Dichroic glass is a special type of glass in which the iridescent color is obtained by depositing thin layers of metals or their salts on its surface. These are, in effect, optical interference filters. These deposited layers have a precise thickness, controlled with nanometric precision, which determines the final color of the dichroic glass. The combination of different metals, or the thickness applied, creates a variety of chromatic effects, such as iridescent, shimmering and multicoloured reflections. It is created using special equipment in which an ultra-high vacuum is created. The glass to be coated is placed in a chamber where it can be subjected to two different types of deposition: evaporation or sputtering.
Evaporation: In the vacuum chamber, the metal material that will form the deposited film is heated until it reaches its evaporation temperature. Since there is a high vacuum, the vaporized metal atoms do not encounter any resistance and will be deposited on the glass, creating a thin and very adherent film. The evaporation rate and the distance between the source and the substrate control the thickness of the film.
Sputtering: An ionized gas (plasma) bombards the metal target, causing the emission of atoms that are deposited on the glass. The thickness of the film is controlled by adjusting the plasma current, the gas pressure and the distance between the target and the substrate. The commonly used materials are:
Titanium (TiO2): produces yellow, orange, red and purple colors.
Niobium (Nb2O5): produces blue, green and yellow colors.
Silicon (SiO2): produces neutral and transparent colors (used for protection).
Other metals such as Chromium, Aluminum, Zirconium and Magnesium, or their salts, can also be used to obtain a variety of colors and shades. Specific software can be used to simulate the optical properties of dichroic glass and to predict the effect of different combinations of materials and thicknesses. New deposition technologies, such as chemical vapor deposition (CVD) and physical vapor deposition (PVD), are emerging to improve the properties and versatility in production. Research is also focused on the development of dichroic glasses with advanced properties, such as photoluminescence and electrochromism.
Optical properties.
The main characteristic of dichroic glass is its dichroism, or the ability to assume different colors depending on the angle of observation and the incident light. Another very important characteristic is the chromatic purity; the observed color has a very narrow wavelength band making the final color very pure and bright. The intensity and variety of colors are depending on the thickness and combination of metal oxides used in its production.
Applications.
Dichroic glass is a versatile material and is widely used to achieve iridescent and metallic effects in glass. It is mainly used to create mosaics, jewellery, souvenirs, stained glass (Guggenheim Museum in Bilbao, designed by Frank Gehry) and design objects (Dale Chihuly and Jeff Koons). Dichroic glass is a fascinating element for the decoration of fused glass. It can be used with different techniques, the main ones are incalmo and pâte de verre. In the incalmo technique, dichroic glass is superimposed on transparent or colored glass and then fused together. It can be used in different forms: sheets cut to required size and incorporated into the fusion of transparent or colored glass. Strings: thin strips of dichroic glass that can be used to create lines, curves and complex designs. In the pâte de verre technique, dichroic glass is broken into small fragments of different sizes and colors which are then mixed and fused together with glass granules. The melting temperature and thermal expansion of the dichroic glass must be compatible with that of the base glass used, to avoid breakages during cooling or even after a long time from production (thermal stress). A sample test, using the thermal shock test, can help to determine the best combinations and firing conditions.
The melting temperatures are around 800-850°C.
Compatible glasses: borosilicate and float.
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