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Firestain with Cupronil?

Good Morning!

I usually use My T-Flux from Rio with sterling silver for flux and firestain prevention, but recently bought some Cupronil to try. Ouch!!! I never have problems with firescale or stain, but now have a few pieces that seem to.

Anyone else with this experience?

I usually cover the silver piece I am working on with an equal (by eye only) mixture of boric acid and alcohol. I keep the mixture in a mason jar with a flip top that can be easily sealed. I put the pieces to be solder in the jar and shake it. Take the pieces out and put them on the soldering block making sure to reseal the jar. Once you know that you are free of any of the mixture other than what is on the pieces, light them with a soft flame to burn off the alcohol and melt the boric acid so that it covers the pieces in a fine white coating. This is a your barrier flux to, hopefully, prevent firescale. This is a must do step if you are thrying to solder pieces that are alread prepolished. I use Handy Flux and occassionally Mt-T-Flux on the joints that I am soldering. I keep the Handy Flux in the jar that it came in and spoon out a small amount at a time into a smaller mason jar that can be sealed. Mix it with water. Distilled is best but I usually forget only to wonder later why I have all this distilled water hanging around. Mix the Handy Flux to a creamy consistency and keep the jar sealed. I keep liquid flux in a small squeeze bottle with a metal tube to dispense it. Sometimes you just have to start over with new flux, new or cleaned solder and cleaned and scrubbed soldering blocks. I usually take the softer soldering blocks out and scrub them on an old brick to get the gunk off of them and get them as level as possible. I try to buy two solderite boards at a time when I may only need one. This way I always have a new one when the old one is just too messy to work on. I also keep pieces of hard and soft charcoal blocks around to shape with an old file when I need a solderin gig. You can also buy hard and soft solderite boards. I have never used Cuprinol or Firescoff, they are too expensive and what I do seems to work. You can also make flux from borax and water or a borax cone. A lot of fine gold jewelers just use borax and water. One final suggestion is Prips. It is a type of barrier flux similar to alcohol and boric acid. When I use it, I dispense it with a small blow pipe device that artitst use to dispense powders in a liquid carrier. Lots to talk about here and much of it is old school stuff that is inexpensive and just works. There is aso a lot in the archives about fluxes. In the end there are no asolutes, or at least very few. We are all just looking for what works for us and it may not be the same for everyone. Good luck…Rob


may i ask? did you warm the piece first?
after piece is warmed, and you spritz on the flux, it should immediately dry/ frost over…and then you continue on with your soldering…

the directions seem simple, but for some reason, I always have problems in the beginning by over spraying the piece, while i am heating it, re-wetting what dried!:rofl: and wondering why its not dry/ frosted.:joy:

it is great stuff once i get my muscle memory working!
the metal remains amazingly clean!


Julie…Are you talking about boric acid and alcohol or Prips?..Rob


prips…i have cupronol too…i just seem to have a hard time hitting the spritz sweet spor…

for boric acid and denatured alcohol, i will just light it up first, or sometimes air dry it…i dint gave oroblems with that…


When I am soldering wide pieces with a lot of smooth surface I will warm the work up to straw color and then submerge it in water and borax( Warm water stir in borax until it is a saturated solution. No more Borax will dissolve in the water) I take out the work and let it air dry on the warm brick. The dry white covering is a barrier coat. Then i will solder with any flux I have. Most often it is Boric Acid and Alcohol.

I have not had a serious worry of stain since using this process. Thank you Tim McCreight.


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Thanks, Rob! I have pretty good luck with Rio’s My T-Flux, but have wanted to try the boric acid/alcohol mixture out of curiosity. You use denatured alcohol, I assume? Any health concerns with this above and beyond what one should be concerned about with whatever might be in My T-Flux?

My main issue that led to this post was the surprise firestain that happened when I used Cupronil. I did warm the piece prior to spraying with Cupronil and likely warmed again and sprayed again, as I typically do with My T-Flux.

