The inherent strength of aluminium extrusions means that they can be manufactured as quite slim profiles.
Also, secondary glazing is generally designed so that its vertical and horizontal profiles line up with the outer frame, transoms (horizontal) and mullions (vertical) of the host window or door. The frames can therefore appear almost invisible from outside.
For most suppliers and installers, the aluminium extruded frames are white as standard but can generally be supplied to match some woodgrain finishes and any official RAL colour.
RAL refers to a colour matching system first developed in the 1920’s to allow architects, manufacturers and specifiers to refer to a distinct colour by a code name so that everyone would understand exactly which colour it was meant to be, without having to exchange samples.
The system has been developed over the years to include both matt and glossy surfaces and is divided into various classifications.
In common parlance, RAL refers to the RAL Classic classification, and includes powder coating, a system of colouring aluminium.