by Dr. Sagar N. Venkateswaran
A mirror, according to current usage, is created when a reflecting layer of silver is deposited on to glass and prevented from tarnishing by protective coatings. By applying silver to the glass using "silvering solutions," reflective surfaces (and conductive surfaces for electroforming applications) of various shapes and forms can be obtained. For example, on flat surfaces flat mirrors are formed while on curved surfaces the familiar "security mirrors" are produced. In this article we focus on the production of mirrors on glass. Minor modifications to this process can produce reflective and conductive surfaces on other surfaces such as plastic, wood, etc. Note also that the introduction of contaminants cantarnish the silver selectively and create the so-called "antique mirror."
We will now discuss briefly the steps involved in the manufacture of mirrors. The chemicals used to form a silver surface on glass are called "silvering chemicals" and are usually sold in concentrates that are diluted before application. The solutions can be applied either by pouring or spraying. The steps are nearly the same whether the chemicals are applied manually or automatically using conveyors. Depending on the method of application and usage, silvering chemicals are formulated to produce bright reflective surfaces of silver with high efficiencies.
The quality of the glass is critical to mirror quality. For example, undulations on the glass surface can produce mirrors with distortion. While this effect can be appealing to children, it is not usually attractive to adults. The use of "mirror-quality" float glass can obviate this problem.
In the first step, the glass is polished using an abrasive slurry of white rouge, for example. This removes the first few thousand layers of the glass surface, thus exposing a virgin surface upon which the silver will be deposited. Polishing is followed by rigorous rinsing to remove all foreign material. Water quality is an important consideration. Impurities in water can contribute to silver tarnishing. The impurities can also cause silver to "sludge," forming a silver mud, meaning that less silver is available for silvering. For this reason de-ionized water (DI water) is used for dilution of the silvering concentrates and for all subsequent steps.
Polishing and rinsing are followed by "tinning," a process wherein the glass is exposed to a solution of stannous chloride. The tinning solutions may be sprayed or poured onto the surface. The importance of tinning can be gauged by the fact that too little or no tin on the glass surface can lead to pinholes on silver, while too much tin can cause the silver to blacken. Our tinning solutions are proprietary and contain additives to ensure that the entire glass surface is wetted to form a complete layer of tin and thus prevent pin holes in silver. Tinning is followed by rinsing with DI water to remove excess tin from the surface. Now the glass is ready for silvering.
A mixture of silver and reducer is sprayed using separate nozzles that are aligned such that sprays mix on top of the tinned glass. The reaction between the silver and reducer forms the reflective silver surface.
Silvering using the pouring method is not much different. In the pouring method, the diluted solutions are mixed before being poured onto the tinned glass. Silvering solutions such as ours contain proprietary additives that ensure silvering takes place only on the tinned surface and do not sludge. Activators are also added to control the rate of silvering.
The shiny, silver mirror obtained from the previous step is rinsed with DI water to remove any excess chemicals. All subsequent steps fall under the category of "protective coating" for silver. Typically the silver coating is protected by a coat of copper followed by one or more layers of mirror-backing paint for additional protection.
Any contaminant that has managed to get past the mirror-backing must get past copper to attack silver. Copper is, in a sense, the last defense and is allowed to corrode to protect the silver.
Copper coatings may be applied in either of two ways: (1) by spraying using nozzles carrying diluted copper solutions and reducer, and (2) by spraying a paint containing copper powder. The former spray method is popular with manufacturers with conveyorized silvering lines, while the latter and less-expensive method is recommended for smaller silverers.
The copper-backed mirror is rinsed with DI water and dried before application of the mirror-backing paint. [A clear protective coat of paint at this stage, without the copper, results in what is called a "one-way mirror."] One or more coats of the mirror-backing paint are applied by curtain coat, roller or by spraying, according to paint specification. The paint can be air-dried or force-dried depending on paint used and the mirror manufacturer. Once it is determined that the product passes all required specifications, it can be sold rightfully as a mirror.
Dr. Sagar N. Venkateswaran is vice president and co-owner of Peacock Laboratories, Inc. in Philadelphia, PA. The firm, a consultant and supplier for electroconductive, decorative and reflective applications, is dedicated to developmental research in mirror manufacturing.
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