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An ordinary lab heating mantle often has a white fabric coating for the flask compartment, which looks like an inorganic fiber:

enter image description here

Image source

I'm pretty sure that for some time in the past asbestos was used for that; these days I think it's some kind of woven fiberglass (assuming the heating element produces elevated temperatures up to about $\pu{400^\circ C}$), but I'm having hard times finding an exact chemical composition.

Also, what kind of chemicals can react with this coating? I guess something like molten $\ce{NaOH}$ and the melts of alkali metals can cause some damage, but I'm not sure.

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    $\begingroup$ Those older ones are often made of woven fibreglass, traditionally with aspestos behind, or nowadays usually some sort of ceramic. More modern isomantles use an aluminium mesh instead of the fabric, again,with a ceramic insulator behind. $\endgroup$
    – NotEvans.
    Commented Jul 30, 2017 at 7:47
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    $\begingroup$ @NotEvans - Good answer - how about reposting your comment as the answer? $\endgroup$
    – iad22agp
    Commented Jul 31, 2017 at 14:05
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    $\begingroup$ From one of the patents, "The textile-like materials referred to are generally fabrics made from quartz fibers or from glass fibers, or asbestos fibers, so to withstand the sometimes substantial temperatures developed by the heating blankets and heating mantles" $\endgroup$ Commented Apr 10, 2021 at 5:16

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Early laboratory heating mantles use glass fiber yarn knitted material with a heat resistant naturally occurring fibrous mineral called asbestos.

Modern heating mantles have found their alternatives
among the metals like aluminium with ceramic or porcelain for insulation which helps in prevention of conduction of thermal shocks to glassware.

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