Once the silver layer is formed on the glass, methods to protect the silver layer from oxidation are employed. A layer of copper is then deposited directly onto the silver. Copper can be applied in two ways: chemically or galvanically. Recent technological advances have lead to the development of copper free protective films, which also prevent silver oxidation.
Once the metal layers are attached to the glass, they are covered by a protective mirror backing paint. The mirror backing paint protects the metal layers from corrosion and from mechanical scratching. The paint can be applied either by passing the glass through a curtain of paint or by passing glass in contact with a roller paint coater. There are many mirror backing paint products available from a number of suppliers. They offer paint systems that are applied as a single coat or double coat. Both coating systems are effective.
Tinted mirrors are produced using the methods described above. The silver coating is applied to one of the various tinted glass substrates available on the market. Tinted mirrors are generally used in decorative applications where color and diminished light reflection are desirable.
Quality requirements for silvered annealed monolithic clear and tinted flat glass mirrors are provided in the ASTM document C 1503 Standard Specification for Silvered Flat Glass Mirror.
Tempered mirrors are
manufactured using fully tempered glass as the substrate. There are
optical characteristics inherent in tempered mirrors, including roll
distortion and the lack of a quality surface for silvering.
Safety backed mirrors are known as Organically Coated Mirrors in the CPSC 16 CFR 1201 and ANSI Z97.1 standards. These are manufactured by applying a sheet of adhesive backed polyethylene material to the back of annealed mirrors. The backing material does not prevent breakage of mirrors, but lessens the potential of injury on impact by retaining the fragments.
Pyrolytic mirrors are highly reflective coated glass products with performance characteristics approaching that of silvered mirrors. This product is promoted for use in shower doors and other areas where moisture can affect the substrate of silvered mirrors.
Transparent/two-way mirrors are composed of reflective glass products, and as such are not silver mirrors. Transparent mirrors are manufactured by both the pyrolytic deposition and vacuum deposition coating processes. Heavy density coatings are offered on clear and gray tinted glass.
Transparent or two-way mirrors are designed to permit vision through one direction while giving the appearance of a standard mirror from the opposite side. Their major application is to permit undetected observation for study or surveillance in interior conditions such as learning centers in schools and universities, medical and psychiatric clinics, and security stations in casinos or high-traffic retail stores.
The transparent mirrors work by reducing the visible light transmittance through the glass. To ensure proper performance the room lighting design and surrounding conditions must be carefully planned and executed. The glass surface in the subject room must appear to be standard mirror. In order to achieve this condition, the coated surface should be toward the subject room and the lighting ratios tightly controlled. For applications utilizing clear glass, manufacturers recommend a lighting ratio of 10:1 subjectís side to observerís side. If the lighting ratio drops to approximately 5:1, the subject may detect movement or silhouettes through the mirror. If 10:1 lighting ratios cannot be maintained, a gray transparent mirror should be specified. Lighting ratios of 5:1 can be successfully used for gray transparent mirror products.
Design considerations call for bright contrasting colors in the subject room and dark, non-contrasting colors in the observer room. Light color surfaces or objects may be noticeable to the subject. The design of the observation room should also prevent sudden light ratio changes. Special care must be taken if transparent mirrors are used on more than one wall.
The above information is from the GANA Glazing Manual, 2004 Edition - the most frequently referenced resource in the architectural glass and glazing industry. The Glazing Manual is an excellent addition to any technical library. Go here to order a copy of the manual or CD-ROM. For further information on this and other GANA reference documents visit the PUBLICATIONS section of the GANA website.
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