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Glass and Light
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SCIENTIFIC SECTION



The interaction of light with glass which, in this exhibition, is the basis of our visual perception is explored in various ways. Due to the reflection or absorption of electromagnetic waves, the reality and the objects that constitute it are revealed to our eyes. The different types of interactions between a beam of light and a piece of glass are shown in the following scheme.



There are thousands of glasses, each with a different chemical composition that influences their physical properties. In general glasses are transparent to the visible light, while they absorb wavelengths greater than 4 µm (infrared) and smaller than 350 nm (ultraviolet). This is due to the fact that glasses have a low coefficient of absorption of radiations in the 390-700 nm range. A light beam is scattered when crossing a glass in which small particles (crystals, gas bubbles, …) with a refractive index different from that of the glass matrix. When the number of particles is small and the object is thin, the glass appears translucent, but when the particle number is high, the glass appears opaque and in this case, if it scatters all wavelengths with roughly equal strength, it looks white. Light reflection can be increased or decreased by applying on the glass surfaces thin films (coatings) of elements, oxides or other compounds with a refractive index higher or lower than the refractive index of the glass. When reflectance is different for various wavelengths, the glass will appear coloured. Depending on the glass composition different visual effects can be obtained. If colorants are employed the transmitted light of a large variety of colors can be achieved. If thin films of certain oxides are deposited on glass a dichroic effect can be observed, where the color changes with the angle of vision. Finally, when some rare earths are introduced in the glass composition, they can emit light of different colours under UV-light depending on the element that was used. In this exhibition, a section is dedicated to the interaction of light with glass and several interactive experiments are displayed for the public. An extraordinary example of the variety of colours obtainable for glass mosaic tesserae is shown in the panel of the Angelo Orsoni factory (Venice, Italy) created more than a century ago with the purpose of astonishing visitors at the International Exhibition in Paris, 1889. Several dichroic glasses are also exposed, where colour varies according to the observation angle. In another interactive experience the light from a halogen bulb and from a LED bulb is decomposed using a prism showing the differences between the two light sources. Several glasses with rare earths are also exhibited, which under natural light are colourless, but under UV-light, several luminescent colours can be observed.

Orsoni Panel – Glass Colour
Glass colour is a complex result of various parameters. It depends on the light source, the glass light absorption, reflectance properties of the surfaces, angles of illumination and viewing. Colours vary in several different aspects, including hue (shades of red, yellow, …), saturation, brightness and gloss. Without a deliberately-added colourant, colour of glass depends largely upon the presence of iron (a contaminant of raw materials). Glasses contain iron in both oxidation states, generating a range of yellows, greens, bluish-green hues, the so-called “natural” colours of glass, which depend on the iron concentration and its oxidation state. To remove colour and obtain colourless glass a chemical decolorizer such as MnO2 or Sb2O5 is added. The decolorizer converts Fe2+ (strong green-blue) to Fe3+ (week yellow) according to the reaction: Mn3+ + Fe2+ → Mn2+ + Fe3+ Physical decolorizing alters color towards neutral gray by adding red (selenium) and blue (cobalt) components to the glass. Manganese is both a chemical and physical decolorizer, since the purple color due to a small excess of Mn3+, neutralizes the residual yellow of Fe3+. Different types of chromophores can be used to colour glass:

a) Ions
When ions of transition elements such as chromium, manganese, iron, cobalt and copper are dissolved in glass, it looks coloured. Colour depends on the ion, its oxidation or reduction state, its coordination number, concentration and glass composition. Ions like cobalt and nickel are dissolved in the glass only in the form of ion 2+. Small concentrations of cobalt oxide (0.025 to 0.1%) yield blue glass. Depending on concentration, nickel gives blue, or violet, or even black glass. Lead crystal glass with added nickel acquires a purplish colour. Ions like copper, iron, manganese and chromium can change their oxidation state in glass. 2 to 3% of copper oxide produces a turquoise colour, or green when associated with oxidized iron, however Cu+ is a colourless ion. Chromium in the oxidized form Cr6+ is a very powerful yellow colouring agent, while in the form Cr3+ it yields dark green colour. Another kind of colouring ions are rare earths which can also give fluorescent colours. For example uranium (0.1 to 2%) gives glass a fluorescent yellow or green colour. When used with lead glass with very high proportion of lead, uranium produces a deep red colour. Particular colours that cannot be obtained in bulk glass are given by superimposed layers of different coloured glass.

