<p>So while doing some research I stumbled upon a <a href="https://reprap.org/wiki/Glass_Nozzles#Step_1_Assemble_Parts_and_Tools" rel="nofollow noreferrer">wiki page on reprap</a> from a few years back where the user was creating a glass nozzle to replace the brass and PTFE assembly.<a href="https://reprap.org/wiki/Glass_Nozzles#Step_1_Assemble_Parts_and_Tools" rel="nofollow noreferrer">1</a></p>

<p>Does anyone know the theory behind this? Glass is a great insulator so I could see how that would be beneficial for the heat break part but I can't see how it is appropriate for the nozzle as this is normally brass which is a good conductor.</p>

<p>Surely the glass takes much more energy to heat up?</p>

<p>On a side note I've seen similar projects using ceramic instead.</p>

Ceramic I can understand - very strong, great thermal range capability.  Glass not so much - you'd need some seriously careful annealing at least.
In either case the material is much harder than brass, or even steel, so you could presumably use tougher tools to unclog, etc. as needed.  If you're using materials loaded with wood or metallic particles, the glass/ceramic tip will be less likely to degrade than brass.
BTW, glass being a thermal insulator means it may take
longer
to heat up, but the
energy
required is probably less . The specific heat of glass is on the order of .84 J/gm-K . Compare with brass at  0.38 , but keep in mind the rate at which brass will shed heat into the air vs. glass.  In either case the energy is tiny compared with the thermal mass of the hotend assembly.