The chemistry of coloured glass

Question 1

Excited electrons in higher lying states

Answer 1

fall back down to lower lying atomic orbitals and emit a photon of light with energy, E, frequency, ν and wavelength, λ.
E=hν C=λν

Question 2

1st main method for coloured glass

Answer 2

Transition metals or rare earth metal oxides are added to the glass. The metal ions absorb certain wavelengths of light, colour is from non-absorbed wavelengths

Question 3

2nd main method for coloured glass

Answer 3

Heat treatment forms colloidal particles that are suspended throughout the glass

• Colloidal particles scatter light of particular frequencies causing colour
• Colour is caused by the size and dispersion of gold particles
• Metallic gold (10ppm) gives a ruby red colour
• Metallic gold (<10ppm) gives a cranberry colour
• Selenium, from pink to intense red
• Ruby gold glass is a lead glass with added tin

Question 4

3rd main treatment for coloured glass

Answer 4

Addition of already coloured particles to the glass e.g. tin oxide gives milk glass and dark coloured inclusions give smoked glass

Question 5

4th main treatment for coloured glass

Answer 5

Colour caused by scattering

• The tyndall effect in opalescent glass: it appears blue from the side by orange light shines through
• Glass containing two or more phases with different refractive indices if “tyndall effect”
• Sizes of the phases must be similar or larger than the wavelength of visible light
• Scattered light is blue and violet
• Transmitted light is yellow and red

Question 6

uranium as an addition to glass

Answer 6

Uranium (0.1 to 2%) can be added to give glass a fluorescent yellow or green color. Uranium glass is typically not radioactive enough to be dangerous, but if ground into a powder, such as by polishing with sandpaper, and inhaled, it can be carcinogenic.

Question 7

photochromic lenses

Answer 7

Silver chloride (AgCl) and copper (I) chloride (CuCl) crystals are added during manufacture
In presence of UV-A light (wavelength of 320-400nm)
Cl- -> Cl + e- Ag+ + e- –> Ag
Oxidation reduction

Ag atoms cluster darken the lens and block the transmittance of light

Degree of darkening is dependent on the intensity of the light

Reduce up to 80% or transmitted light

Copper (I) chloride reverses the darkening process when the lenses are removed from the light
Cl + Cu+ ——> Cu+2 + Cl-
Cu+2 + Ag ——> Cu+1 + Ag+

Lenses become transparent again as the silver and chloride atoms are converted to their original oxidised and reduced states

Question 8

braggs law and diffraction

Answer 8

constructive interference

destructive interferences

Question 9

A X-ray diffraction patterns is a plot of the intensity of X-rays scattered at difference angles by a sample

Answer 9

The detector moves in a circle around the sample

• The detector position is recorded as the angle 2theta
• The detector records the number of X-rays observed at each angle 2theta
• The X-ray intensity is usually recorded as counts or as counts per second

To keep the X-ray beam properly focused, the sample will also rotate
- On some instruments, the X-ray tube may rotate instead of the sample

Question 10

each phase produces a unique diffraction pattern

Answer 10

A phase is a specific chemistry and atomic arrangement.

Quartz, cristobalite, and glass are all different phases of SiO2

• They are chemically identical, but the atoms are arranged differently.
• As shown, the X-ray diffraction pattern is distinct for each different phase.
• Amorphous materials, like glass, do not produce sharp diffraction peaks.

Question 11

The diffraction pattern of a mixture is a simple sum of the diffraction patterns of each individual phase

Answer 11

From the XRD pattern you can determine:

• What crystalline phases are in a mixture
• How much of each crystalline phase is in the mixture (quantitative phase analysis, QPA, is covered in another tutorial)
• If any amorphous material is present in the mixture

Question 12

Experimental XRD data are compared to reference patterns to determine what phases are present

Answer 12

The reference patterns are represented by sticks

The position and intensity of the reference sticks should match the data
- A small amount of mismatch in peak position and intensity is acceptable experimental error

Question 13

glass transition for amorphous materials

Answer 13

glass transition is not the same as melting

Question 14

glass transition temperatures for different materials

Answer 14

A given sample does not have a unique value of Tg because the glass phase is not at equilibrium.

