Yes, most underground soil/drain is earthenware or now plastic. The famous pottery company Royal Doulton started off doing drains as Doulton & Lambeth in salt glazed pottery which is quite hard wearing.Not sure what my pipes are made of, pretty certain it's not iron, looks like clay. Reckon they were put in around 1970 when the cottages were converted en masse from one down, two up farmworker's dwellings with the toilet down the garden, to having indoor bathrooms and an actual kitchen (the house two down from me still has the massive iron range in the living room where they will have cooked and lived until the kitchen extension was built on the back).