Nasty looking pours! Why?

[nickmarch]

First timer here!

I did not refine but did start with solid 14k gold.

When I pour into the mold the top (the part that does not touch the mold/the part you can see while the gold is still in the mold) looks like copper, is bumpy and very bad looking. The gold in the mold(the part you can't see when the gold is still in the mold) is very nice! What am I doing wrong or how can I fix the problem?

I tried stirring. Not stirring. Borax, no borax, Anhydrous Borax, Anhydrous Borax and Ammonium Chloride mixed together. Nothing works for me!

If I melt one ounce and pour it happens. If I melt 5 ounces and pour five seperate one ounce bars it happens. If I melt five ounces and make one five ounce bar it happens.

No matter how many bars I pour the thickness of the bad part is pretty much the same. A five ounce bar will have pretty much the same thickness of bad looking copper like stuff as a one ounce bar. It's making me crazy!

I tested the copper looking stuff and it tests at 14k???

Any ideas? Suggestions?

Any help is appreciated!

 

[Anonymous]

I have not done this but I think your melt is not hot enough because copper has a higher melting temp
and your cooling is too slow.
Now, I know they are alloyed and have what should be a common melting point, but, I can not help but think
the copper is solidifying slightly giving up heat to keep the gold liquid causing a portion of copper to
start to crystalize out.

If I am mistaken I am sorry and I am sure someone with karat casting will correct me and your proceedure.
Also, 14k could have stuff other than copper and gold messing things up. I think that is why jewellers perfer to
alloy on site for the pour.

Jim

 

[g_axelsson]

When pouring or casting carat gold the base metals will oxidize. To get the yellow nice golden color you have to treat the gold with an acid to remove the copper oxide. It's called to pickle the gold.

This is a ring I made a couple of years ago. This picture was taken just minutes after I cast it, only removed from the form.
View attachment 1

This picture is from later that evening after pickling and polishing. The polished parts are shiny while the back side is bright yellow. That is the color it had after pickling.

The ring is made of 18k gold and weighs 10g. Remelted scrap gold, not refined, that will be a later project.

If you want to pour a shiny bright golden bar you need to have it refined or polish it afterwards.

/Göran

 

[Anonymous]

that looks like a way to hide my gold alloy with some copper and pour it dirty, looks like nothing. I like that better than leaving it
as a brown powder jar.

Jim

 

[nickmarch]

Goran,

Before I melted the gold I chopped it up and boiled it in nitric acid for a few hours. The acid turned green. After pouring and seeing the nasty cooper look I boiled the bars for hours again in nitric acid. Nothing happened. The acid turned a slight green.

My bars do not look like what you showed in your pictures. My camera batteries are dead. I'm going to get some batteries tomorrow and post a picture.

Thanks for the advice!

 

[g_axelsson]

Pictures would be great, it is really hard to get the finer details from a short description.

To boil the gold in acid before the melt isn't helping a lot. The only thing you achieve is to remove some of the base metal on the surface, soon the gold will shield the base metals from the acid and stop any further reactions. That is why you have to alloy down the gold before inquartating. As for pickling acid, I don't know how effective nitric acid is for copper oxides. The classical acid used by goldsmiths is sulphuric acid as it attacks copper oxide when boiling and doesn't give off so much fumes.

/Göran

 

[nickmarch]

I'm not sure what order the pictures will be in. The one shows the shiny top with the copper looking jaggedy stuff on the side. The shiny side actually looks a lot better than the pic.

The other shows the bottom with the copper looking, jaggedy look. The copper look is not deep, it just coats the surface.

The copper looking stuff tests as 14k gold and did not disolve in nitric acid.

I did not mention that I also boiled the gold in hydrochloric acid before melting and after pouring. It didn't do anything after the pour but did turn green when it was first boiled.

Any ideas?

Thanks again!

 

[Harold_V]

Two things come to mind.

You are over-heating the gold, oxidizing the base metal(s) beyond an acceptable level.

You are using an oxidizing flame instead of a neutral or reducing flame.

