[time-nuts] Lifetime of glass containers

Poul-Henning Kamp phk at phk.freebsd.dk
Tue Jun 16 11:52:06 EDT 2009


In message <4A37AD7F.1090806 at erols.com>, Chuck Harris writes:
>J. Forster wrote:

>To me, it would seem that playing with a blob of molten glass in
>a fire, and spreading it out, or rolling it would be a more natural
>step in the progression of making glass windows than blowing
>a bubble.

The problem is that you don't get very clear glass using any kind
of surface, because pretty much everything you can think of using
for the base will contaminate the glass, either as particulate
matter or chemically.

The history in reverse is something like:

Some (french ?) guy figured out that tin (Sn) did not chemically
react with glass and used a liquid bath of it as base that a "float"
glass process which produces clear consistent glass with very few
refractory variations.  In float glass, imperfections are mostly
particles which cause "sombreo" like refractory imperfections.
Practically all windowglass is produced this way today.

Before the float process, drawn glass was used, basically pulling
a membrane of glass out of bath of molten glass.  This required
quite a bit "fingerspitzgefühl" and special buildings which could
control the temperature gradient very precisely.  The resulting
glass very often had straight lines of refractory variations in the
drawn direction and significant thickness variations.  I belive
this process has never been done manually or with animal power,
only with machinepower.

Before the "drawn" glass, window glass was "blown" by picking up a
lump of molten glass on a steel rod and twirling it as fast as
possible (often on special "tables" consisting of two steel bars
set in parallel), making the centrifugal force draw the solidifying
glass into a disc.  Thinking about pizza dough here is entirely
appropriate.  The disc were subsequently cut into rectangular pieces,
and the glass is easily recognizable because of the numerous and
significant arc-shaped refractory variations.  In general it was
very hard to get a diameter above approx 2 feet with this method.
To my knowledge, this process has never been mechanized, power
from horses has been used.

Archeologists have found two kinds of *flat* glass predating this,
one bearing evidence of simply being molten glass poured out over
a flat, possibly heatet, marble slab.  Some of this glass has had
the resulting opaque layer polished off, at great expense and effort.

The other kind is the "rolled glass", where molten glass has been
poured out through a narrow slot and subseqently rolled further,
like dough for a flatcakes can be recognized by having both sides
having an opaque layer.

Poul-Henning

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