The chemistry of coloured glass Flashcards
Excited electrons in higher lying states
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
1st main method for coloured glass
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
2nd main method for coloured glass
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
3rd main treatment for coloured glass
Addition of already coloured particles to the glass e.g. tin oxide gives milk glass and dark coloured inclusions give smoked glass
4th main treatment for coloured glass
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
uranium as an addition to glass
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.
photochromic lenses
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
braggs law and diffraction
constructive interference
destructive interferences
A X-ray diffraction patterns is a plot of the intensity of X-rays scattered at difference angles by a sample
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
each phase produces a unique diffraction pattern
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.
The diffraction pattern of a mixture is a simple sum of the diffraction patterns of each individual phase
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
Experimental XRD data are compared to reference patterns to determine what phases are present
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
glass transition for amorphous materials
glass transition is not the same as melting
glass transition temperatures for different materials
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
vitrification
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
physical fit
Rarely found in real cases
Needs a perfect fit in 3 dimensions
Requires microscopic match of marks
Photograph documents fit
edge thickness - physical measurements
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
curvature - physical measurements
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
order of forensic analysis of glass
physical properties
optical properties
elemental properties
how wide do samples need to be for side by side comparison?
> 5mm
colour
Side by side comparison with comparison fragment of same size and thickness as sample
side by side comparison
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
density (mass/volume)1
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
how much sample do you need to measure density of glass?
5mg
comparison using a density gradient column step 1
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)
comparison using a density gradient column step 2
glass is added and particles will sink to the portion of the tube that has a density of equal value and remain there suspended
comparison using gradient density step 3
the density distribution pattern of glass particles can be obtained and compared to other specimens with the same method
what does the density of glass range between
2.4 g/cm3 to 2.8 g/cm3
window glass does not have uniform density so what?
need to take samples from different positions in window
tempered glass has different densities
on surface (more dense) and interior (less dense)
iron(II) oxide as an addition to glass
iron(II) oxide bluish-green glass used in beer bottles. With chromium dark green colour, used for wine bottles.
sulphur carbon and iron salts as additions to glass
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.
colbalt as an addition to glass
cobalt (0.025 to 0.1%) yield blue glass.
copper oxide as an addition to glass
2 to 3% of copper oxide produces a turquoise color.
nickel as an addition to glass
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.
chronium as an addition to glass
Chromium yielding dark green
cadium and sulphur as an addition to glass
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.