The measured value of Tg will depend on the molecular weight of the material, on its thermal history and age, on the measurement method, and on the rate of heating or cooling

Question 15

vitrification

Answer 15

slower cooling results in a lower temperature at which Tg occurs and a more highly dense glass

increased time for structural relaxation

Tg = change in heat capacity

Question 16

physical fit

Answer 16

Rarely found in real cases

Needs a perfect fit in 3 dimensions

Requires microscopic match of marks

Photograph documents fit

Question 17

edge thickness - physical measurements

Answer 17

A micrometre is used to measure accurately the edge thickness of the glass fragments

The broken edges to find out at which point, the crime exhibits matched with any portion of broken glass

Question 18

curvature - physical measurements

Answer 18

A spherometer measures the radius of curvature of the glass fragments having curved surface. The radius of curvature of the fragment is calculated using the formulae
R = (I2/6h)+(h/2)
Where, I = the mean distance between the legs of the spherometer
h = height of the curved surface

Question 19

order of forensic analysis of glass

Answer 19

physical properties
optical properties
elemental properties

Question 20

how wide do samples need to be for side by side comparison?

Answer 20

> 5mm

Question 21

colour

Answer 21

Side by side comparison with comparison fragment of same size and thickness as sample

Question 22

side by side comparison

Answer 22

Very difficult to differentiate colour fragments that are small, dirty or among debris

Place on edge on a white surface and view in natural light

View in UV as some glasses fluoresce various colours and incandescent light (tungsten) to distinguish colours

Record colours on spectrophotometer or colorimeter to give evidence in court
- Otherwise evidence is speculative

Heat or chemical treatment of glass can change its colour

Question 23

density (mass/volume)1

Answer 23

Rarely performed in forensic laboratories replaced by refractive index measurements, faster, more accurate and more precise

Density of large glass fragments can be measured by water displacement

Glass densities are determined by an ASTM standard method involving floatation in liquids

Uses toxic liquids, the composition of the liquid is carefully adjusted by the addition of drops of bromoform or bromobenzene until the glass chip remains suspended in the liquid medium

• At this point, the standard/reference glass and liquid each have the same density
• Densiometer measured liquid density – precision 0.0001g/mL

Question 24

how much sample do you need to measure density of glass?

Answer 24

5mg

Question 25

comparison using a density gradient column step 1

Answer 25

glass tube filled with liquids of different densities (9:0, 8:1, 7:2, 6:3, 5:4, 4:5, 3:6, 2:7, 1:8, 0:9)

Question 26

comparison using a density gradient column step 2

Answer 26

glass is added and particles will sink to the portion of the tube that has a density of equal value and remain there suspended

Question 27

comparison using gradient density step 3

Answer 27

the density distribution pattern of glass particles can be obtained and compared to other specimens with the same method

Question 28

what does the density of glass range between

Answer 28

2.4 g/cm3 to 2.8 g/cm3

Question 29

window glass does not have uniform density so what?

Answer 29

need to take samples from different positions in window

Question 30

tempered glass has different densities

Answer 30

on surface (more dense) and interior (less dense)

Question 31

iron(II) oxide as an addition to glass

Answer 31

iron(II) oxide bluish-green glass used in beer bottles. With chromium dark green colour, used for wine bottles.

Question 32

sulphur carbon and iron salts as additions to glass

Answer 32

Sulfur, carbon and iron salts, is used to form iron polysulfides and produce amber glass Manganese added in small amounts to remove the green tint given by iron. One of the oldest glass additives, and purple manganese glass was used since early Egyptian history.

Question 33

colbalt as an addition to glass

Answer 33

cobalt (0.025 to 0.1%) yield blue glass.

Question 34

copper oxide as an addition to glass

Answer 34

2 to 3% of copper oxide produces a turquoise color.

Question 35

nickel as an addition to glass

Answer 35

Nickel, depending on the concentration, produces blue, or violet, or even black glass. Nickel together with a small amount of cobalt was used for decolorizing of lead glass.

Question 36

chronium as an addition to glass

Answer 36

Chromium yielding dark green

Question 37

cadium and sulphur as an addition to glass

Answer 37

Cadmium and sulphur forms cadmium sulfide and results in deep yellow color, used in glazes. Cadmium is toxic. Together with selenium and sulphur it yields shades of bright red and orange.