Prips vs cupronil vs boric acid and alcohol

I am aware of argentium but have been warned against it and would love continuum but it costs too much and so I would like to keep this post about standard, silver/copper sterling.

With that said, I have been looking for a solution to the issue of firestain and after some research I gather than it comes down to flux and not overheating your piece. However which flux I cannot tell. Prips flux seems to be very popular amongst orchidians as is cupronil, which however does a better job stopping firestain as a firecoat? Most say firescale has been nearly or entirely eliminated with a generous coat of prips flux. Does commercial or homemade prips flux differ in composition or effect?

I would also like to know about simple boric acid and alcohol as a firecoat. I assume better than nothing but not nearly as effective as prips or cupronil?

Does heating to hard soldering temperatures (around 750-780 Celsius) enough to be considered “overheating your metal” and causing firestain?

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I have used them all. I now keep a sealed jar of borax and alcohol nearby. If you go to the archives you can find out more on how to make it and use it. You heat your metal until the flux flows and then stop. The quicker the better…Rob

Sorry…Boric acid and alcohol. Haven’t had my coffee yet…Rob

I have had some coffee and will start over so as not to confuse you. Firescale is a fact of life if you work in sterling. I have never tried any of the other silver alternatives. Over the years I have tried all of the various products that are made to suppress firescale and some of them work well. They are expensive. I have settled on boric acid and alcohol because it is cheap. I keep it in a sealed mason jar where I can get at it quickly and then seal the jar. It gets real exciting if you manage to set your jar of boric acid and alcohol on fire. That is why you need to be able to seal it quickly. A lot of my work gets an all over rigorous sanding and grinding when I am done soldering. This removes most firescale, so I don’t coat it first with boric acid and alcohol. I do use it when I am soldering pieces together that were previously finished to prevent or reduce firescale as a result of the final soldering operation. That step is also usually done with a lower melting point solder. This helps to prevent firescale too. I also use boric acid and alcohol when I am working on little pieces like earring elements, filigree, chains and finding. The use of any of these products depends a bit on what you make and how you make it, so there is no absolute one way, which is what you seem to be looking for. You need to find your own way. As for how hot to get your solder, you heat the entire piece first and then zero in on the joint. When the solder flows, you got it hot enough. Try to do this as quickly as possible. If you have pitting in your solder joint, you probably got it a bit too hot. Hard solder is the least likely of the solders to pit. The trick is to prepare the joint well. It needs to be clean and tight. I use use Handy Flux or borax and water. Use a lot of it. Watching solder flow under liquid flux is a pretty sight. There will likely be some flux residue left on your piece. It is very hard and difficult to remove. I usually just give the piece a good soak in my rinse water. Think a cup of coffee or walk around the block. The glassy residue will dissolve off and leave a nice clean metal surface. If you don’t use them, get an optivisor. It will open a whole new world for you. Don’t use too much solder as you will just need to clean it up later. I am using wire solder more and more, especially on small pieces like filigree. I either draw it out smaller of roll it in my rolling mill to control how much solder is heated when you touch solder. It takes a while to learn how to do this. I also use wire solder snippets if the joint is not flat, like soldering two pieces of round wire together side by side. The wire solder piece fits the joint and is heated more evenly than if you try to use a flat snippet in the round joint. I think that they are actually called pallions. Again, you need to find you own way. There is no one right way to do most of what we do. My brother and I make some of the same designs. While the end result is the same, we go about it differently. Time for more coffee. Good luck…Rob


I love/ use boric acid/ denatured alcohol barrier flux as well. Both it and Prips flux are easy to make and much less expensive than the pre-mixed store bought barrier fluxes such as cupronil or firescoff (both great products!)

I had read that the Prips flux (boric acid, borax, TSP) stays active longer and at higher tempuratures than simple boric acid/ denatured alcohol…i think it is the borax(?)…it’s all spelled out in the archives somewhere…

(I was having problems getting it to mist and coat properly while heating/warming the piece up…i just kept changing spray bottles until I found one that worked well for me…trigger spray bottle worked better than finger pump down…better aim?..finger pump mist direction kept shifting/ bottle turning in my hand…?)

you can buy and try them all…Prips is less expensive and has the ingredients to perform well.


Very kind of you to put the effort in to not confuse me. :slight_smile: So from what I can tell all of them work decently, sand pieces to remove what firescale does form and use strong pickle for a long time to remove flux residue. Thanks.


I forgot to ask…what are you making?

