In message <4a37ad7f.1090...@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 -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 p...@freebsd.org | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence.
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