Here is a small step-by-step guide How to Prepare Expanded Clay Pebbles for Growing Cannabis. Clay Pebbles are a very popular and excellent planting medium.
Clay is chemically neutral, stable in its structure and ensures excellent drainage and aeration of the roots. The ph-value of the expanded clay (hydrocorrel) is approx 8 when purchased so it ideally has to be adjusted so that its Ph is typically 5.5 to 6.
This is how the Clay Pebbles are Prepared: