Complex sentence: In some instances, these wires may even be aluminium, a practice which was common in the 1960s and 1970s, following a massive increase in the cost of copper as a raw material.
Split into simpler sentences: In some instances, these wires may even be aluminium, a practice which was common in the 1960s and 1970s. Essentially, this decision followed a massive increase in the cost of copper as a raw material.