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In the following image, the reflection of the light source is just off the top edge of the piece above the craze cluster. Though most of this piece is covered with crazing, you can only see the crazing near the light-source reflection. The crazing shown is not visible unless the lighting is just right.
Acrylic sheet is manufactured by polymerizing liquid acrylic monomer between two sheets of glass, or by extruding melted acrylic polymer through a die. During polymerization or cooling, the hardening acrylic shrinks. Because the dimensional change is relatively quick, the molecules scrambling to rearrange themselves into a smaller space don't always arrive at a stable symmetrical position before the matrix solidifies. This creates invisible stresses, usually near or on the surface of the material.
When these unbalanced sites are subsequently exposed to energy stressors such as mechanical strain, reactive chemicals, or ionizing radiation such as ultraviolet light, the disorder is increased, which increases the stress. When the stress at a site exceeds the strength of the surrounding material, a tiny short crack forms to relieve the stress. Cracks form in a distinctive pattern that manifests the previously invisible shape of the stresses. This system of stress cracks is called "crazing."
SpecialChem, a specialty chemicals manufacturer, defines crazing as follows: "The so-called crazing phenomenon corresponds to the formation and extension of a craze network in a polymer matrix. A craze is a kind of crack but bridged by fibrils of oriented matrix polymer chains, normal to craze surfaces."
Crazing normally results from exposure to these stressors over a long period of time. However, shorter periods of exposure at a higher intensity will also cause crazing. The ionizing energy in the electron beam used in creating a Lichtenberg figure causes the same stress as many years of ultraviolet radiation, but in just a few seconds.
The electron beam does not actually create the craze inducing stress. It acts on stresses that were created during the acrylic manufacturing process. Therefore, the location of the crazing is determined by conditions during manufacturing, not by proximity to the electron beam. For this reason, the crazing is not necessarily on the same side of the block as the solarization. However, we have usually observed crazing only on one side of the sheet, and rarely on the other side to any significant extent.
Crazing fissures are very narrow, but deep relative to their width. Because of their shape, they are sometimes difficult to observe unless the lighting is just right. They are best observed under high-angle dark-field point-source illumination. Since crazing usually only occurs on one side of our pieces and the recommended lighting for observing the figure is exactly opposite that needed for observing the crazing, the crazing does not detract from the beauty of the Lichtenberg figure.
To look for crazing, try the following procedure:
If you are fortunate enough to find crazing, and you have access to a low power microscope, you might find it interesting to observe the crazes under magnification. Below is an image of nearly the full length of two crazes at 30X magnification. The crazes on this piece are all about 0.127 inches long, which just happens to be close to the 0.13 inch field width of this microscope.
Because the scale of crazing is usually close to the wavelength of light, you may observe a phenomenon called "dichroism". Dichroism is the ability of a material or structure to selectively reflect or transmit specific wavelengths (colors) of light. Through constructive or destructive interference of the light waves, white light is reflected from the crazes in a rainbow of colors. In the two images below of the same craze cluster at different magnifications, the colors are different because the viewing angles are slightly different, another characteristic of dichroism.
Crazing is a result of random conditions that occur during acrylic manufacturing, so we cannot predict when, where, or to what extent it will occur. Many pieces have several crazes on one side. A few have many crazes in clusters which are easy to see, as in the images above. However, we cannot ensure that your Lichtenberg figure will have this interesting bonus feature. Since we documented the crazing for a study on a number of pieces still in our inventory, for a limited time only we can honor requests for crazed pieces at no extra charge.