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The question stems from my search on staining metals; in particular I commonly encountered "Can you dye a molten metal" questions, with the usual, simple answer of "any pigment will decompose or burn in contact with molten metal."

Well, I got some Wood's metal, the bismuth/tin/lead/cadmium alloy with melting temperature of 80 °C. Great most of pigments can easily survive 80 °C. But I have zero hope they'd mix and give a nice uniform tint instead of forming granules on the surface. Then the train of thought went: Well, maybe a solvent that will mix, providing an interface? Wait... What can mix with molten metals at all?

And I drew blank. Other than other metals, forming alloys - of course most materials will decompose, burn up, evaporate on contact with most metals in liquid state. But there's mercury, there's quite a few metals and alloys with melting temperature low enough most substances will survive the contact just fine, and I have no clue if anything will mix. I know of iron-carbon alloy, aka steel, so "nothing" is not the answer. But as I know polar dissolves in polar, non-polar dissolves in non-polar, detergents facilitate mixing of the two due to their dual nature... I completely don't know how molten metals fit into the picture.

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    $\begingroup$ Other elements also dissolve. Not only carbon but Si, B and others. $\endgroup$
    – Mithoron
    Commented Apr 18 at 15:19
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    $\begingroup$ Somewhere on some binary phase diagram you will find basically any element being soluble with a 'metal' in the liquid phase (well, maybe not noble gases). Complex molecules (such as a dye) are likely to get ripped apart. Then there is the question of what a dye would do if incorporated in a metal phase (or even grain boundary) - it quite likely will no longer function as a dye when embedded in a metal. $\endgroup$
    – Jon Custer
    Commented Apr 18 at 16:03
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    $\begingroup$ I would like to see an answer that works at room temperature rather than very high temperatures. $\endgroup$ Commented Apr 20 at 2:53
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    $\begingroup$ Melted silver is known to dissolve aerial oxygen that gets expelled during solidification, causing melted silver splashes and solid silver surface irregularities. $\endgroup$
    – Poutnik
    Commented Apr 22 at 7:39

3 Answers 3

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There are some examples of salts soluble in liquid metals. Above $\pu{1051 K}$, liquid bismuth is completely miscible with $\ce{BiCl3}$.
More about these experiments here: https://www.osti.gov/biblio/4212609

Others are usually either less soluble or less metallic after mixing.

But as I know polar dissolves in polar, non-polar dissolves in non-polar, detergents facilitate mixing of the two due to their dual nature... I completely don't know how molten metals fit into the picture.

Since there are no polar molecules in metals (delocalized electrons will cancel out any dipoles), 'detergent' analogy is: instead of maximizing the difference in polarity of the functional groups on opposite ends of a molecule, find a functional group that is able to interact with metal without reacting fully (that depends too strongly on both temperature and the choice of metal to have a general rule.) In practice, for $\ce{Ga}$,some thiols can be used.

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    $\begingroup$ Indeed, many of the salts have solubility. $\endgroup$
    – Jon Custer
    Commented Apr 18 at 20:44
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Metals with a low thermodynamic tendency to form oxides can dissolve an appreciable amount of oxygen, or one could call it said oxides, in the melt.

Copper is the archetype of such metals. Not only does it exhibit appreciable oxygen solubility in the melt, this solubility also impacts it in practice because oxygen is deliberately introduced (through roasting of the ore) during smelting.

At 1150°C the phase diagram taken from Figure 3 of Reference 1, reproduced below, shows an oxygen solubility of about 1 wt% in the copper melt. As a weight percent figure it appears unimpressive, but 1% by weight amounts to converting almost 10% of the metal to the oxide $\ce{Cu2O}$ with which the metal is in equilibrium. The oxygen/oxide solubility in the liquid is enough to set up a measurable eutectic depression on the temperature axis.

enter image description here

Reference

  1. Pohja, Rami & Vestman, Heikki & Jauhiainen, Petra & Hänninen, Hannu. (2003). Narrow Gap Arc Welding Experiments of Thick Copper Sections. 41 pages.
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Liquid metals can be used as solvents for synthetic chemistry, much like water or organic solvents. Given the high temperatures required for melting most metals, the reactions are typically done at high temperatures and have the goal of synthesizing inorganic products. Here's an example of boron nitride being dissolved in a molten mixture of nickel and chromium at 1500° C and then subjected to a flow of nitrogen gas to make higher quality hexagonal boron nitride crystals than what they started with.

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