Non-Chemical Steel mold stain?

[autumnwillow]

I'm getting this weird stain on top of my pure gold when pouring. I am not sure if my gold isn't just that pure or there is something else going on.
I pour all kinds of metals in this steel mold. Scrap jewelry, fine silver and pure gold before.

It is only about the last two refinements where I got this weird stain on the top of the gold bar.
The first one I thought that it was just not refined properly.
The second one was dubious cause I'm pretty sure I did it properly so I decided to digest the pure gold bar in a 50/50 nitric, and the stain on top was removed.

Now this is weird, if my gold was contaminated with something that nitric can digest, it should have alloyed with the gold when I melted it. Why was it only a layer on the top of the bar?

I'm thinking it could be my steel mold, but what's causing it?

 

[FrugalRefiner]

I'm getting this weird stain on top of my pure gold when pouring. I am not sure if my gold isn't just that pure or there is something else going on.
I pour all kinds of metals in this steel mold. Scrap jewelry, fine silver and pure gold before.

This would be the first source of contamination that comes to mind. The scrap jewelry, in particular, could have any number of metals in it that remain on your mold between pours, and the could contaminate your gold.

The second one was dubious cause I'm pretty sure I did it properly so I decided to digest the pure gold bar in a 50/50 nitric, and the stain on top was removed.

Now this is weird, if my gold was contaminated with something that nitric can digest, it should have alloyed with the gold when I melted it. Why was it only a layer on the top of the bar?

Why do you assume it was only a thin layer on top? Pickling is used when making jewelry to remove fire stain (oxidized base metals) that can appear after soldering. The base metals exist throughout the piece of jewelry. They become oxidized at the surface because their exposed to oxygen. The pickling bath removes the oxides from the surface, but the base metals still remain in the alloy. As you said, the nitric bath removed the stain from the top, but that doesn't necessarily mean there's not more contamination inside.

I'm thinking it could be my steel mold, but what's causing it?

How do you prepare your mold? Do you soot it or use a mold wash?

Dave

 

[autumnwillow]

1st quote: Come to think of it I only started pouring the jewelry scrap into molds recently. How does it contaminate the mold? What can I do to clean it up? I tried to brush it with a steel brush and soap. Same result.

2nd quote: You are right about this. Only the exposed surface gets oxidized. I'm also thinking that maybe my re-used ammonia wash has degraded. I'll try to make a button next time, see if it still gets oxidized on the exposed surfaces. If it doesn't then its definitely the mold.

3rd quote: I soot with acetylene.

How do you multi quote like that?

 

[Lou]

[ quote ] remove the spaces and put the text you want to quote inside the brackets [ /quote]


Ideally you would use a separate mold for fine gold and silver. That's my recommendation.

The acetylene soot makes a good barrier.

 

[goldsilverpro]

If you're using acetylene to soot the mold, I would say the melting dish is the more likely culprit. I usually kept 4 or more melting dishes, marked with a fire assay crayon - 1 for pure gold; 1 for pure silver; at least 1 for impure gold; and at least 1 for impure silver.

 

[FrugalRefiner]

How do you multi quote like that?

See Including Quotes In Your Posts.

Dave

 

[autumnwillow]

I would say the melting dish is the more likely culprit.

I did use a fresh clay crucible on the second refinement that I did because I ran out of silica crucibles, got the same results.

Only two probable causes here, my gold was either not really pure or something wrong with my mold.
I can figure out how to fix my refining process but how about the mold? How does one clean it up? I do not want to spend on another one. The silver I pour is just used for inquartation anyway. So it doesn't really have to be poured on a mould.

 

[autumnwillow]

I steel scrubbed my mold with soap and applied lots of soot.
I checked my ammonia whether it could still dissolve silver chloride, apparently it didn't so I made a fresh one.

And I ended up with a very nice shiny metallic gold bar! =D