The mirror and sandblasting techniques used on commemorative coins essentially involve treating the metal surface in different ways, altering how light reflects off it. This, in turn, produces distinct effects on the three-dimensional appearance of the design in terms of detail clarity, texture, and visual focus. This difference isn't simply a matter of "bright" or "dark," but rather, the interaction between light and the metal surface creates a different spatial perception of the design's lines, contours, and textures, ultimately guiding the viewer's perception of the three-dimensional effect.
The mirror finish, achieved through high-precision polishing of the surface of commemorative coins, achieves a nearly flat and smooth finish. When light strikes it, it produces a regular, specular reflection—much like a mirror reflects light, reflecting it in a fixed direction without scattering due to surface roughness. This reflective property allows the treated areas to clearly display the fine lines and edges of the design, making them particularly suitable for depicting the detailed text, emblems, or facial details on commemorative coins. When the viewer's line of sight is at the right angle to the reflected light, the mirrored area will appear bright and lustrous. This sheen, combined with the lines of the design itself, creates a sharpness in otherwise flat lines, making them appear as if they were slightly raised from the coin's surface, enhancing the clarity and refinement of details. For example, the year number or theme text engraved on commemorative coins, after being mirrored, eliminates the blurry transitions between the strokes and instead displays a clear demarcation between light and dark, making the text appear more three-dimensional and easier to read, avoiding the loss of three-dimensional effect caused by blurred details.
In contrast to mirroring, sandblasting involves using a high-pressure airstream to spray fine abrasive particles onto the surface of commemorative coins, creating countless tiny pits and bumps in the metal, giving it a delicate matte finish. This rough surface diffuses light—light scatters in all directions upon striking the surface rather than being reflected in a fixed direction. As a result, sandblasted areas lack the intense luster of a mirrored surface and instead exhibit a soft, even, dark texture. This texture makes sandblasting particularly adept at creating a sense of volume and contrast in texture. It's particularly well-suited for creating high-relief features on commemorative coins, such as portraits, architectural silhouettes, or scenic compositions. When sandblasting the raised areas of the relief, the diffused reflection creates a matte finish that creates a distinct contrast between the raised areas and the surrounding areas. Meanwhile, the sunken areas, where light is less scattered, appear darker. This contrast in light and dark clearly highlights the contours of the design, allowing the viewer to instantly perceive its three-dimensional form. For example, in the case of a portrait on a commemorative coin, the facial contours and the depth of the hair are accentuated by the varying shades of matte, creating the illusion of a face with realistic relief rather than a flat print. This sense of volume is the core contribution of sandblasting to the three-dimensional effect.
The two techniques also differ in how they direct visual focus, indirectly influencing the perception of the three-dimensional effect. The bright sheen of a mirror finish is visually appealing, drawing the viewer's attention to the mirrored details. The clarity of these details allows the viewer to perceive the three-dimensional structure of the design from a local perspective to the overall structure. The matte finish of a sandblasted finish, on the other hand, is more discreet and doesn't steal the viewer's attention. Instead, it serves as a backdrop or focal point, highlighting the details of other finishes, creating a "dark background with bright details" or "matte main body with layered details" effect. For example, many commemorative coins utilize a combination of sandblasted main body and mirrored details: the main design (such as figures or buildings) is sandblasted to enhance its volume through diffuse reflection, while the text and decorative patterns on the edges are mirrored to enhance the clarity of the details through mirror reflection. This combination creates a complementary effect between the volume of the sandblasted areas and the detail of the mirrored areas. The viewer can perceive the overall three-dimensional form of the design through the sandblasted areas while also appreciating the intricate details through the mirrored areas, resulting in a more comprehensive and richer three-dimensional effect.
Furthermore, the impact of these two techniques on the 3D effect is tailored to the design and theme of the commemorative coins. For commemorative coins that emphasize refinement and commemorative significance, such as those featuring historical relics or works of art, the mirror finish's ability to render detail better reproduces the fine patterns of the artifacts or artworks, allowing the 3D effect to serve the purpose of "representing authenticity." For commemorative coins that emphasize solemnity and emotional resonance, such as those featuring major historical events or heroic figures, the volumetric and matte finish of the sandblasting process creates a calm and profound atmosphere, allowing the 3D effect to serve the purpose of "emotional expression." Regardless of the specific purpose, the differences between the two techniques are not mutually exclusive. Instead, through their varying control of light reflection, they combine to create a 3D rendering system for the commemorative coins' designs, focusing on both detail clarity and volumetric layering. This allows the coins to possess a tactile quality while retaining distinct, legible details, ultimately achieving a 3D effect that "visually layered and perceptually profound."
