Once the master lacquer has been cut, it must undergo metallisation to become conductive and serve as the cathode (negative electrode) in the subsequent electroforming process.
This metallisation involves coating the front surface of the lacquer with a thin layer of metallic silver. To achieve this, two pressurised reagents are sprayed in converging streams so that their chemical reaction takes place directly on the rotating surface of the lacquer, which spins at a constant speed.
The underlying chemical reaction is similar to the Tollens’ test, and involves the following components:
- Silver nitrate (AgNO₃) – the substance that is reduced;
- Glucose (C₆H₁₂O₆) – the reducing agent;
- Concentrated ammonia (NH₃) – the complexing agent
During the reaction, silver ions (Ag⁺) are reduced to metallic silver (Ag), while glucose is oxidised to gluconic acid. This process is not spontaneous; it requires an alkaline environment and a sufficiently high temperature.
The use of concentrated ammonia prevents the formation of silver oxide and helps maintain a relatively stable reduction potential for Ag⁺.
The full chemical reaction is:
and produces Metallic Silver (Ag), Gluconic Acid (C6H12O7), Ammonia (NH₃) and Aater (H2O).
To accelerate the reduction of Ag⁺ to metallic silver, the lacquer is first immersed in a solution of tin(II) chloride (SnCl₂) before the reagents are sprayed.
For the reaction to proceed correctly, the lacquer surface must be thoroughly cleaned. This includes pre-degreasing and rinsing using effective but non-invasive methods and products.
The resulting silver layer is extremely thin — just a few microns thick — and serves solely to provide electrical conductivity for the electroforming of the first nickel layer, known as the "metal master".
There are additional preparatory steps that, while not technically complex, are essential. Among them is the removal of the lacquer coating from the centre of the back of the disc, exposing the aluminium core.
The lacquer disc is built on an aluminium substrate. For proper electrical contact during electroforming, the lacquer must not only be metallised on the front but must also maintain electrical contact with the motor shaft, which is connected to the negative terminal of the generator (the cathode). This contact is established through the exposed aluminium on the back of the disc.
After the first metal master is separated from the lacquer, the silver layer remains adhered to the nickel that has formed on top of it. However, due to its fragility, the silver should be chemically removed before continuing with further electroforming steps to prevent the replication of any surface defects.
Several effective methods exist for dissolving silver; one widely used technique involves the combined use of ammonia and hydrogen peroxide, resulting in the following reaction:
It is a reaction that occurs progressively:
- ammonia reacts with metallic silver to form a soluble complex in which the silver is only partially oxidised.
- hydrogen peroxide accelerates the oxidation process, allowing for complete dissolution of the silver layer.
Silvering