep borax from fluffing up when heated?.. it’s a hydrated mineral that expands… the hard way is to heat it until it melts down after all the fluffing up, then grind up the dehydrated borate into a powder… it will rehydrate if exposed to moisture and fluff up again