What is this "Crust", "Flux", "Slag" on my Ingots!!??!!

[rybak97]

I need some help from the forum on identification and removal of the "crust" that formed on the top of my ingots when I poured them.

I was using a graphite crucible, heated to 1070C, using very minimal borax (crystals were very pure) and using a graphite mold pre-heated with a MAPP torch. The ingots were poured under flame to remove and oxygen that could oxidize the surface. When I poured the ingots I could see something floating on top of the molten surface and when it solidified the “crust” shown in the photos remained.

It looks like flux to me. It has a cloudy, glassy appearance and a slightly yellowish tint when viewed at the right angle.

These pictures were taken this morning AFTER soaking the ingots in a 10% sulfuric solution over night, and the “crust” is still there. I haven’t yet tried to use the 30% solution over heat as I've read on the forum yet, but will probably try that tonight.

Any ideas?? Thanks All.

 

[Anonymous]

Well, according to you, you used some borax/flux on a graphite mold. As far as I know, you don't need any borax/flux when you're pouring into a graphite mold.

To remove the flux, use diluted HCL and heat it for about 30+ minutes. That will remove the flux, but you have to rub it off. Basically, you want to use about 10 - 15 ml of hydrochloric acid with 200 ml of water... put on heat and let it loosen up.

Kevin

 

[rybak97]

Well, according to you, you used some borax/flux on a graphite mold. As far as I know, you don't need any borax/flux when you're pouring into a graphite mold.

To remove the flux, use diluted HCL and heat it for about 30+ minutes. That will remove the flux, but you have to rub it off. Basically, you want to use about 10 - 15 ml of hydrochloric acid with 200 ml of water... put on heat and let it loosen up.

Kevin

I was using a small amount of Borax in the graphite crucible, not on the mold. I just sooted the mold with a torch.

I don’t have HCL right now, is there a non-refining source I where I can pick some up today? I got the 30% Sulfuric at Discount Auto as battery acid and I was planning on using that. Will it work instead of HCL?

 

[mikeinkaty]

I use 35% Muriatic Acid (HCL) available at Home Depot, most hardware stores, Lowe's, and any building center. It's used for cleaning concrete.

I use a torch and silica crucibles for melting. For the crystals I use a clean crucible that has only been lightly glazed. If the crucible is clean and melting 99.99 crystals I see no need for flux. I have gotten bars like in your pic but to the best of my memory I got them only when using flux or when there was too much residual flux in the crucible. Judging by those two partial bars it looks like they or the mold were not hot enough when poured. I have found that using my torch to smoke the mold gives the bars a little more time before solidifying. The soot, as well as providing a small air gap so air won't get trapped under the silver, seems to also provide a little insulation. Trapped air generally comes to the surface causing imperfections. Mike

 

[rybak97]

I use 35% Muriatic Acid (HCL) available at Home Depot, most hardware stores, Lowe's, and any building center. It's used for cleaning concrete.

I use a torch and silica crucibles for melting. For the crystals I use a clean crucible that has only been lightly glazed. If the crucible is clean and melting 99.99 crystals I see no need for flux. I have gotten bars like in your pic but to the best of my memory I got them only when using flux or when there was too much residual flux in the crucible. Judging by those two partial bars it looks like they or the mold were not hot enough when poured. I have found that using my torch to smoke the mold gives the bars a little more time before solidifying. The soot, as well as providing a small air gap so air won't get trapped under the silver, seems to also provide a little insulation. Trapped air generally comes to the surface causing imperfections. Mike

On this thread they guy used sulfuric to clean the bars, thats why I was trying it:

http://www.goldrefiningforum.com/phpBB3/viewtopic.php?f=50&t=17582&p=178518&hilit=crystal+pattern#p178518

I can go to Lowes and get the muriatic acid pretty easy. Did you heat it? How long did you soak the ingots?

 

[Anonymous]

I was using a small amount of Borax in the graphite crucible, not on the mold. I just sooted the mold with a torch.

