Frances, about your question regarding fiber blanket vs.
vermiculite: Ceramic fiber acts as a “heat mirror”, but does not
itself absorb heat. What heat it does absorb it loses very
quickly. It’s function as an insulator for hot glass is that it
simply reflects the heat of the hot piece back into itself, so
the piece cools more slowly. But the blanket can’t really be
heated up from an external source and contribute any heat to the
glass (which would allow the glass to cool even more slowly).
Vermiculite is an insulator that is more like traditional clay
materials: it will absorb and retain heat, and give it off
slowly. If you place a piece of hot glass in cold vermiculte, you
will thermal shock the glass because the cold vermiculite will
absorb its heat. But if you place hot glass in hot vermiculite,
the glass will cool very slowly, much more slowly than it will
in fiber blanket.
In the 80’s a lot of glass people discovered fiber blanket for
their annealing kilns as a good alternative to asbestos. But in
the last decade there has been some evidence that the ceramic
fibers also do nasty things to your lungs. There has also been
evidence that the vermiculte may be harmful as well. So
safety-wise, it may be a toss-up.
Personally, I place a can of vermiculite in a large stainless
steel mixing bowl, then surround the can with fiber blanket in
the bowl. I cover the top of the can with a piece of blanket to
keep the heat in, and preheat the whole arrangement on a hot
plate. So I get the best of both: the hot vermiculite insulates
the hot glass, and the blanket never gets hot but acts as a
shield so the vermiculte stays evenly heated throughout the can,
and cools more slowly. The top piece of blanket also keeps the
vermiculte dust down.
I make very large beads and lose none of them to annealing
problems. (I also do run them through a precise annealing cycle
in a kiln after I have a large batch.)
I hope this addresses your question.