Once you have oxidized the base metal(s), there is no recovery aside from refining, or you can dilute the bad gold with lots of fresh gold-----but that's a little like washing your car with muddy water.

If you expect to pour gold of decent quality, it must be melted only as required, using only as much heat as is required. A neutral or reducing flame is VERY helpful.

You should expect to see what is commonly termed "fire coat" on the surface of cast alloy gold items. It should be readily removed by dilute sulfuric acid, or it can be removed by a process known as bombing, which uses cyanide and H2O2 (hydrogen peroxide). It's very fast.

Harold

 

[nickvc]

I will assume you are melting in a crucible and then pouring into an ingot mould, when you have the gold molten add a small amount of flux and look for the metal to clear on the surface ie looks shiny give it a stir heat the mould and coat it in wd40 recheck your melt to see its still clear then pour into your mould in one smooth movment including the flux, alow it to cool and dip the bar in cold water carefully this should loosen and probably remove the flux. If you have copper oxides in the melt you will not get a really good finish and as harold V said the only way to make it look better is to dilute it with more clean metal.

 

[LeftyTheBandit]

Would it not be simpler just to inquart and make 24k?

It seems it would be quicker.

Do you have either silver or copper to add to it to bring it to 25% gold, If you do melt it in and treat it with nitric. Seems a lot simpler.

 

[Montecristo]

Two things come to mind.

You are over-heating the gold, oxidizing the base metal(s) beyond an acceptable level.

You are using an oxidizing flame instead of a neutral or reducing flame.

Once you have oxidized the base metal(s), there is no recovery aside from refining, or you can dilute the bad gold with lots of fresh gold-----but that's a little like washing your car with muddy water.

If you expect to pour gold of decent quality, it must be melted only as required, using only as much heat as is required. A neutral or reducing flame is VERY helpful.

You should expect to see what is commonly termed "fire coat" on the surface of cast alloy gold items. It should be readily removed by dilute sulfuric acid, or it can be removed by a process known as bombing, which uses cyanide and H2O2 (hydrogen peroxide). It's very fast.

Harold

So are saying using a flame with less oxygen and taking a little longer to melt the scrap, will produce a better, more homogenous pour?

 

[nickmarch]

Someone told me too and I did melt ,make shot, smash the shot flat, soak it in hot hydrochloric, rinse, soak in hot nitric, rince, melt with flux and pour. It worked!

 

[Harold_V]

So are saying using a flame with less oxygen and taking a little longer to melt the scrap, will produce a better, more homogenous pour?

That's correct, although once you've oxidized the base metal there's no turning back.
Be patient when you melt, and use a slightly reducing flame. No worse than neutral, for sure.

Depending on the fuel type, it may be difficult to know the nature of the flame, but you can make that determination by heating a gold alloy and making observations. . You can oxidize or reduce the surface simply by adjusting the fuel. It might pay you to do just that, taking note what the flame looks like as it changes from oxidizing to reducing.

The other thing to remember is that you can melt alloyed gold only so many times without causing damage, which is not true if you melt in an inert atmosphere. Rarely does the home refiner have that luxury.

Harold

 

[goldenchild]

If you're actually willing to refine the gold I would say to get it molten as 24k. Here you wont have to worry about firescale. Then right when you are ready to pour into the ingot mold add the base metal(s) you choose. The molten gold should suck up the base metal(s) extremly fast and they will have very little exposure time to get oxidized. Not only that but if adding say silver, it will make it much easier to pour as adding silver will keep the gold from hardening so quickly. If you've ever poured 24k you know that the pour has to be perfect. All in one quick shot. Another thing to remember that might not of been mentioned is to make sure that the borax you are melting in and your ingot mold are clean! If unclean, it will drag all kinds of garbage into your gold. I think you would get a much nicer ingot doing it this way.

 

[nickvc]

A word of warning on putting cold metals into molten metal be very careful ,if you have reasonable amounts molten do not put quantities of cold metal in the results can be VERY DANGEROUS,i think its thermal shock, the whole lot can explode......i know i did it once and was lucky enough to realise what i had done and dived backwards away from the shower of molten metal.....You dont do it twice!