Re: [Phys-l] glass

["John Clement" <clement@hal-pc.org>]

The information is all in the Wikipedia:
http://en.wikipedia.org/wiki/Glass

Plus other articles.

The most familiar type of glass, used for centuries in windows and drinking
vessels, is soda-lime glass, made of about 75% silica (SiO2) plus Na2O, CaO,
and several minor additives. Often, the term glass is used in a restricted
sense to refer to this specific use.

Crown glass was an early type of window glass. In this process, glass was
blown into a "crown" or hollow globe. This was then transferred from the
blowpipe to a pontil and then flattened by reheating and spinning out the
bowl-shaped piece of glass (bullion) into a flat disk by centrifugal force,
up to 5 or 6 feet (1.5 to 1.8 metres) in diameter. The glass was then cut to
the size required.[1]

Crown glass is type of optical glass used in lenses and other optical
components.
 
Crown glass is produced from alkali-lime (RCH) silicates containing
approximately 10% potassium oxide. It has low refractive index (?1.52) and
low dispersion (with Abbe numbers around 60); crown glass is one of the
earliest low dispersion glasses.

Flint glass is optical glass that has relatively high refractive index and
low Abbe number. Flint glasses are arbitrarily defined as having an Abbe
number of 50 to 55 or less. The currently known flint glasses have
refractive indices ranging between 1.45 and 2.00. A concave lens of flint
glass is commonly combined with a convex lens of crown glass to produce an
achromatic doublet lens because of their compensating optical properties,
which reduces chromatic aberration (color defects).
 
With respect to glass, the term flint derives from the flint nodules found
in the chalk deposits of southeast England that were used as a source of
high purity silica by George Ravenscroft, circa 1662, to produce a potash
lead glass that was the precursor to English lead crystal.
 
Traditionally, flint glasses were lead glasses containing around 4-60% lead
oxide; however, the manufacture and disposal of these glasses are sources of
pollution. In many modern flint glasses, the lead can be replaced with other
additives such as titanium dioxide and zirconium dioxide without
significantly altering the optical properties of the glass.
 
Flint glass can be fashioned into rhinestones which are used as diamond
simulants.

A concave lens of flint glass is commonly combined with a convex lens of
crown glass to produce an achromatic doublet. The dispersions of the glasses
partially compensate for each other, producing reduced chromatic aberration
compared to a singlet lens with the same focal length.

Apparently there are 2 types of crown glass.

Lead glass is a variety of glass in which lead replaces the calcium content
of a typical potash glass.[1] Lead glass contains typically 18-40 weight%
lead(II) oxide (PbO), while modern lead crystal, historically also known as
flint glass due to the original silica source, contains a minimum of 24%
PbO.[2] Lead glass is desirable owing to its decorative properties.
 
Technically, the term crystal is not applied to glass, as glass, by
definition, lacks a crystalline structure. The use of the term lead crystal
remains popular for historical and commercial reasons, and originally stems
from the Venetian use of the word cristallo to describe the rock crystal
imitated by Murano glassmakers. This naming convention has been maintained
to the present day to describe decorative hollow-ware.[3]

I believe it is the common soda lime glass which tends to have a greenish
cast.  And Russia produces the highest lead content glass which has a very
high refractive index.

And they debunked the common myth that glass flows.  It seems the glass was
never uniform thickness in very old houses so it was usually installed with
the thick end down for stability and water shedding reasons.

John M. Clement
Houston, TX


> On May 9, 2011, at 8:37 PM, Anthony Lapinski wrote:
>
> > I am teaching about refraction, and gave out a table of refractive
> > indexes. A student asked about two of the entries:
> >
> > fused quartz	1.458
> > crown glass		1.523
> >
> > She wanted to know what the difference was. I wasn't exactly sure,  
> > but I
> > believe that crown glass is "clear" and used for eyeglasses, while  
> > fused
> > quartz has a greenish tint due to impurities. Is this true? 
> I thought
> > crown glass was more "pure" and thus clearer. Not sure what fused  
> > quartz
> > is used for.
> >
> > Can anybody help with this?