What kind of glass can I purchase that will melt in a Bunsen burner flame?
Is there any way to treat glass such that the melting point is lowered? I tried adding some sodium hydroxide but the glass shattered when heated.
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asked Jul 20, 2014 at 20:54
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2$\begingroup$ I know that soda lime glass has a low melting point, but I don't know what that is. Glass shatters when the stress is great enough, like when there is a large temperature difference in the glass. You could continue to heat what you have until it melts. If it hasn't melted within 15 minutes, I would think your temperature is not high enough. $\endgroup$– LDC3Jul 20, 2014 at 21:05
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$\begingroup$ I've tried heating a soda lime glass lab slide in a flame before--it just throws off slivers of glass and eventually shatters. Ironically, the glasses I have seen successfully heated and melted are borosilicates, which have a much higher melting point, but a lower thermal CoE, so they can more easily be heated in a flame. TL;DR low melting point alone might not be a good candidate for melting in a bunsen burner; you also need it to be tough enough to withstand the uneven heating of a flame. $\endgroup$– chipbusterJul 20, 2014 at 21:20
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$\begingroup$ What is your end goal here? Some crafts stores sell frit glass which melts below most flame temperatures, but it's sold in a powdered form $\endgroup$– chipbusterJul 20, 2014 at 21:28
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$\begingroup$ I want to make Prince Rupert's drops. $\endgroup$– Brinn BelyeaJul 20, 2014 at 23:56
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2$\begingroup$ I found this youtube video that shows how to make the drops. Natural gas burners (Bunsen burner) don't have enough heat to melt the glass; you need to use a propane torch. youtube.com/watch?v=5zxZkK2aJig $\endgroup$– LDC3Jul 21, 2014 at 0:55
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1 Answer
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There are a few options for low-melting glasses that will easily melt at the temperatures of an air-gas torch (i.e. a Bunsen burner). I suspect that either soda-lime or lead glasses would be the soft glasses of choice for your application (making Prince Rupert's drops).
Soda lime glass should be an adequate choice. For some applications, it can be prone to cracking if not properly annealed, however.
Lead-based glasses are among those with the lowest melting points of any glass, and probably the lowest applicable to your application. Annealing is simple and can just be done with the torch itself. Lead glasses also have a large "soft" temperature range (the range of temperatures below the melting point at which they are still somewhat pliable). This is seen as a disadvantage in many use-cases, where sagging prior to sufficient cooling/hardening is a problem. For your work, I would think this would either be a non-issue or possibly could even expand the possibilities for making cool looking drops ;)
Given the availability and affordability of these solutions, further treatment to reduce the melting point just seems like a waste of good glass-working time.
