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I hope this question belongs in here; I cannot think of any other better place to ask it.

Which kind of material is used in dichroic halogen bulbs to attach the bulb itself to the reflector and keep it in there? Looks to me like a ceramic or some kind of terracotta, but I wish to know the exact kind and if it is available for home use or, in case it isn't, a good recommendation as a domestic substitute. Of course, I understand it's main property is, appart from its hardness, the ability to be fireproof and/or refractory, due to the high temperatures being reached in there. For this same reason, something like epoxy resin or anything plastic based probably wouldn't work as it would melt or even burn.

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  • $\begingroup$ Most halogen reflector bulbs of this type (now nearly obsolete due to more efficient LED bulbs) contained a quartz inner tube but did not connect that tube directly to the outer reflector. Usually only the metal terminals of the inner tube were connected to the outer shield which, in turn, were anchored into the base with the certamic. $\endgroup$
    – matt_black
    Mar 11 '21 at 22:17
  • $\begingroup$ Yes @matt_black; is that ceramic the one I'm referring to $\endgroup$
    – Pere
    Mar 13 '21 at 10:01
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Actually, aside from heat resistance, a primary qualification is that the adhesive cement thermal expansion coefficient match that of the quartz (or glass or alumina) envelope over the temperature range of storage and operation, to avoid cracking the envelope (the metal fixture is more flexible and "forgiving").

There are a number of cements in use, such as, "aluminum phosphate refractory cement... a mixture of aluminum phosphate... phosphoric acid [and] alumina". Glassbond sells a variety of adhesives, such as a phenolic/melamine adhesive for glass lamps with operating temperature below 300°C. Sauereisen has a long history making high-temperature adhesives, including those of silicate and of magnesium oxysulfate composition. One could try making these cements at home or in the lab, though they're readily available commercially. You could experiment with refractory furnace cement, though it likely would have a different coefficient of expansion, cracking the envelope.

BTW, I remember, in school, attaching thermocouples to incandescent lamps with a variety of cements... and watching the glass envelope form small star-shaped cracks at each joint, for many of those adhesives, as the lamp warmed. Sorry, but I don't remember which formulations actually succeeded past a few on/off cycles.

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