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sinohe6

New member
Joined
Feb 9, 2013
Messages
2
Hello my friends,
I'm new member here and this is my first post for my big problem. I'm a goldsmith and recently I start use enameling in my artworks, for making enamel on gold or silver we must fire the enamel in furnace to 800'C 1500'F, when i apply enamel on pure silver or pure gold everything is fine but the problem starts when that I use gold 750 or 18K , because the gold alloy contain silver, copper, zinc and many other metals that I don't know and when fired piece in the furnace the gold color turn to black for oxidation.

Is there any chemical process or any process that remove undesirable alloy metals from all of my casting 750 gold surface and achieve to thin layer of Pure gold on all of my piece to avoid oxidation when fired to 800'C ?

Please Help me my friends
 
I would recommend the website http://www.ganoksin.com/ It contains a lot of information about gold alloys, enameling, plating... and so on. They also sell pdf:s of scanned old books for a rather decent price.

Maybe your solution would lie in fire gilding, repeatably annealing - pickling - polishing to raise the gold contents on the surface.

Disclaimer, I haven't tested any of these techniques myself, only read about them.

Göran
 
I think the best solution would be to avoid the oxidation, or "fire", in the first place by bright annealing the piece. You want to exclude oxygen from your furnace during the enameling by replacing it with an inert gas.

Dave
 
I'm no expert by far in this area, but it being 18k gold i would think you could use nitric acid to leach the top layer of base metals from the surface without affecting the color. Don't know.
 
Thank you my friends for your guidance,

I read all the ganoksin forum article about metal cleaning and until now do not find any helpful process,

after firing the metal in kiln and happening the darkness in the piece i could remove the oxidation with acid process and polishing method but the problem is the gold surface under the enamel goes to dark and because the enamel melted on the gold we don't have access for cleaning this part of the metal.

I need a process that remove all alloy 750 gold to achieve a thin pure gold in all surface of the piece before enameling that when i heat the piece there is no change on metal color because pure gold have no color change with high temperature.
 
I can only see three alternatives that would stop oxidation.
1. Remove oxygen - fire the piece in a controlled atmosphere without oxygen.
2. Remove the copper - Repeating annealing - pickling - polishing, until you have a thick enough zone of pure gold.
3. Cover the surface - plate the surface thick enough with pure gold

One problem is that metal atoms moves (it's called diffusion) when it is so hot so the surface have to be quite thick. (compare thickness of gold plate on CPU lids for protection against oxidation while soldering, and that's not even close to 800 C)

A combination of point 1 and 3 would be to put down a thin layer of clear enamel first, using a reducing flame. That way you would protect the surface for the next firing with your final design.

Göran
 
I would take a small sample piece of the gold you're using an put it in a beaker with 1 part water and 1part nitric acid. Heat it just under a simmer for about 1 hour. Then wash the piece with hot water or steam a couple of times to clean the surface good. Then try firing that and see if it works.
 
What i do at work is very similar to the problem you are having. The problem sounds like the environment you are firing the glaze in. You somehow have to fire the piece in the absense of oxegyn... you can do this by using a high vaccum (cryogenic pumps) or using an inert gas like argon to displace the oxygen. Another tactic you might be able to use would be to find something to 'seal' to gold with that will stand up to the heat of firing, and give you the transparancy you need. At work, when we do silver coatings, we have to put an overcost of SiO2 to prevent the silver from oxidizing.

I do not think acid baths are going to help you much as I am assuming you want the metal to be really shinny... acid baths would dull the finish of you gold. if that is not a problem, then and acid bath before firing might work for you.
 
Can he have the karat ring plated with pure gold? Much like plating Silver with Rhodium. That may keep the surface from oxidizing during the firing.
 
The Incas used a very weak gold and Copper alloy which was part of the reason the conquistadors thought they had so much Au in the first place.
They would cast or work the alloy then fire the finished article,This would oxidize the copper which would make the item look black.
And not having any strong acids they would use a solution of Alum to take away the copper oxide and leave a pure Gold finish.
It looked very good and If it ever showed a sign of ware they would just repeat the process.
I thought it was a lovely low tech solution when I read the article.
Please note that was a paper I read several years ago and I have never tried it my self.
 
That was what I was describing in my first post, although I got the name wrong. It's called depletion gilding, not fire gilding. http://en.wikipedia.org/wiki/Depletion_gilding

and you should burnish the surface to get a better finish and a more compact surface.

... it's been a while since I studied jewellery making so my vocabulary is a bit off.

Göran
 

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