What is the black graphite looking slag that is formed when sterling silver is melted with borax? Is there a way to remove it from the surface of the sterling silver? It appears to be indestructible!

The melting is related to jewelry casting. All melting was done in an electric Ney muffle furnace. Here is a melting dish with some half-melted buttons of sterling silver.

Half melted buttons in a melting dish

I can't remember why I didn't completely melt this. I assume the green is salts of copper from the sterling silver. Notice how the black slag erodes the walls of the melting dish.

Here is a closeup of the erosion.

Closeup of erosion

This erosion property ate through the bottom of my furnace! UGGG! It'll be $600 to replace the muffle. I heated to 1,900 Fahrenheit for about one hour and this is what I saw when I opened the furnace to get the crucible with the molten sterling silver, but with glowing white-orange light.

How I found the crucible

Here I have the crucible laying on its side next to the hole.

Crucible next to the hole

Here is a close up of the hole and in one of the images I'm lighting it up with a flashlight. I'd say it's about 1.5 inches in diameter and 1.5 inches deep. There are 2.25 Ozt of sterling silver down in the hole somewhere because the eroding black slag ate holes through the wall of my crucible.

Hole with shop lights Hole with flashlight

A closeup of the erosion in front of the main hole. These two erosion spots were from the previous melt cycle with the same crucible. I saw that the black slag was penetrating the crucible but I figure it was just minute capillary action and it'd be ok. Notice that the black slag seems to be working like a catalytic reaction because the amount of borax in the crucible is only about the volume of one grain of white rice. These two depressions were not depressions at the end of the previous melt cycle they were just flat black areas of the same size and shape.

The two depressions near the hole with flashlight

Here is a view on the outside of the crucible. I assume one of those holes is where the silver leaked out. Erosion on the outside of the crucible

A view inside the crucible. In person, the black slag has a metallic sheen to it. It is very similar to the dark grey metallic sheen of graphite. Inside crucible

What is this black slag?

  • $\begingroup$ It is unnecessary and distracting to use edit statements. The history of the post is visible to everyone who is interested. $\endgroup$ Commented Nov 27, 2019 at 13:07
  • $\begingroup$ I have seen copper alloy penetrate a crucible like that when held at melting temperature for over an hour. $\endgroup$ Commented Nov 27, 2019 at 16:32
  • $\begingroup$ Hi Martin, I rolled back your edits because you removed my joke at the end. :) Also, my edit statements are necessary because someone who had seen my original post would not know that I had made changes. They would only know if they specifically looked to see if I had made changes. James $\endgroup$ Commented Nov 28, 2019 at 20:17
  • $\begingroup$ blacksmith37, thanks for this thought. I'll keep this in mind. I still never found out what the black slag is. What I did was I made my own crucible out of stainless steel and inner refractory. When the black slag eats through the inner refactory it is stopped by the stainless steel liner. Then, when things cool down I fill in the erosion holes with more refractory. The stainless steel liner would work by itself self but the molten sterling silver cools down too quickly without the thermal insulating property of the refractory, which is about 15 mm thick. $\endgroup$ Commented Nov 28, 2019 at 20:34
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    $\begingroup$ Please take the tour to learn how this site works. We do care a little bit less about the few people who might have seen your post before, but more about the many people who might come to see it in the future. Your question should stand by itself anytime, without the history of edits being a requirement to know. This is a science site, and while some of us appreciate a joke, the questions is not a good place for it. For some casual fun visit Chemistry Chat. To notify people about your comment, put @ in front of their username. $\endgroup$ Commented Nov 30, 2019 at 14:45

5 Answers 5


"Indestructible" by what means exactly? How did you try to "destroy" it? Provided that sterling alloy is an alloy of silver and copper, the first guess is that your slag is primarily a copper (II) oxide. Depending on the actual composition of the alloy, it may contain other 3d-metal oxides. The formation of borates (not of silver, but of less noble metals) cannot be excluded, for borax is frequently used as a constituent of the high-temperature fluxes for spectroscopic and calorimetric applications. To what extent the borates are formed depends on the temperature and duration of your melting experiment.

You should be able to dissolve your slug in a boiling mineral acid of your choice. Personally, I'd try the $\ce{HCl}$ solution, or $\ce{HNO3}$ if you do not care that silver dissolves as well.

The following may be irrelevant to the question asked, but, based on my experience and some textbooks, people first try to purify silver and then melt the pure metal, not the other way around. The methods of purification are aplenty, but most of them are based on dissolving the silver-containing alloy in $\ce{HNO3}$. Then $\ce{AgNO3}$ can be transformed into $\ce{AgCl}$, which, in turn, can be reduced by metallic zinc in acidic environment or formaldehyde - in basic.

  • $\begingroup$ See images and new information concerning how the black slag has erosive properties to refractory materials. Indestructible in that heating it to a very high degree with a propane-oxygen torch causes it just glow with a white light similar to how carbon behaves. Also, soaking for hours in room temperature 10% solution hydrochloric acid or sodium bisulfate seems to only turn it to a powder conglomeration that needs to be brushed vigorously to remove it from the surface of the sterling silver. $\endgroup$ Commented Jan 11, 2019 at 5:35
  • $\begingroup$ I just remembered something that could be a clue. In addition to the borax, I used this product, riogrande.com/product/… Just a minute amount, maybe one or two sprays. I now know that I didn't need to use this but at the time I thought it couldn't hurt. Maybe it did hurt A LOT! HAHA! :D $\endgroup$ Commented Jan 23, 2019 at 22:09
  • $\begingroup$ The reference does not say anything about the composition of this product. But my experience tells me that it could be NH4Cl, or ammonium chloride. It protects metallic surfaces from oxidation during soldering. $\endgroup$
    – Maurice
    Commented Nov 28, 2019 at 20:41

Voffch mentioned that borax is a flux. Quite true; borax beads are used for determination of various metals, and copper or silver would give characteristic colors, like green and whatever silver gives. Fluxes are also solvents for metal oxides, like the refractory material of the crucible. While molten alloys should be containable by refractory crucibles (I'm surprised by blacksmith37's comment!), if the crucible is not properly chosen, I suppose it just might not do its job of containing the molten metal.

The borax was probably added to provide a cover for the molten alloy, but was too powerful a solvent for the crucible. There are many materials for crucibles, such as clay, alumina, graphite, and silicon carbide. There are many articles on the internet which address the specific issue of melting silver and sterling silver. Perhaps a better crucible would allow the use of borax (recommended for silver) or a different flux could be chosen.

The black slag has been called fire-burnt copper, an oxide which is very hard product when cooled; probably also combined with the borax.


I have experienced a situation that resembled yours when I got in a hurry and melted my silver nitrate right after filtering. So the silver was still saturated with nitric acid. My thought was it would just burn off before the Silver started to melt. Well the result looked like what you have shared.


When I was working with silver a back powder was often formed.

I have found out, that this black powder was silver oxide. Don't know if it is silver oxide in your case, but it likely is.


The black slag is copper oxide and silver oxide (both are black compounds). If you're trying to refine the sterling silver, i.e. remove the copper and other base metals, then you're approaching this incorrectly. You could theoretically remove the base metals via pyro, but you'll lose a ton of silver to the slag. I'd recommend a hydrometallurgical approach first.

Additionally, the crucible you're using is a fire assay crucible. It's made such that oxides will seep through it. Given the large amount of slag, it's no surprise it broke.


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