Removal of borax coating

[Primus778]

I have a chunk of .999 silver with a thick coating of borax from a bad melt. I can't break it off and I don't want to sand it off. Tried to remelt it but can never get rid of borax coating.

Other than sanding this junk off what can be done to refine the silver back to pure .999?

Thanks guys

 

[ericrm]

with gold the easy way is to remove boarx with hot dilute sulfuric acid... i dont know how well silver will do as it disolve in sulfuric acid... but i think i would be worth the try maybe you wont loose much...

edit: my mistake aparently sulfuric acid dosent dissolve silver... your good to go

 

[Geo]

silver is ok in hot diluted sulfuric acid. it will not dissolve. try auto battery electrolyte and a good pyrex container. be careful. WARNING : hot sulfuric acid will dissolve flesh and bone and cause blindness on contact with eyes. be very careful when heating sulfuric acid but it will remove the borax without harming the metal.

 

[butcher]

Silver will not dissolve in dilute sulfuric acid, even if boiled.

Now concentrated it is a different story, as I understand it the silver can dissolve from persulfate formed, similar to how gold dissolves in the concentrated sulfuric acid cell where persulfate forms at the anode, persulfate can also form from boiling concentrated sulfuric acid to the point SO2 gas leaves solution like in this dangerous process--- where silver can be dissolved in concentrated boiling sulfuric acid, page 53 Hoke's book speaks of its use instead of using HNO3, for parting silver and gold (low copper or lead content), large lots done in cast iron pots.

 

[kadriver]

I use about 10ml concentrated sulfuric acid in about 200ml distilled water (tap water probably ok) in a pyrex bowl.

Then I place that in a corningware casserole dish on low heat - but be careful because the water will evaporate away which will concentrate the acid.

The dilute acid very effectively dissolves the stubborn flux, and leaves the bars looking fresh and bright. I have found that higher concentrations tend to discolor the surface of the bars.

Once I was removing flux from a bar. I put the bar in the solution and turned on the heat, then went back out in my shop. An hour later I remembered and rushed back inside where I found the whole house full of white sulfuric vapor.

The water had boiled away and the acid had turned to a vapor cloud. The silver bar was erroded (but not too bad) and the pyrex bowl was as dry as a bone.

I put on my 3M vapor respirator and went inside to open all the windows and doors to let the vapor out of the house.

Here is a shot of some newly poured 5 troy ounce silver bars bathing in dilute sulfuric acid (the acid is Liquid Fire from Ace Hardware).

kadriver

 

[Primus778]

Thank you guys! I will give it a try and report back

 

[Primus778]

Yes, the aforementioned method suggested by kadriver worked perfectly.

I actually used regular battery acid, which I think is around 30% sulfuric acid, and slowly heated it to between low and low-medium. Most of the borax dissolved and the rest was wiped away.

Auto zone carried the battery acid in a small pack for around $8; somewhat pricey for the amount I got but I have plenty left for the future.

Thanks to all who posted.

 

[MysticColby]

kadriver, I'm glad I'm not the only one to make that mistake. Don't forget your solutions on the hotplate!
I find that most of any borax stuck to a silver ingot will come off if you dip the silver ingot into a bowl of water soon after pouring it (make sure it's fully solidified, use tongs, don't drop it!), until it stops sizzling. heat shock cracks the borax glass.

 

[Anonymous]

This is an interesting thread.

Thanks to kadriver, MysticColby, butcher, Geo and ericm for your information. Now I know what to do in case this happens to me... well, it did, I still have some gold stuck in a crucible that has borax coating it. It's only about 3grams still stuck, yet, I didn't have the propane torch I now have.


Kevin

 

[Palladium]

For gold i use heated diluted Nitric to remove the flux. I don't like Sulfuric, even diluted.

 

[Anonymous]

For gold i use heated diluted Nitric to remove the flux. I don't like Sulfuric, even diluted.

Thanks for the info. Earlier this year, I took the crucible and started breaking it up. It's one of the 1oz or less size crucible.

I'll give that a try.

Kevin

 

[joon5225]

A safer solution to remove borax is to use citric acid in warm water. A Crock pot for example, will help speed this up.

 

[goldsilverpro]

An old refiner's trick. For small amounts of slag on the surface of a bar (doesn't work well in depressions), I hold the bar under running water and gently tap the slag with a stainless rod about 6" long and about 3/16" in diameter. The slag breaks off quickly and cleanly. If you're careful, it won't scar the metal. If any fragments remain, rub them gently with your thumb, under the running water. I've noticed that, if you rub a soft gold or silver bar when dry, it will scratch it. Under running water, it won't.

When weak sulfuric turns the slag cloudy, it will usually come right off under running water, with a little rubbing.

 

[WillGreen]

I wouldn't use sulfuric acid for this, after all, it's not terribly cheap (perhaps better in the USA, I do not know).
Many, even most, goldsmiths use sodium bisulphate or just 'Sparex' - Sparex 1 is for steel, I have never used it, Sparex 2 is for precious metals.
It works better if you heat it up a bit - warm water will do, don't let it boil, the fumes are not good for you.
Every jewellery tools supplier sells Sparex and it keeps a long time, but the problem is that Sparex is not pure sodium bisulphate - sometimes it creates some brown muck in a slow cooker. I do not know how this is called in America, but you can buy stuff to put in your swimming pool at your local Loewe's - it's cheap and it's pure sodium bisulphate and it solves all problems real quick. At least, that is what people have told me.

Best,

Will

 

[arcadron]

You can use caustic soda. Use couple of flakes while the silver is still very hot (near to red hot). The flakes will melt on the bar and wash away the remaining flux. USE SAFETY GOGGLES

 

[arisas]

silver is ok in hot diluted sulfuric acid. it will not dissolve. try auto battery electrolyte and a good pyrex container. be careful. WARNING : hot sulfuric acid will dissolve flesh and bone and cause blindness on contact with eyes. be very careful when heating sulfuric acid but it will remove the borax without harming the metal.

Geo what do you mean "try auto battery electrolyte" if you just put the borax silver chunk at a pyrex plate on top of heat the borax will still dissolve right leaving behind the silver only right....?

 

[jason_recliner]

Auto battery electrolyte, a.k.a. car battery acid, is approximately 30% sulfuric acid.

 

[arisas]

Auto battery electrolyte, a.k.a. car battery acid, is approximately 30% sulfuric acid.

Thanks for the info jason