Hello, Can anyone please help me know better why pits sometime occur in my solder seams? I imagine it might be due to overheating but I would love to have a clear understanding of if this is so and why! In particular, I am referring to some seams on bezels in silver, that were not particularly large pieces. The torches I have are the Swiss torch, with Oxygen and Propane.
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You are likely getting the solder hot enough to vaporize some of the zinc. Are you using hard solder to solder your initial bezel seam? Hard solder has the least amount of zinc among the typical three solders that we use and melts at the highest temperature of the three. You need your torch to be hot enough to get in and get out. The longer that you heat the solder in the joint, the more likely it is that you will drive off zinc. Make sure that the joint fits perfectly and that it is clean. You might also coat the entire bezel with a solution of boric acid and alcohol, burn off the alcohol and then coat the seam with a high temperature flux. You aren’t alone. I suffer from this problem once in a while and I have to go back and apply the basics making sure that I don’t skip one or two in my haste to get it done. This can also happen after the fact as you solder additional seams in the piece. Additional seams should be soldered with lower temperature solders to avoid heating one seam when you work on another enough to cause the solder to flow again in the original seam. If you can, don’t do any finishing on the first bezel seam until you are done with all of your soldering. This might allow you to finish into the seam to a point where the solder doesn’t have pits. If you do find pits, you might try to flatten them out with a rotary burnisher. You can buy them or make one from an old bur. You torch is fine. Actually, it is very nice. Others may have different ideas, but these are mine. Good luck…Rob
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Thank you! Yes, I am using hard solder for the most part. Thank you for this explanation and your suggestions, it’s very helpful and greatly appreciated!
Best wishes,
Barbara
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Probably not zinc vaporization in your solder. Zinc vaporizes at 1665 F, that is 25 F higher than the melting point of sterling silver at 1640 F.
Dirt, and other contaminants can be vaporizing at soldering temperatures but more likely is oxygen that is being absorbed by the molten solder coming out of the solder as it begins to solidify. Silver has a crazy ability to absorb oxygen while it is molten but can’t hold it as it returns to the solid state and it comes out of solution as bubbles. Apply enough flux to protect the joint and keep the joint area covered and keep you soldering times as quick as possible to prevent absorbing too much oxygen.
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All super good advice from Rob, but I would like to emphasize one point he said: “You need your torch to be hot enough to get in and get out.”
Once the solder flows, take the heat off.
All the best.
~AJ
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Jim…You know a lot more about what is happening chemically and metallurgically than I do. I just know how to avoid pits (most of the time), not why. So thanks for the clarification. I also really like looking at your work. Thanks…Rob
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Thank you Rob,
There are multiple reasons for bubbles. At higher temperatures zinc vaporization can be a big issue for solder seams at temperatures above sterlings melting point.
One thing that I am curious about. When I cast an ingot of sterling silver, usually in the 2 ounce range and 1” X 2”X maybe 4mm thick, as I roll it out to around 16 gauge, I will get what appear to be small blisters after annealing and pickle. They roll out, but reappear after the next pickle. I am curious what might cause these blister. Thanks…Rob
Hi rob,
oh, the answer is in the brepohl book…lemme look…
juiie
i think there are posts on this too…
What Jim Binnion said. I’ve done a LOT of silver work in my career. The magic key is cleanliness and a good flux. I have found that liquid flux burns off quickly just and doesn’t work as well as a good paste flux. When silver soldering I first dip the piece is denatured alcohol and powdered boric acid and then flame off the alcohol. I dim the lights so that I can see how hot things are and start off with a slightly bushy flame and wave it quickly back and forth over the piece I am working with. When it starts to get a warm glow I then tighten my flame and focus on the seam. Silver is kinda like a transactional romantic relationship. It like things hot clean and shiny.
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Blisters like you refer to are trapped gas. There are several ways you can end up with them in your ingot. First guess is trapped oxygen that comes out of solution as the metal solidifies. Silver can contain as much as 20 times its volume in oxygen when molten. However it cannot hold it in solution as it returns to solid state. If that gas is trapped by metal solidifying around it remains as bubbles in the metal matrix. Turbulence in the pouring of the ingot can also trap gas in the ingot. Other means of trapping gas in the matrix are not typically going to be found in studio produced ingots.
As you roll the metal down the thickness of the metal around the bubble is reduced. When heating the gas bubble tries to expand, if the metal surrounding it gets thin enough it will form a visible bubble distortion on the surface of the metal. When you roll it out it flattens the bubble back down into the surface. When you heat it to anneal the gas again expands and the bubble reforms.
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I guess that the best way to avoid it is to cast under an insert gas. Something that I don’t have the ability to do other than to switch to my electric furnace and keep the cover on it. Are you describing actual bubbles of just an area that has gas mixed with the metal?
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Though I doubt its the issue at play here I thought I would mention that pickling acids can attack the zinc in solder so fabricated pieces left in the pickle overnight may come out visibly pickled.
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overheating of molten silver alloy and pure silver absorb 16-20X in volume% of oxygen, as mentioned in the prior post. I haven’t had much trouble with silver ingot casting with absorbed oxygen. using casting flux with powdered charcoal will help the silver from taking in oxygen. Stirring the molten metal with a graphite rod lets out the dissolved bubbles. Make the pour quickly.
I use an oxyacetylene steel cutting torch head to melt. Lots of heat output and high temperature.
There is a lever on the side of the torch handle. When pushed, the lever open a jet of oxygen into the silver melt. “fire refining” is method that I’ve used to make fine silver for bezels. the jet of oxygen blows into the molten metal, burns out the copper which is trapped in the flux. As the metal cools, bubbles of oxygen form within the molten silver and pop on the surface of the melt. This is called “spitting silver”..The flux will turn brown with copper oxides. removing the flux and repeating a couple of times until the flux clears creates a complete burn out of copper leaving fine silver that can be poured in a mold and rolled out. This process can be dangerous as when the silver “spits” droplets of molten silver can cause flammables to catch fire. Hoiwever, the droplets are small and heavy, they only fall next to the crucible. I have done this only outside with a fireproof table top using cement board. Fire refining will not get to.999 fine, 98-99% fine. Fine enough to make a higher melting point silver and a more soft and ductile silver, good enough for bezels.
oxygen absorption is not the usual way to create bubbles. I have to agree with that.. Dust and other contaminants are far more common. choosing a pickle solution is also important. sulfate acid pickle (Sparex) is less corrosive than organic acid pickle (oxalic acid).. Leaving overnight to pickle is not a good way to go. In the old days, I used Saniflush. Saniflush was sodium acid sulfate (NaHS04)… it was just as effective and way way much cheaper than Sparex! The new Saniflush safer chemicals, took out the acid sulfate and can’t use for pickling. Zinc is the bane of all things silver and brass. Working with mixed metals including brass and copper applique is attractive but has its own problems with the zinc content of brass creating its own solder. Copper will also create its own solder by moving the contact melting point towards the eutectic melting point of silver and copper. I used an air propane torch with a lower flame temperature than gas and oxygen. Enough high temperature to work with and cheaper than fixed gas torches.