Fundamental of Glass

8/3/2019 Fundamental of Glass

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Fundamentals of Glass


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Physical Properties of Glass

Physical and chemical properties.

Metric and British systems.

Celsius (Centigrade) vs Fahrenheit. Mass vs weight.

Density

Refractive Index

Crystalline vs amorphous solids.

Double refraction and birefringes.


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Physical property: describes the behavior

of a substance without having to alter thesubstances composition through achemical reaction

Chemical property: describes the

behavior of a substance when it reacts orcombines with another substance

Fahrenheit scale: the temperature scale usingthe melting point of ice as 320 and the boiling

point of water as 2120, with 180 equal divisions ordegrees between them.


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Celsius scale: the temperature scale using themelting point of ice as 00 and the boiling pointof water as 1000, with 100 equal divisions ordegrees between

Weight: a property of matter that depends on

the mass of a substance and the effects ofgravity on that mass

Mass: a constant property of matter thatreflects the amount of material present

Density: a physical property of matter that isequivalent to the mass-per-unit volume of asubstance


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Refraction: the bending of a light wave as itpasses from one medium to another

Refractive index: the ratio of the speed oflight in a vacuum to its speed in a givensubstance

Crystalline solid: a solid in which theconstituent atoms have a regular arrangement

Atom: the smallest unit of an element; notdivisible by ordinary chemical means. Atomsare made up of electrons, protons, andneutrons plus other subatomic particles


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Amorphous solid: a solid in which the constituent

atoms or molecules are arranged in random ordisordered positions. There is no regular order in

amorphous solids.

Birefringence: a difference in the two indices of

refraction exhibited by most crystalline materials

Dispersion: the separation of light into its

component wavelengths

Tempered glass: glass to which strength is addedby introducing stress through the rapid heating and

cooling of the glass surfaces


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Becke line: a bright halo that is observed

near the border of a particle immersed in aliquid of different refractive index

Radial fracture: a crack in a glass thatextends outward like the spoke of a wheel

from the point at which the glass wasstruck

Concentric fracture: a crack in a glass

that forms a rough circle around the pointof impact

Mineral: a naturally occurring crystallinesolid


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Density-gradient tube: a glass tube filled

from bottom to top with liquids of

successively lighter densities; used to

determine the density destruction of soil


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Properties of Matter Chemical Properties

a characteristic of a substance that describes the

way the substance undergoes or resists change

to form a new substance

Physical Properties

a characteristic of a substance that can beobserved without changing the substance into

another substance


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Physical Properties Extensive Properties

depend on the amount of sample

volume, mass

Intensive Properties

do not depend on the amount of sample

melting point, density


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Density The ratio of the mass of an object to the

volume occupied by that object

g/cm3 (solids); g/mL (liquids)

d = m/V

Densities of solids & liquids are often

compared to the density of water sink or float

Varies with temperature


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Refraction The bending that occurs when a light wave

passes at an angle from one medium to

another (air to glass)

bending occurs because the velocity of the

wave decreases


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Refractive Index (ND) The ratio of the velocity of light in a

vacuum to the velocity of light in a given

medium ND (water) = 1.333

light travels 1.333 time faster in vacuum than inwater

An intensive property Varies with temperature and the light

frequency


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Double Refraction Crystals refract a beam of light into two

different light-ray components

extraordinary ray refracted (bent)

ordinary ray

path unchanged

Causes a double image to be seen

No double refraction with isometric crystals


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Birefringence The difference between the two indices of

refraction

for calcite: 1.486 & 1.658

birefringence for calcite is 0.172

Use in identifying crystals


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Dispersion Occurs when

an incident parallel beam of light to fans out

according to the refractive index of the glass for

each of the component wavelengths, or colors.


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Basics of Glass One of the oldest of all manufactured

materials

A simple fusion of sand, soda & lime (all

opaque)

produces a transparent solid when cooled


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What is Glass?

