Re: Saltwater Caps

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Original poster: "Dr. Duncan Cadd by way of Terry Fritz <twftesla-at-uswest-dot-net>" <dunckx-at-freeuk-dot-com>

Hi Adam, All!

>My question is this:
>Has anyone ever determined whether clear glass caps
>are better than brown glass caps? My limited knowledge
>of glass thinks that clear glass has less impurities
>than colored glass. Thoughts?


Well, supposedly the clear glass is less pure, or at least was years
ago, but whether there's been a change in manufacturing practice is
another one of those "gotchas".

Brown and green glass are/were so coloured because of the natural
impurities (mainly iron) in the raw materials.  What they used to do
(and maybe still do) was make a batch of glass, coloured by these
impurities, and add some pyrolusite (a name meaning "to cleanse by
fire") - a naturally-occurring mineral which is crude manganese
dioxide.  This oxidised the coloured impurities to make the glass
clear - i.e. it's basically a high temperature bleaching process
conducted in the melt.  However, the glass made by this process always
had a greenish tinge in thick section, e.g. if you looked down the
neck of a bottle, whereas I've seen a lot of clear glass these days
which really is water white, even in thick section - some of this is
borosilicate (pyrex and the like) but a lot isn't.  Maybe this implies
the bleaching is carried out on the raw materials prior to
glassmaking, or that the raw materials these days are more clean to
start with.  I don't really know.

I suppose the $64000 question is whether the bleached impurities (if
made by that process) are less of a dielectric breakdown risk than the
unbleached impurities.

The presence of colour suggests that the amount of energy E = hv  (h =
Planck's constant, v = frequency) needed to excite the electrons lies
in the visible region of the spectrum, but whether the bleaching
process shifts that to the ultra-violet (higher energy) or infra-red
(lower energy) is another question, as is whether there is any direct
correlation between that energy and the tendency to breakdown.  It
could be that the coloured ions represent a higher dielectric loss and
thus more internal heating, hence a higher probability of fracture.
It could in fact be almost anything . . .

I can recall reading a Victorian amateur experimenting book which
suggested that you should select glass for capacitors on the basis of
how good a static charge you could get on the glass by rubbing it with
e.g. a bit of silk or a dead dog or whatever.  Probably as good a
guide as anything ;-)  It would be interesting to know how the
different colours of glass stack up against each other on the basis of
so simple a test.

Perhaps Antonio has some relevant experience here with all the work he
has done on electrostatic machines, many of which are glass-based?

Dunckx