Silver Paste or Flux?

[Taogem]

I used a 56% silver solder paste combined with the very small solder chips. Very small..... for silver soldering.

I am thumbing through Rio trying to get as many items as I can afford this order, and began to wonder about the use of paste as compared to flux.

In one of the videos it showed both the use of solder paste and then she used flux for a second soldering. I never did get why or when we use flux as compared to the solder paste. She did not comment on that in the video.

Pretty sure need to use the paste solder for silver, but is there an easier method/product that makes it easier to get these small little solder chips into place?

I am getting some sterling wire etc this time. Maybe that is what the flux is for?

Man, I need to take a class....   

[Taogem]

MedicanMan just posted some good info right as or after I posted this.

So think he covered many questions.

Thanks Randy !

[theimage1]

Tao,

The idea of flux is to keep the metal surface clean during the heating and solder flow. Since sterling silver is just less than 8% copper, it has problems with heat. The copper can rise to the surface and it gets oxidized forming copper oxide. (Blackish or reddish color.) The copper oxide hampers solder flowing on the surface, it can make it ball up and stay stationary.

Flux melts over the surface to be soldered and keeps oxygen from getting to the copper. It forms a low temperature glass-like covering that keeps the air out. Some fluxes also have fluoride which converts to acid and actually dissolved the copper oxide and cleans the silver too. These make the solder flow smoother.

Most of the paste solders are just a flux mixed with small particulate solder. The smaller the solder is, the sooner it will melt, and it distributes itself over more area before melting. I never tried the paste solder before. I have always used small snippets of solder and usually paste flux. Once you get used to seeing it, you can see the solder flow and fill in the seams when everything arrives at temperature. Solder will fill in contact seams, but silver solder will NOT fill a gap, like lead solder on pipes will. Thus the pieces must be in contact before soldering.

Silver also tends to be a bit tricky because it has the highest heat conductivity of any metal, copper is second and gold a distant third. This means that when you heat one point on a silver object the heat is moved throughout the object really quickly. Thus you cannot heat one point to the needed temperature for solder to flow until ALL points reach that temperature. Thus it's usually a good idea to use enough flux to cover most of the exposed metal when soldering for the first time (high temperature solder). Since the entire piece will reach the maximum temperature it is likely to fire-scale (create copper oxide) the longer the piece is maintained at flow temperature. (A secondary thing you will notice, is that when you buff a silver piece it get uncomfortable to hold really quickly even though y may only be buffing only one small spot ... good old heat conductivity!)

I have found that the smaller a torch is, the harder it is to solder silver. You need a larger, somewhat softer flame to more evenly heat the piece. The micro-torches with their smallest tips are great for gold where you can heat one spot and expect the solder to work, but they just don't work all that well on silver. (At least in my experience.) You need to bring the whole piece up to temperature and distribute the heat. If you concentrate the heat with a micro-torch on one spot (on silver), by the time the whole piece reaches temperature the one spot will quickly go too high and likely melt. It's really easy to melt a thin bezel sitting on silver backing if you apply too much heat to one area.

I have heard that using the paste solder works well if you are trying to simultaneously solder sever  small thing at the same time. Say you want to solder a dozen little silver spheres to a backing for a design. Of if you have several snippet of metal that need to be soldered to the backing, like thin wires, or the cloisonné wire used my enamelers and stone inlay people.

I saw someplace where there was a question about annealing too. When you heat silver to a dull red the crystal structure of the metal breaks down. As with most things, if you quench (rapidly cool) the metal right after it is heated to the dull red, it will instantly re-crystallize, and fast crystallization causes small crystal size. Small crystals make the metal more pliable. They make it softer. If you heat the metal to dull red and let it cool to room temperature slowly, it grows larger crystals and hence becomes more brittle. (less flexible.)  Pounding on the metal or bending it, or drawing it (as in wire drawing) will also cause it to work harden, effectively it forces the crystal to grow together making it more brittle.

The pickle solution is used to remove light fire scale from the metal. It is an acid based material that can dissolve the raised copper and oxide.  If you really heat the silver and get deep oxide formation then the pickle cannot remove it. It becomes locked in the silver piece and it's a real bear to remove. The exterior fire scale is usually Cu2O (reddish) first, then is converted to CuO (black). If the fire scale gets formed at depth, it is usually Cu2O (Due to the abundance of silver) and cannot be chemically treated, it has to be ground away. ONe reason some people like to solder on charcoal blocks is that the charcoal is converted to CO2 during the process and helps blanket the metal keeping oxygen away. The white color that results after pickling is actually very fine pure silver because the pickle removed the copper, leaving a thin pure silver surface, with only some silver oxide (which is also white.)

That's pretty much my data dump on silver soldering...

[Taogem]

If you really heat the silver and get deep oxide formation then the pickle cannot remove it. It becomes locked in the silver piece and it's a real bear to remove. The exterior fire scale is usually Cu2O (reddish) first, then is converted to CuO (black). If the fire scale gets formed at depth, it is usually Cu2O (Due to the abundance of silver) and cannot be chemically treated, it has to be ground away.
That's pretty much my data dump on silver soldering...

I am running into this. Yesterday, for the life of me could not do a simple solder to join the bezel ends together. After the second try the silver seemed to oxidized to even salvage.

Odd that could not get such a simple solder accomplished.

It also sounds like best practice is in fact to spray or coat somehow the entire piece with a flux. Then I can add the silver solder over that at the joint with the solder pallions.

As in the piece that I now have to trash because I did not use any flux at all.

Hey thanks too for the rest of the good info there too Ron.

[Lavenderfish]

Tao,

There's a product called Cupronil  that was a very recent topic of discussion on the Orchid forum. You can get it from Rio, $6.55 for 4 oz

It works as both a protectant against firescale and as a flux and has gotten rave reviews from everyone who has used it. It's for sure on my list for the next Rio purchase.

[Taogem]

Thanks Carol,

You made it just in time with that as I am just about ready to call in some goodies in a few minutes.

Thanks 

[Taogem]

Tao,

There's a product called Cupronil  that was a very recent topic of discussion on the Orchid forum. You can get it from Rio, $6.55 for 4 oz

It works as both a protectant against firescale and as a flux and has gotten rave reviews from everyone who has used it. It's for sure on my list for the next Rio purchase.

Not sure why, but I neglected to order the Cupronil after all..

What I did order from Rio was a Self Pickling Flux.

I am a bit confused.... Again ! 

I want to use my Silver Paste Solder. Am I suppose to spread the paste over the joint, add the silver solder talians, and then squirt the whole thing with the Self Pickling Flux ?