…you are interested in different alloys…I think they each have pros and cons…depending on what you are making…

some result in less firestain after casting…or heating…
some result in good working properties for casting, but not fabrication

so, the choice of alloy also may come down to what you want to do with the metal…cast…fabricate…fuse…enamel…engrave…set stones…high polish…etc…

Well, I usually use standard sterling which a I alloy myself using .925 Ag and 0.075 Cu. However I tried alloying with cartridge brass (not from cartridges) which worked out to 0.925 Ag, 0.0525 Cu and 0.0225 Zn and it seemed very slightly softer and should be better for firescale, right?

I will weigh in as well. I find scale to be a nuisance but it is part of working in Sterling and I have learned to live with it. Depending on what I am doing I use boric acid and alcohol as a flux and as a barrier coat, separately and together. Sometimes I use it as a coat and use Battern’s or a green alternative to solder the joint. I drop the Battern’s flux on the joint with syringe or a dropper. If the piece has a wide open flat surface that will get a hi polish I will warm the metal to a straw color and I will quench it in a borax and water suspension. I will then let the piece dry leaving the borax behind before I do any soldering. Any ingots that are to be rolled I do an aggressive brass brushing between annealing, forging, and rolling. Depending on the work I will sand, file, or grind as needed.

People ask me why I don’t use any of the other alloys that are said to be scale resistant. They don’t work well in my process. Hot argentum is fragile and I do a lot of manipulation of molten or near melted metal.

Don Meixner

I did not mention pickle in my reply. Pickle is a subject in and of itself. Assuming that you are using some sort of pickle that is warmed, possibly in a crockpot, it will remove some firescale. How long you keep a piece in pickle is a challenge. Long enough to remove firescale, but not so long that you cause damage to joints. Leave a scrap piece in pickle over night and see what it does. Practically speaking, you want to keep your time in the pickle pot short enough to keep you moving on to the next operation. Again, you need to learn your own way. The long time in a liquid that I wrote about above was rinse water to remove glassy flux residue. Once you pickle, but your piece in rinse water and leave it there for a while to remove the residue and just generally clean the piece. Staying on the theme of being cheap, sparex is expensive. Buy pH down from the local store that sells pool chemicals. There are also many other greener alternatives that you can read about in the archives. Try one, you may like it. Good luck…Rob


I am not sure what you mean by “which worked out to 0.925 Ag, 0.0525 Cu and 0.0225 Zn and it seemed very slightly softer and should be better for firescale, right?”

Why would this be better for firescale?

Firescale is easy to remove, with pickle, as it is a surface layer of copper oxides.

The more tenacious problem is firestain, which is subsurface, and which can only be eliminated by the remove of metal to below the level of the firestain, therefore it is always better practice to try to avoid it rather than have to remove it.

I did a copy/paste of an older post from Peter Rowe about firescale/ firestain that is very interesting. It is on Ganoksin under your previous post about firestain


Sorry a I want clear, What I meant was that the sterling now had a slight zinc content making it slightly softer but with a lower copper content as well as some zinc in it it should resists firestain, tarnish and firescale better than standard sterling. It definitely seems to resist firescale better.

I used for a ring that required three soldering operations. One to solder band closed, another to solder on a decorative bead and a third to solder on a second decorative bead.

I fluxed it with standard borax and water (borax cone) and used a hard solder for all joins. It shows a very thin coating of firestain and firescaled little to nothing. Is this the amount of firestain to be expected for this design or did the slight zinc content reduce the firestain?


You did not mention if you used any barrier flux? you only mentioned the flow flux (borax cone)

You asked:
“Is this the amount of firestain to be expected for this design or did the slight zinc content reduce the firestain?”

I would not know the answer to that question, but I would venture to guess that, when properly done, the ring your referenced should be able to be easily fabricated without firestain, in sterling silver.

another thought…I am thinking that when dealing with copper alloys, you might want to rely less on the alloy content, and more on perfecting your fluxing/ annealing/ heating/ soldering skills…?


I was using only the barrier flux. I produce these types of rings often using standard sterling and easily remove firestain. I am wondering if the minor firestain that does form would be less in the sterling with zinc than in regular sterling. Thanks for answering my questions! :slight_smile:

Not barrier, flow flux. I am little tired today…

Not only is it softer, but the melting point is lower than standard sterling. Essentially what you’ve done is to make hard solder (which also seems to tarnish less, or at least differently than sterling). Standard hard solder has 3% zinc; your alloy, which is 2.25% Zn is probably close enough to work the same way, But pity the repair person who has to try to fix it - the whole thing could melt into a puddle when they attempt a repair.

I didn’t notice the melting point was lower. It was certainly higher than argentium…I don’t know and will keep it in mind. Thanks.

How are you measuring meting point temperatures?..Rob

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I didn’t measure, but I soldered with standard silver hard solder (which can’t be used on Argentium because the melting point is to low) and it soldered like normal.

Cupronil all the way!!!