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Color Center Formation - SolarizationEach sculpture starts as a crystal clear block of acrylic. It is then subjected to such incredible energy that three feet of concrete is required to protect the technician. Such high energy often causes tinting of the material, especially on the side from which it is exposed to the electron beam. Such changes in color and light transmittance is called solarization in the glazing industry, and color-center formation in physics.
Poly(methyl methacrylate) (PMMA), the technical name for acrylic, exhibits a decrease in light transmittance through absorption which is obvious in the wavelength range of 350-600 nm. Most of this absorption is in the range of 400-500 nm, which corresponds to the violet and blue portion of the color spectrum. A decrease in blue light is perceived by our eyes as an increase in yellow and red light. Thus, solarized acrylic has a beautiful yellow to amber tint.
Solarization is most visible when viewed through an edge.
It is barely visible when viewed through the front or back side. Many pieces in our inventory have less solarization because they have faded over time.
There are a number of color centers with different electronic configurations, absorptions, and behaviors. Each type is identified by a single-letter name either in recognition of its discoverer or following the whim of the moment.
The most likely color center responsible for solarization of PMMA is the F-center, which is an electron trapped in a negative ion vacancy. (The "F" stands for Farbe, the german word for color.) However, physics literature as late as 2000 attributes PMMA solarization to "an unknown color center" and identifies some behavioral differences from the F center. So, solarization of PMMA might still remain a mystery.
Color Center Annihilation - BleachingThe PMMA color centers can be annihilated by means of thermal annealing (bleaching) and several other processes. There is also some spontaneous annihilation at normal temperatures over long periods of time, particularly near the surfaces of the block, which results in the fading and thinning of the solarization band and a subtle shift in hue. In order to preserve this interesting effect, we no longer bleach our figures.
WARNING: DO NOT attempt to use thermal annealing to rapidly bleach your own figure. Acrylic gives off highly flammable fumes when decomposed by overheating. These gases are potentially explosive if allowed to collect in an unventilated area. Most kitchent ovens do not have accurate controls to prevent overheating and melting, and do not have adequate ventilation to prevent the dangerous build-up of explosive fumes.
The solarization does not affect the presentation of your Lichtenberg figure under recommended lighting.