b) Nano-particles (colloids)
Colloidal suspensions in a glassy matrix of metallic Cu (dark red), Au (ruby red) and Ag (yellow) particles produce strong colours. “Striking”colours refers to colours that form their colloidal coloring centers during a heat treatment to help aggregation of the metal atoms into colloid particles 10-100 nm in size. Selenium imparts a reddish colour, caused by selenium nanoparticles dispersed in glass. When used together with cadmium sulfide, it yields a brilliant red colour known as "selenium ruby". Cadmium together with sulphur forms cadmium sulfide and results in deep yellow colours. Colloidal particles of metallic copper (or cuprite Cu2O) produce a deep red transparent glass. Metallic gold, in very small concentrations (around 0.001%, or 10 ppm), produces a rich ruby-coloured glass (ruby gold or "Rubino all’oro"). Silver can produce a range of colours from orange-red to yellow. Silver stains (or luster-painted, where copper is associated to silver) are obtained by applying over the glass surface a silver compound (sulphates, chloride, …) dispersed in a clay and then fired at about 650 °C for a few minutes and then slowly cooled. The method results in the exchange of silver with alkali ions from the glass, and subsequent reduction of silver to metal and growth of silver nanoparticles.

c) Pigments (coloured inclusions)
When microscopic phases with a refractive index different from the glassy matrix are uniformly and abundantly distributed, the incident light will be scattered and reflected in all directions (diffusion) and the glass will appear opaque white. This effect can be obtained by separation of small crystals during cooling (calcium antimonate crystals separate as the transparent melt is cooled), or introduction of a finely dispersed refractory non-soluble compound during melting (tin oxide crystals are added to the batch in form of tin-lead calx prepared by firing a mixture of metallic tin and lead). The presence of increasing amounts of crystals dispersed in a coloured glass matrix gives progressively clearer hues. When microscopic crystals absorbing selectively part of the visible spectrum (pigments) are uniformly and abundantly dispersed in a glass matrix, the incident light will be selectively absorbed, scattered and reflected in all directions. The glass will appear coloured and opaque. Cuprite (Cu2O) in the form of large crystals colours glass in a deep red hue, while in the form of small crystals colours glass in an orange hue. Finally, metallic copper micrometer particles give a brown-red glass. Yellow pigments are prepared in form of lead-tin, lead-antimony and lead-tin-antimony crystals in high-lead glass and then dispersed in a glass melt. The number of hues can be extended by addition of iron and zinc. An extraordinary example of the variety of colours obtainable for glass mosaic tesserae is the panel of the Angelo Orsoni factory (Venice, Italy) created more than a century ago with the purpose of astonishing visitors at the International Exhibition in Paris, 1889. This panel is also in this exhibition. A smaller panel is also presented with metal leaf tesserae which are composite materials made of a leaf of gold, silver or their alloys (less than 0.5 micrometer thick) hot fixed between two glass layers. The glass embedded in the mortar (the support) is 5 to 10 mm thick, while a thin sheet of blown glass (the cartellina, usually 0.3 to 0.7 mm thick) protects the fragile metal foil and adds to its brilliance. The colour of the metal leaf tesserae depends on the colour of the metal leaf, the colour of the cartellina and also of the support. 

