the chemistry of coloured glass Flashcards
what happens with excited electrons in higher lying states
fall back down to lower lying atomic orbitals and emit a photon of light
coloured glass method 1
- 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
additions to glass
- iron(II) oxide bluish-green glass used in beer bottles. With chromium dark green colour, used for wine bottles.
- 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.
- cobalt (0.025 to 0.1%) yield blue glass.
- 2 to 3% of copper oxide produces a turquoise color.
- 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.
- Chromium yielding dark green[6]
- 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.
- Uranium (0.1 to 2%) can be added to give glass a fluorescent yellow or green color.[8] 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.
coloured glass method 2
- Heat treatment forms colloidal particals that are suspended throughout the glass. ‘striking colours’
- 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 golf <10ppm) gives a cranberry colour
Selenium, from pink to intense red
- Ruby gold glass is lead glass with added tin
photochromic lenses
- silver chloride (AgCl) and copper (I) chloride (CuCl) crystals are added during manufacture
- In presence of UV-A light (wavelengths of 320–400 nm)
- 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% of transmitted light
- copper (I) chloride reverses the darkening process when the lenses are removed from 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 oxidized and reduced states.
coloured glasses method 3
- Addition of already coloured particles to the glass e.g. tin oxide gives milk glass and dark coloured inclusions give smoked glass
coloured glasses method 4
Colour caused by scattering
- Glass containing two or more phases with different refractive indices of ‘tyndall effect’
- Sizes of the phases must be similar or larger than the wavelength of visible light
Scattering light is blue and violet
Transmitted light is yellow and red
Braggs law and diffraction
- the peaks and troughs aline differently for constructive and destructive interference
constructive- peaks and troughs line up
destructive- troughs meet peaks
X-ray diffraction
plot of the intensity of x-rays scattered at different angles by a sample
how does the X-ray diffraction work
- the detector moves in a circle around the sample
– The detector position is recorded as the angle 2theta (2θ)
– The detector records the number of X-rays observed at each angle 2θ
– 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 of x-ray dffraction produces a unique diffraction pattern
- A phase is a specific chemistry atomic arrangement
- Quartz, cristobalite, and glass are all different phases of SiO2
- They are chemically identical, but the atoms are arranged differently.
- the X-ray diffraction pattern is distinct for each different phase.
- Amorphous materials, like glass, do not produce sharp diffraction peaks.
The diffraction of a mixture is a simple sum of what
the diffraction patterns of each individual phase
from the XRD pattern what can you 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
why is experimental XRD 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- the glass transition is NOT the same as melting
Look on power point to see graphs
crystalline region
• Below Tm: Ordered crystalline
solid
• Above Tm: Disordered melt
amorphous region
• Below Tg: Disordered amorphous solid with
immobile molecules
• Above Tg: Disordered amorphous solid in which portions of molecules can wiggle around
glass transition temps for different materials
material - Tg (degrees. celsius)
Fused quartz- 1,200 soda lime glass- 520-600 ZBLAN fluoride glass- 235 time rubber- -70 polypropylene- -20 PVC- 80 Teflon - 115
why does a given sample not have a unique value of Tg
the glass phase us not at equilibrium
what will the measured value if Tg depend on
he molecular weight of the material, on its thermal history and age, on the measurement method, and on the rate of heating or cooling
what does slower cooling (vitrification) result in
lower temperature at which Tg occurs and a more highly dense glass
- Increased time for structural relaxation
you can measure Tg using
differential scanning calorimetry (DSC)
physical fit
- Rarely found in real cases
- Needs a perfect fit in 3 dimensions
- Requires microscopic match of marks
- Photograph documents fit
thickness
Provide information where it comes from e.g. temepered glass, beer bottle
Provides limited information
Physical measurement
Edge thickness- a micrometre is used to measure accurately the edge thickness of the glass fragments. The broke edged to find out at which point, the crim exhibits matches with any portion of the broken glass
Curvature- a spherometer is 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
forensic analysis of glass
see power point for flow chart
colour
- Side by side comparison with comparison fragment of same size and thickness as sample
- Samples need to be >5mm wide
- Very difficult to differentiate colour on 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)
- 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
- need 5 mg of sample. Measurement of small, irregular or dirty fragments of glass may be inaccurate
comparison using a density column
glass tube is filled with liquids of different densities (bromoform and bromobenzene)
glass is added and particles will sink to the portion of the tube that has a density of equal value and remain there suspended
the density distribution patter of the glass particles can be obtained and compared to other specimens with the same method
density of liquids
- Methylene Iodide (CH2I2) ~ 3.32 g/cm3
- Bromoform (CHBr3) ~ 2.85 g/cm3
- Bromobenzene ~ 1.495 g/ cm3
- Acetylene Tetrabromide ( CBr2CBr2)~ 2.96 g/cm3
- Sodium Polytungstate ~ 2.89 g/cm3
- Na6H2W12O40 x H2O (water soluble)
what is the density of most glass range
ranges between 2.4g/cm3 to 2.8 g/cm3
window glass does not have uniform density
need to take samples from different positions in the window
tempered glass
has different densities on surface (more dense) and interior (less dense)- Need to test a number of samples
