The invention relates to methods of altering the appearance of decorative objects, such as gemstones, by coating the decorative objects with thin film coatings that provide color via optical absorption to improve the appearance of the objects.
A number of processes have been developed to improve the appearance of gemstones or to create simulated gemstones. For example, methods of diffusing ions into gemstones (e.g., diffusing ions of titanium and/or iron into sapphire, or diffusing ions of cobalt into topaz) have been disclosed. These diffusion methods, however, traditionally have been limited to specific ions and specific substrates. Moreover, diffusion methods typically involve extremely high temperature, which frequently causes breakage or damage of the gemstones. Diffusion methods characteristically cause the added ions to become part of the crystal surface with no distinct boundary. In fact, diffusion methods commonly leave a gradient of ion concentration in the substrate (e.g., in a gemstone). Diffusion methods typically require long processing times, commonly more than a day. Reference is made to U.S. Pat. Nos. 2,690,630 and 4,039,726.
Nuclear radiation has been used to produce color centers in gemstones, giving a body color that in some cases can be improved with heat treatment. Cyclotrons and neutron bombardment are routinely used to impart blue color in colorless topaz. This method does not involve coating the stone. Rather, it produces color centers throughout the stone. A disadvantage of this method is the requirement for a “cooling off” period to allow the topaz to radioactively decay to a safe level. Traditionally, it has only been possible to obtain shades of blue with this method. Impurities in the gemstone (and the nuclear process used) determine the particular shade of blue that is obtained. Thus, it is difficult to obtain a consistent color on any given lot of gemstones.
Rhinestones and Carnival Glass have reflective coatings layered on one or more surfaces of a clear substrate. The coating is usually silver or some other highly reflective material utilized to apply a mirror coating (usually silver or aluminum) onto the back (e.g., the pavilion) of a faceted glass gem. In such a coating, substantially all of the light is reflected without passing through the coating. U.S. Pat. No. 3,039,280 is representative.
Commonly-assigned U.S. Pat. No. 5,853,826, issued to Starcke et al., discloses desirable coatings for enhancing the optical properties of a decorative object, such as a gemstone. The coatings impart in the decorative object a desirable colored appearance, wherein the color of light reflected from the decorative object to a viewer changes with angle of observation.
Tavelite™ is a product produced by depositing thin multiple layers on a transparent substrate to produce an interference effect. The coating is believed to be deposited, at least in some cases, through a process that involves high temperatures. When gemstones are coated at high temperatures, considerable breakage can occur.
U.S. Pat. No. 6,197,428, issued to Rogers, assigned to Deposition Sciences, Inc., is believed to disclose the coatings and deposition methods that are used for some of the Tavelite™ products. The Rogers patent teaches an optical interference coating that is applied over substantially the entire surface of a gemstone. The coating comprises alternating layers of materials with relatively high and low refractive indices. The coating is said to be composed of materials that are substantially free of absorption of light (i.e., visible radiation). In particular, the optical interference coating is said to impart in the coated gemstone perceived color that is dependent on the angle of incidence and the relative positions of the object and the viewer.
Layered coatings on a surface of a gemstone have been provided to increase the “fire” of the stone. These techniques involve a first coating of a highly refractive material, with respect to the gemstone's index of refraction, followed by a second coating of a different highly refractive material. The layers are designed so that the light reflected at each interface of each layer causes an optical interference effect. Coatings of this nature are described, for example, in U.S. Pat. No. 3,490,250.
Aqua-aura™, a product of Vision Industries, is a surface treatment providing a single moderately saturated color. The surface treatment is proprietary, but is believed to involve a gold-based coating that is deposited by spraying at high temperatures. The Aqua-aura stones have a metallic sheen and a substantial dichroic appearance. For many applications, it is desirable to provide coated stones that do not have a dichroic appearance, as stones of this nature have a particularly natural appearance.
Atmospheric pressure chemical vapor deposition has been used to deposit films of titanium oxide by thermal decomposition of a titanium compound (usually TiCl4) in air.
Colored lacquers have been painted onto the pavilion of gemstones to give the stones a colored appearance. Unfortunately, these lacquers tend to have poor durability, and have been found to come off easily.
It would be desirable to provide durable coatings that can be applied at low temperature to gemstones and other decorative objects to impart in the decorative objects a body color that appears substantially constant at different angles of observation.