I don’t have HCL right now, is there a non-refining source I where I can pick some up today? I got the 30% Sulfuric at Discount Auto as battery acid and I was planning on using that. Will it work instead of HCL?

Using Borax on a melting dish, (which shouldn't be used at all unless it's an electric or furnace melting) isn't needed at all. As a matter of fact, you don't need flux at all on graphite unless you're positive you're got contaminants in the mix.

On another note, I have never known of a good melt/pour when the melt started with a graphite melting dish (unless it was a furnace the dish was in). Start using a clay dish or silica dish for melting.

Kevin.

 

[rybak97]

I was using a small amount of Borax in the graphite crucible, not on the mold. I just sooted the mold with a torch.

I don’t have HCL right now, is there a non-refining source I where I can pick some up today? I got the 30% Sulfuric at Discount Auto as battery acid and I was planning on using that. Will it work instead of HCL?

Using Borax on a melting dish, (which shouldn't be used at all unless it's an electric or furnace melting) isn't needed at all. As a matter of fact, you don't need flux at all on graphite unless you're positive you're got contaminants in the mix.

On another note, I have never known of a good melt/pour when the melt started with a graphite melting dish (unless it was a furnace the dish was in). Start using a clay dish or silica dish for melting.

Kevin.

I’m using a Hardin melting furnace with a graphite crucible, not a dish. I don’t think the furnace is the problem. I should not have used ANY Borax on a pour of such pure silver and certainly not to glaze the graphite, which is not necessary. Regardless, this is what happened and I’m trying to find methods of removing the borax from the ingots. I’ve tried 10% Sulfuric over night and the borax wasn’t even soft in morning. I’ve tried 31% sulfuric over low heat for about 30mins and the flux seem to soften, but not enough for me to rub it off without scratching the ingot.

I went and got the 35% muriatic from Home Depot and will try that next. Any other ideas would be appreciated.

 

[philddreamer]

A boil in 20 to 30% sulphuric will take the borax glass off. I boil the borax glass when I'm recovering the PM's and stones and also when I'm cleaning the melting dishes. It works everytime!
Take care!
Phil

 

[mikeinkaty]

I use 100ml of liquid-fire (98% Sulfiric acid) in 900 ml of distilled water in a crock pot on the low heat setting and it gets the borax off of my silver in 6 hours. I do use flux when pouring anode bars. The temp gets close to boiling. Haven't measured it though. The Crock Pot brand is good because they use a pot inside that is impervious to sulfuric acid. Another Goodwill purchase for $6.

Oh yeah. Occasionally I get pink stuff on top of my anode bars that sulfuric will not remove. I hit it very lightly with a wire wheel on my grinder and it goes away. My scale will go to a tenth of a gram and when using the wire wheel to do this the weight does not change that my scale will detect. Mike

 

[samuel-a]

Brian

What you are seing is not flux.
What you are seing is a different surface pattern of the silver itself. Let me explain:
When you soot your mold, you basically apply a layer of loose carbon. Now... when you pour, some of that carbon finds its way to the surface of the liquide, much like when you pour water on dry dirt, if you look at the surface, you will always find some floaters.
With this in mind, lets remember that you supply a reducing environment via tourch ontop of the melt while pouring. The carbon that floats around on the surface does not get the chance to burn away before solidification of the metal. The bar solidify with this unburnt carbon, thus, leaving the rough surface... When you remove the tourch, it is burnt away... leaving you pondering and wondering.

There are few things you can do to prevent that:
Since you are using a graphite mold, apply minimal sooting. Iron mold would need thicker layer.
Use a slightly air enriched flame over the mold while pouring. If using iron mold with thick soot layer, wait 1-3 seconds until surface carbon burn away and then apply slightly air enriched flame.

In this case (pure metal), the flame is to prevent Oxygen absorption and minimize collapse (pipe).
The slower the solidification across the bar, the less contraction.

Problem with small bars, that they solidify very fast and you usually could not hit these two birds... with smaller bars, you can pass the protective flame bed.