Dichroic glasses
Glass may be prepared with mono- or multilayer coatings to provide special colour effects. Colored, dichroic, mirror glasses have one or several coatings (metals, metal oxides, or nitrides) in the nanometer-range. Dichroic glasses presented in this exhibition are formed by a special optical coating applied to glass. This coating is formed by a series of thin films alternately with a low and a high refractive index. The periodic structure formed by these layers, causes by interference, selective reflections and transmissions of light. The intensity of the effect depends on the number of layers. The thickness of each layer on the glasses is about 100 to 150 nm and the number of layers varies from 14 to 28. Notice that the colour varies according to the observation angle. The dichroic glasses are used for several applications, namely in anti reflective coatings, filters, sun glasses, decorative panels, etc. The dichroic glasses used for artistic applications, have usually alternate layers of silica and zirconium or tantalum oxide.

When the light goes through glass - glass decomposition of light
When a beam of light goes through glass part of it is reflected and part is transmitted. It is possible to observe a change in the direction of the light beam, due to the different refractive indices of air and glass. The refractive index of glass is the ratio of the speed of light in vacuum to the speed of light in glass. The refractive index of lead glass (crystal glass) is higher than that of common soda lime silicate glass, which is the reason why it is more brilliant. Light is an electromagnetic radiation whose propagation can be represented by an electromagnetic wave. The white light is composed of all the frequencies in the visible range, going from about 7.7 x1014 to 3.8x1014 Hz (corresponding to wavelengths from about 390 to 780 nm). A glass prism bends a beam of white light and the angle changes with the wavelength. This means that the light is decomposed into its constituting colours, resulting in a rainbow, with colours changing from violet to red, as it is possible to observe in the following scheme.

The composition of artificial light depends on the source. For example the filament in the halogen lamp is heated by the electrical current until it becomes incandescent; most of the energy is emitted in the infrared region which is not visible. The resulting visible spectrum is uniform with more energy in the higher wavelengths (red). In the case of LED it emits blue light (450 nm wavelength); a coating on the LED is used to convert the blue light to a broad-spectrum of white light. The resulting visible spectrum has two main peaks (blue and yellow).

 

The light decomposition emitted by a led lamp is shown below.

 

Luminescent glasses with different rare earth oxides
One of the many processes by which a molecule can dissipate its energy, when a photon is absorbed, is called luminescence, light emission of ultraviolet, visible or infrared photons from an electronically excited species. A luminescent material can be fluorescent or phosphorescent. Fluorescence is the emitted radiation originated from an excited state that has the same spin multiplicity, as the ground state (S1 ◊ S0 relaxation). On the other hand phosphorescence is originated from a de-excitation of an excited state with a different spin multiplicity, as the ground state one (T1 ◊ S0). A major difference between this two luminescence types is the characteristic decay times, since in fluorescence if the source of excitation is turned off the emission decays so fast (10-9 – 10-6 seconds), as to be immediate for the human eye. In phosphorescent materials the emission decays much more slowly, and can be observed by the naked eyes sometimes during several hours. Luminescent glasses are commonly synthesized using rare earth oxides. These glasses display interesting luminescent colours which can be tuned by changing the element and the composition of the glass matrix. Rare earths, according to IUPAC (International Union of Pure and Applied Chemistry), correspond to the elements 21 (Sc), 39 (Y) and from 57 (La) to 71 (Lu). The Lanthanides is a term (Ln) used to designate the elements from 57 (La) to 71 (Lu). Contrary to what the term “rare earth” suggests, some of these elements exist in relatively large quantities and are commonly used for several applications, for example in electroluminescent devices, biomedical applications, chemical sensors, catalysts or even in decorative and art objects. Soda-lime silicate glasses produced with different rare earth oxides were displayed in this exhibition. Under natural light no colour could be observed, however under UV-light (in this case using UV LEDs, ca. 365 nm), several luminescent colours could be obtained: red, blue, green and yellow due to the incorporation in glass of europium (Eu2O3), cerium (CeO2), terbium (Tb4O7) and dysprosium (Dy2O3) oxides, respectively.

 

 

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