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
