Collyweston slates have been made for hundreds of years by mining the fissile rock, known as log, and in the winter, when a there was the promise of a cold spell, laying it out in the fields for the frost to split it. During the late 20th century there were less cold winters. Consequently the traditional craft declined and many historic roofs were replaced with fake slates. But there were still plenty of skilled slaters so what was needed was a method of frosting the log without having to rely on a cold winter. This would also make continuous rather than seasonal production possible.
The slate log lies on a bed of hard sand which was removed with a pick and the log supported on pillars of waste stone. After undermining the log to a depth of twelve or fifteen feet the pillars were knocked out and the log allowed to fall.
In the Winter the log was laid out in the fields and kept wet throughout the days and nights. Once the frost had opened the fissile layers the process was completed with a cliving hammer.