gold "disappears" while melting

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A

alvaschein

Active member
Joined
Feb 13, 2013
Messages
37
Location
Switzerland
Hi all

I even come of melting down about 1.7 g of gold in a crucible when something very strange happened.
First I put the gold to the primed crucible and then added a bit of flux (Auropurifax: mixture of K2CO3, KNO3 and Na2B4O7·10H2O). The melting was OK until suddenly it happens: I heard a sound like from a firecracker and all the gold has gone. On the bottom of the picture you can see the shadow of the gold button. I couldn't find any trace of gold on my working space. The whole work of recovering the gold for the birds!
IMG_1971.JPG
I kept with my mouth open - what was this?
What happened? Did I heat it too much? Or was it the flux I used the first time? I really don't know. It's the first time I see it happen.
Has anyone an idea? Thanks very much.

Roy
 
Roy,

I could be way wrong on this, but it is possible you experienced a steam explosion. You mention that you started with a primed crucible. I am assuming you mean one which had been heated and coated with molten borax. Then you added your gold powder and some premixed flux containing potassium carbonate, potassium nitrate and borax. Notice the formula for the borax - Na2B4O7·10H2O. The 10H2O at the end of the formula is water molecules bound up in the matrix of the borax. When heated, this water turns to steam. This is what causes borax to puff up when it is first heated before it flows into a liquid.

It is possible that there was some of this borax at the bottom of the crucible, somewhat insulated from the heat as you started your melt. At some point, it is possible it reached the critical temperature that allowed the water to flash into steam, causing a steam explosion under your gold. A steam explosion could easily propel your gold out of your crucible.

It looks like there are some small prills of metal in your dish, notably some on the left side of the dish near the top. These appear to have tails on them as though they were thrown from the center of the dish with some force.

Another possibility is if you had not preheated your dish before adding your gold and flux. If the dish had not been preheated before use, it is possible there was some moisture trapped within it. Heating could again have caused a steam explosion under your melt.

That's just a guess. Hopefully others will offer their opinions as well.

Dave
 
i was going to mention what appears to be flaking in the melting dish. a sure sign of moisture explosions.
 
The crucible was hot enough before I put the gold in it. My guess is the new type of flux. Working with borax alone I never had effects like this.
I think I'll make some tests melting copper with this flux to get used to it.

Roy
 
Don't discount the possibility of a flashback in the torch. What were you using for a heat source?

A comment about the quality of the gold you attempted to melt (please bear in mind, I always strived for the cleanest possible gold, as mine was used by my customers). I wonder what your objective was. The discoloration of the flux indicates that your gold was quite dirty---as if you had recovered foils and just melted without even one refining process.

If you hope to have clean gold, do not rely on flux to do the purification. In spite of everything you may read in that regard, it will raise the quality only marginally-----and lead to great frustration, assuming you care about seeing your gold a pretty yellow color, without signs of oxides.

It should be noted that melting gold requires no flux. If you properly prepare a melting dish (that's what you used. It is not a crucible), the trace of residual borax should be all that is required. If you need more, the gold is not pure and should be properly processed. That, of course, depends on your objective. If it's just to melt so the gold can be sold to a refiner, that may not be true.

Harold
 
I think Harold was right on about it being a flashback from the torch. There was the firecracker sound. Had it been moisture I think it would have happened much earlier in the melt.
 
With over a hundred melts of refined gold and/or silver I have yet to use or need any flux addition to get a perfect melt.
As long as your melting dish is properly seasoned their is more than enough residual flux in the dish to allow for a complete melting and pour.

As for the pop, burning acetylene (carbon) might be having a negative reaction with the nitrates, similar to adding diesel to certain nitrates. I don't know the specifics of this combination to rule it out, but worth checking on.
Maybe one of our chemists would know if this is possible or not.

edit to add;
Most of your gold is still in the dish as hundreds of tiny balls stuck in the dish, clearly visible when you enlarge the picture. Now the fun part will be getting them all melted and flowing back into one ball. Been their, done that :shock:
 
What type of torch gas did you use? Propane, Mapp, acetylene?
What type of tip? Cutting tip, rose bud, ?
Did you hold your torch real close to concentrate the heat? Say within an inch or two from the material in the dish.
When you were heating it did you hold the torch sort of in a vertical position and concentrate the heat in a downward fashion from above, or did you hold the torch down low to the dish and heat it from a horizontal sideways position?
 
Hello everyone
I believe palladium is speaking of an
oxygen deficient area above and inside
your melting dish

Try welding a few pieces of metal
you'll notice when the torche is held straight up
and down and gets too close to the metal it will pop
and usually hiss untill turned off or relit if possable

And yes definitely as loud as a firecracker

As for a moister or steam explotion yes as said it would
have happened much earlier before melt ,and you
Would not see any gold ,as the bottom of your dish would
be gone
Hope this helps steyr223 rob
 

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