 

[rybak97]

Brian

What you are seing is not flux.
What you are seing is a different surface pattern of the silver itself. Let me explain:
When you soot your mold, you basically apply a layer of loose carbon. Now... when you pour, some of that carbon finds its way to the surface of the liquide, much like when you pour water on dry dirt, if you look at the surface, you will always find some floaters.
With this in mind, lets remember that you supply a reducing environment via tourch ontop of the melt while pouring. The carbon that floats around on the surface does not get the chance to burn away before solidification of the metal. The bar solidify with this unburnt carbon, thus, leaving the rough surface... When you remove the tourch, it is burnt away... leaving you pondering and wondering.

There are few things you can do to prevent that:
Since you are using a graphite mold, apply minimal sooting. Iron mold would need thicker layer.
Use a slightly air enriched flame over the mold while pouring. If using iron mold with thick soot layer, wait 1-3 seconds until surface carbon burn away and then apply slightly air enriched flame.

In this case (pure metal), the flame is to prevent Oxygen absorption and minimize collapse (pipe).
The slower the solidification across the bar, the less contraction.

Problem with small bars, that they solidify very fast and you usually could not hit these two birds... with smaller bars, you can pass the protective flame bed.

Thanks for the reply, your advice is very appreciated. Since these pictures were taken Ive tried another batch with the same results.

This time I didnt soot the mold at all and they all dropped out very easily. I still got the same surface pattern as on the first run, even without the soot. Perhaps this carbon is coming from the graphite crucible?? Ive used both propane and MAPP torches on the surface while pouring, with no difference is the surface appearance.

I dont know what else to try, except for using a ceramic crucible? Im still trying to find a drop in ceramic replacement for my graphite crucible, dont know why someone doesnt make them!!

 

[samuel-a]

Try to pour without any flame over the mold.
And remember to heat the mold currectly.

btw, see if that suits you:
http://www.amazon.com/gp/product/B009CYBIDI/ref=ox_sc_sfl_title_2?ie=UTF8&psc=1&smid=A30N7QCN0C3D7A

 

[rybak97]

Try to pour without any flame over the mold.
And remember to heat the mold currectly.

btw, see if that suits you:
http://www.amazon.com/gp/product/B009CYBIDI/ref=ox_sc_sfl_title_2?ie=UTF8&psc=1&smid=A30N7QCN0C3D7A

If I don’t use flame over the top of the mold wont it just oxidize and give me a rough surface? Ill give it a go regardless, Ill try anything at this point. I use my MAP torch to heat the mold to at least 50% of the pour temp so that shouldn’t be a problem.

The opening to my furnace is only 52mm, so close!!

 

[kadriver]

I use 10ml of 98% sulfuric acid in 200ml distilled water with low heat to remove flux from the silver bars. This equals a 5% sulfuric acid solution - thats all you need in my opinion. I'm not sure, but I think greater acid concentrations will begin to dissolve the silver and cause the bars to loose their brilliant luster.

I only leave the bar in long enough to soften and remove any flux/borax glass - usually only 5 minutes or so depending on the size and thickness of the adhered flux.

After 5 minutes, I remove the bar with tongs and rinse with warm tap water. I also give a slight rub with my thumb, being careful not to scratch the brilliant surface of the clean metal.

If the flux is still there, then I put it back in the 5% dilute sulfuric acid bath and let it sit for a few more minutes.

Sulfuric acid will dissolve silver. I once left a bar in too long (about an hour) and it looked dull and milky instead of bright and shiney.

I believe that the acid dissolved some of the silver off the surface of the bar and exposed the crystal pattern.

It looked so bad (to me anyway) that I remelted it and then it turned out fine.

Here is a picture of one of the bars after using the above process to remove the flux.

kadriver

 

[lazersteve]

A side note to the 5% sulfuric acid is that it keeps for a very long time. I have beaker dedicated to flux removal that stays labeled and covered in my workspace. When I need to remove stray flux from bars, I simply place them in the beaker and let them sit for a short while. Heat will speed the process, but is not necessary. I only change the 5% solution if it gets too dirty.

Steve