There is a lot of discussion on the boards here about borax vs boric acid, and honestly, I find it all bit muddled. And with regard to whether you can mix it with water, alcohol, vodka, everclear, etc. Haven’t seen mixing it with beer or wine, though! That would be going to far, I expect.

I appreciate your reply and expertise!

There is some confusion. I am not a chemist, although I got talked into teaching it one year when the real Chemistry teacher went out on pregnancy leave. As far as I know Boric Acid is a more refined version of borax. All the reading that I do tells me that borax and water make a fairly good flux for joint soldering, especially gold but not as a barrier flux. Boric acid and alcohol are what you use for a barrier flux. There are more expensive commercial products that do all of this, but I just reply on boric acid and alcohol. My shop is well ventilated, so I don’t worry a lot about alcohol fumes. I worried more about radon when I discovered that it had gone up to 12 pc/l over the years. I have since remediated it. Look at the archives and you should find some consistent information. I have both of Tim McCreight’s books and they also a help. but are a little confusing regarding what to use borax and wter for. My go to soldering flux is Handy flux. It’s a bit messy, but works well especially on the bigger pieces that I make where I might be using a lot of heat that would burn off other fluxes. In the end, we each use what works for us, but I lke to experiment once in a while too. Good luck…Rob

Boric acid is B(OH)3 but borax is B4Na2O7-10H2O. The latter is a strange eight member ring with four borons and four oxygens with some other oxygen and hydroxide ions added on. So they are different. IDK whether borax at some point in heating would turn into boric acid or release B+ ions…I suspect so, but my chemistry is rusty (BA in Chem, '68). Prip’s flux is borax and boric acid with TSP (trisodium phosphate) and Cupronil is borax and boric acid with disodium phosphate added instead. I’ve made up my own Cupronil, but haven’t had a chance to use it yet…should work OK. Got the amounts to mix off the Cupronil MSDS data sheet. Boric acid is roach powder and Borax is the 20 Mule Team laundry stuff. Got DSP off ebay. Heat up the water and disolve the chemicals in it as for making Prip’s. QED -royjohn

Borax and boric acid are similar in that both will melt onto your metal blocking oxygen, thus preventing fire stain and fire scale. Boric acid is a bit more active, attacking existing oxides a bit more, though neither is very active at this, which is part if why others form coatings that don’t quickly become ineffective with prolonged heat or a lot of oxides. Handy flux is an example of a much more active mix, where fluoride and other additions more aggressively attack oxides, but in trade, will burn out/become saturated with oxides, etc., and stop doing its job. So great as soldering flux, but not so good as a protectant of your metal. Between boric acid and borax, boric acid melts and becomes active at a lower temperature, protecting your metal before any oxides can form. Its commonly used dissolved/mixed with alcohol, where burning off the alcohol leaves a light coat on the metal, as well as stones, some of which (primarily diamond) it will protect. Borax melts at a higher temperature, and is less active at dissolving oxides, but historically was used more than boric acid when working silver. You get, for protective purposes, the best of both. Prips flux is a mix of 3 parts Borax, 2 parts each of boric acid and sodium phosphate, dissolved in tap water. Use the more common TSP, trisodium phosphate. It acts as a wetting agent to let the other two form a uniform protective coat against fires are and fire stain on silver. Unfortunately, it also seems to make Prips flux unsuited as an actual soldering flux. Use it, but then add a good soldering flux just to the seam.


Peter – thank you! This is a great post. Very clear about what to do and what to use, both as a soldering flux and and a protectant. I think part of my confusion about the formulas, borax, boric acid, etc stems from the fact that the word “flux” is used interchangeably for soldering flux and as a firescale/stain protectant. Is this correct usage? Are others confused by this?

Thank you!

I am beginning to wonder if the bottle of Cupronil I have might be defective… I think I will toss it and fill it with My-T Flux. I do really have good luck with My T Flux, btw, both for firescale/firestain prevention and as a solder flux. Not sure why I thought I should try something else. But your post gives me some room for thought, and a desire to continue to experiment!

Thanks to everyone else who posted too!


I’ve had problems before with cupronil until I realized you have to shake the bottle/mix it well before each use…