I have tried 4 experiments at various temperatures to melt obsidian. At varying temperatures it just bubbles up and I don't seem to reach a melting point.
Do you know what gases obsidian might contain and would they be the cause of the bubbles?
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6$\begingroup$ Obsidian doesn't have a particular chemical formula. It is a mixture. So there is no definite melting point. $\endgroup$– MaxWMay 14, 2017 at 23:09
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$\begingroup$ When you say that it "bubbles up" that sounds like melting to me, so I'm hoping you can clarify your meaning. Also, do you know at about what temperature you first see this bubbling, and also what was the highest temperature you used? $\endgroup$– airhuffMay 15, 2017 at 1:22
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$\begingroup$ I have worked with casting glass(45%lead) in the past which flows into a given shape when melted at 900°c. So I tried melting the obsidian at a simular temp. It did nothing at 1000° but at 1020° it doubled in size due the bubbling and it was grey in colour.I held it for 4 hours hoping it would then flow. It didn't and stayed in its 'frozen' bubble form. Other firings at 1050° and 1100° formed larger bubbles and the colour remained black? $\endgroup$– Jill GoddardMay 15, 2017 at 20:55
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4 Answers
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Obsidian
This is a volcanic glass, formed when rhyolitic lavas cool too quickly for crystals to form. From a chemistry point of view, it's a mixture of silicon dioxide, aluminium oxide, with sodium ,potassium, calcium and iron oxides in various quantities. These lavas will also contain significant volatiles (water and $\ce{CO2}$) held in solution by pressure, although they should be lost at the pressures required for glass formation (higher pressures means slower cooling due to more insulation).
Melting
Note that when molten, rhyolite lavas can have a very high viscosity. So it's possible that you have melted your sample, but just didn't give it enough time to flow. Indeed, with substances like this you will see partial melting - some components will melt at a lower temperature than others. If you are getting bubbles, this would indicate that your glass formed under enough pressure to retain some $\ce{CO2}$ and/or $\ce{H2O}$ in its lattice, and you are now allowing this to be released by heating at surface pressure. This is all a bit speculative, without a closer inspection.
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$\begingroup$ Thanks Andrew for all your information. I think my next attempt will be to refire a piece that has bubbled at 1020°. I'm hoping that now co2 or h20 has been released it may flow? $\endgroup$ May 15, 2017 at 21:10
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$\begingroup$ Might need to be a bit hotter, the volatiles lower the melting point. $\endgroup$ May 16, 2017 at 9:00
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$\begingroup$ I'll add that the volatiles are probably not CO2, because it is lost way before the lava erupts to become obsidian. Most likely dominated by H2O, and even then it's possible it's being held up by minerals like micas and amphiboles, and not in the glass itself. $\endgroup$– GimelistOct 13, 2018 at 0:26
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I cast using Obsidian (experimenting for 7yrs) an amazing material it has so many different reactions
depends on point of origin of material
Size of pieces
Speed of ramp temps
Hold times.
Temperature range 1000-1130c
You can make light large pieces that float or heavy dense work my experience is to treat more like a ceramic material than glass when firing as very different to Gaffa glass ](https://i.stack.imgur.com/R1fI4.jpg)
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Obsidian is mostly silicon dioxide (about 70%), with a good bit of aluminium oxide and then about 10-20% various other oxides.
Melting point for silicon dioxide is 1,710 °C, and for aluminium oxide 2,072 °C. You're going to need a lot of heat for this.
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6$\begingroup$ Welcome to Chemistry.se! While your answer is true, it does not fully address the question. It also does not offer much more than the already given answer. It would be nice if you could expand it a little more. $\endgroup$– Martin - マーチン ♦Aug 7, 2017 at 8:26
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2$\begingroup$ What you say is correct but likely irrelevant. Mixtures often have radically different melting points to their components so speculating about obsidian based on pure components isn't helpful. $\endgroup$ Jul 30, 2018 at 10:22
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Obsidian melts between 700 and 1050 deg C http://www.swxrflab.net/blmpaper.pdf
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2$\begingroup$ I don't think that is what the linked paper says. Unless you mean something different by melting. $\endgroup$ Jul 30, 2018 at 10:21