An extended, 3D network of atoms which

lacks the repeated, orderly arrangement

typical of crystalline materials

The viscosity is such a high value that the

amorphous material acts like a solid


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What is Glass?

glass is formed upon the cooling of a

molten liquid in such a manner that the

ordering of atoms into a crystalline

formation is prevented

materials which form glasses are relatively

rare SiO2 (silica) is the most common example


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Structure of Glass


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Physical Properties At ordinary temp.

internal structure resembles a fluid

random molecular orientation

external structure displays the hardness &

rigidity of of a solid

Does not show a distinct melting point on heating gradually softens

on cooling gradually thickens


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Physical Properties Common Properties

hard

perfectly elastic

brittle

non-conductors of electricity

chemically stable


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Types of Glasses ~a thousand chemical formulations

each has its own combination of properties

more than 700 compositions in commercial

use

Most common type encountered by the

forensic scientist is flat glass

glass used in windows & doors


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Components Formers forms the glassy, non-crystalline structure

fluxes

improve melting properties but impart lowchemical resistance

typically alkali or alkaline earth oxides

modifiers (stabilizers or intermediates)

a material that improves stability

typically oxides of Ca, Al, or Zn


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Common Glass ComponentsFORMERS INTERMEDIATES MODIFIERS

SiO2 Al2O3 Na2O

B2O3 PbO CaO

GeO2 Sb2O3 K2O

P2O5 ZnO MgO

V2O5 TiO2 Li2O

As2O3 BeO BaO


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Soda-lime-glass

Silica itself makes a glass (fused silica)

high mp (3133 oF or 1723 oC)

high viscosity in liquid state

difficult to melt & work

Na2O (soda) lowers melting temp (A flux)

glass lacks durability (soluble in water)

CaO (lime) increases stability


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Borosilicate Glass Over 5% B2O3 added to the silica

a heat resistant glass that expands only ~1/3 as much as

silicate glass more resistant to breaking on rapid heating & cooling

Pyrex

Uses

laboratory ware & thermometers household glassware

sealed-beam headlights


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Lead Glasses

Incorporates up to 80% PbO

Has high refractive index & high electrical

resistivity Suitable for hand fabrication

Uses

crystal tableware

costume jewelry

fine chandeliers

neon sign tubing


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Colored Glasses Metallic oxides or sulfides added to soda-

lime glass

chromium oxide (green)

cobalt oxide (blue)

cadmium or selenium sulfide (red)

Colloidal particles of iron & sulfur producesthe carbon brown beer bottle


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Decolorized Glass General term for the soda-lime-glass marketed as

clear for windows

Color caused by impurities present in the rawmaterials removed or masked

destruction of carbonaceous matter

oxidation of Fe(II) (blue) to Fe(III) (yellow)

NaNO3, KNO3,, BaNO3

Most clear glass is not absolutely colorless

observable by viewing on edge


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L

ightS

ensitive EyeglassL

enses Contain colloidal particles of silver halide

Identified by exposure to uv light


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Glass Production Flat Glass

until late 1950s produced by sheet * plate

processes

primarily produced by the float glass process

today

molten glass is floated over a bath of molten time

produces a distortion-free sheet


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Other Classification Methods Microscopy

float glass is absolutely flat

wine glasses are slightly curved

bottles have microscopic defects from mould

Fluorescence

when excited by uv radiation, many glassesexhibit fluorescence

caused by heavy metals (including tin)


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Fluorescence Can differentiate

between float and

nonfloat window glass Can differentiatebetween differentsamples of float glassin some cases

(a) nonfloat glass or nonfloat side

(b) float side Sample #1

(c) float side Sample #2


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Scanning Electron Microcopy Can detect

Si

Na Ca

Mg

K

very small samples

can be analyzed (50

micrograms)


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X

-ray Fluorescence Can detect major

elements in glass

samples sometimes detects

minor & trace level

components


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Elemental Analysis Many of the trace elements enter the glass

via trace impurities in the raw materials

Comparison of elemental analysis of crime

glass & reference glass

if ranges of elements overlap for every element

indistinguishable

if ranges of one or more elements are different

samples are distinguishable


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Red= flat blue= container black= tableware

RI=1.5177-1.5183

Red= flat

blue= container

black= tableware

Fundamental of Glass

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