Hello - I recently started using borax cone for flux. A couple of times after solder was almost done flowing the flux bubbled up and created a hard, granular deposit on the silver. It was very hard and I had a hard time filing it off. Also, one of the solder joins became pitted. I only use distilled water with the cone. Any idea what I am doing wrong and what is going on? Thank you,
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Pitting can be due to a poorly fitted joint, too much solder, overheating the solder (especially with easy and medium solders. Most common though, is simply dirt. Borax as a flux works well, but it is not highly active. That helps to keep it from burning off, but it also means your work, solder, flux, and tools need to be clean, as the borax glass won’t dissolve or remove dirt and oxides.
Your tough deposit is just borax glass, which is what it is supposed to do. That coating is what prevents oxidation. If it is too thick, use less, but surface tension effects will pull it towards seams, valleys, hard to clean areas. But it is simple to clean off. Where in the dickens did you learn to clean that off with a file? That is what pickling your work after soldering is for. Dilute (5-10%) sulphuric acid solution, generally used hot, but not boiling, is the classic. But a sulphuric acid salt, sodium bisulphate, is safer and widely sold by jewelry supply stores. Sparex us a common brand, though I recommend private label supplier brands, like Rio Pickle or others. Generally cleaner/purer than Sparex and cheaper. You can also use milder acids, like citric acid, which is safer, but can be much slower. I’ve even heard vinegar being used, though I’ve not tried it. Intuition suggests straight borax might be a bit tough for vinegar. If you try it, please be sure to remove the pickles first. 
But whatever you use, please leave your abrasives and files for use as they were intended, on metal.
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Peter…Thanks for your great answer to the OP’s question. I found a long time ago that a good soak, sometimes overnight, in rinse water will remove a lot of ills, especially glassy flux deposits. I often wonder if it is because my rinse water is really a very diluted pickle. It is frustrating when you are polishing to find a small area of flux that didn’t disolve. Your inclination is to grind, sand or file it off. Don’t, you will just create a divot in your just polished surface…Rob
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Another source of sodium bisulfate (Sparex) would be swimming pool pH down or pH reducer. Exactly the same chemical as Sparex, but probably quite a bit cheaper at Wally (Rio Pickle: $11.95/3 lbs; pH Down at Wally: $7.88/5 lbs). You can get it in various sizes, too, so prolly no need to buy more than you need, altho’ it does keep OK. -royjohn
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I use a borax/denatured solution when soldering gold fill. It leaves that same hard residue like old varnish. I do quite a lot of GF and I didn’t want to pollute my pickle, so I found an easy solution… I put a squeeze of Dawn into a bowl of very hot water and let my pieces soak for a few minutes. It has cleaned off the residue completely every time. Then I pickle to remove any firescale.
Thanks so much for your help. I use Rio brand pickle but it did nothing to remove the deposit. The deposit looks like silver. I think you are right–I was using too much borax and I think there must have been some contaminant.
Thank you for responding–I did soak in the Rio Pickle overnight but it didn’t help–I must have some contaminant. Also I think I used too much borax.
Try soaking it in clean water, not pickle. let us know what happens…Rob
The water didn’t help either. I did thin out the borax slurry and it’s not happening anymore.
hi,
re: solder joint pitting- overheating may be burning out the zinc in the solder…?
re: leaving the piece in the pickle overnight…are you using your pickle warm?, or cold?…warm is faster…you probably shouldn’t leave the piece in the pickle overnight…there is a good reason, but i forget what it is…
julie