New generation mirrors are more environmentally friendly.

Recent developments in mirror production technology have resulted in improved corrosion resistance and a more environmentally friendly process. Here Christian Dauby(*) describes how mirror production processes have evolved.

There have been a number of interesting developments in mirror production

technology during the past decade. What has happened is that the process has become a great deal `cleaner' in an ecological sense, whilst there has also been an appreciable improvement in mirror quality, particularly as regards resistance to corrosion - the key to the age of a mirror. This combination of positive effects is unusual. Generally-speaking, a reduction in the environmental pollutant burden created by a product goes hand-in-glove with an appreciable deterioration in the way in which that product performs.

Glaverbel plays a dominant role in developments within these areas of technology, which are the fruit of major efforts on the part of the company's R & D Division. The `traditional' approach to mirror manufacture (the method used by Glaverbel up to 1994, and still used by many companies) involves a number of different stages. Float glass is laid horizontally onto a roller-type conveyor, which is usually referred to as a `silver coating conveyor'. It then undergoes a number of different processes.

Glass cleaning

The glass surface is brushed, mechanically optimising the action of the cerium oxide suspension laid on the glass. This important operation gets rid of dust and other materials, both greasy and non-greasy, that may have found their way onto the surface during manufacture or during transfer to the mirroring shop. These impurities can seriously compromise mirror quality, since they can prevent the deposit of a reflective silver layer or alter the way in which this happens.

Heavy rinsing

The glass then undergoes a heavy rinse under a demineralised water spray, after which it is coated with an acid stannic chloride solution in order to adsorb any complex tin ions on the surface of the glass. These ions will have a catalytic effect during the initial phase of silver coating.

Silver coating

Once the glass has been `sensitised' in this manner, it is rinsed with de-mineralised water and it then passes on to the silver coating section. Here, a layer of silver, which forms the reflective surface of the mirror, is applied using a wet chemical process. An ammonia silver nitrate solution is sprayed onto the glass at the same time as a solution containing a reducing agent (a wide variety of products is available on the market, but suppliers' formulae are closely-guarded secrets). Once the two solutions come into contact and following the action of the tin complexes on the glass surface, silver is deposited and gradually forms a layer weighing approximately 1g per [m.sup.2].