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Often, in electroplating, we use an inert anode, where metal is not dissolved away, and
control the concentration of the depositing metal in the electrolyte solution by direct
additions of the appropriate metal as a salt to the electrolyte.
The amount of metal – in our case gold – deposited is governed by Faraday’s law which
says that : The weight of metal deposited is proportional to the quantity of electricity
passed.
The quantity of electricity is defined as the current (in amps) multiplied by the time (in
hours). The weight of metal deposited for a given quantity of electricity will be different for
different metals which is related to their atomic number and valency through a factor
called the electrochemical equivalent.
This Faraday law is very useful in calculating and controlling the amount (weight or
thickness) of metal deposited on a piece of jewellery. Obviously, at a constant
electroplating current (and salt concentration in the electrolyte), thickness of the
electroplate is directly proportional to the plating time. Double the plating time and you
double the thickness.

Factors Affecting the Electroplating

Process

For decorative applications, we usually require a uniform thickness of electroplate over
our complex shaped item. This can be a problem at sharp edges and recessed surfaces,
for example. We usually also want a bright deposit, with good adhesion to the underlying
item. We do not want the electroplated deposit to be highly stressed with a tendency to
crack and spall. We may want to plate at high speeds and still retain a good uniform bright
surface. We do not want a porous or micro-cracked coating which could allow corrosion
or tarnishing of the item during subsequent wear.
If we are co-depositing more than one metal, i.e. a carat gold, we also want good control
of composition – a uniform gold content over all the surface and throughout the thickness,
for example.
So how do we control these factors? Well, we achieve this through a number of ways:
 Control of electrolyte formulation and pH (a measure of acidity or alkalinity)
 Control of anode surface area and position
 Control of electrical conditions
 Control of temperature
Firstly, the electrolyte. A good electrolyte will contain the metal (or metals) to be deposited
in solution in a sufficient concentration. In cyanide based gold baths, this will be in the
form of gold potassium cyanide salt. It will also contain other additives to give good plating
properties, These include, for example, additives to improve:
1. the throwing powerof the bath which means good uniformity of thickness over the
piece being plated.
2. the brightnessof the deposit. Special brighteners are added to assist.
3. the internal stress in the deposit. These additives control the build-up of stress to
prevent cracking and spalling.
4. the chemical stability of the electrolyte and may include buffering agents to
control pH which is a measure of the acidity or alkalinity of the electrolyte.