I spoke to mother. She says that in Scandinavia they used a spoon/spatula to remove the bulk, then to fine clean, water and birch twigs.
The birchtwigs : you take a small handful of thin fresh birch twigs, remove the leaves. Bend in half and tie off the end with another birch twig.
Lasts for several weeks. She said it was almost as good as those 3M squares, better than those on Teflon.
She also said that the peasants usually had two cauldrons of metal, one for soups and stews (salty food) and one for porriges
She said that they were not cleaned much, as they were used daily.
If a meat stew was made, the next food was a soup made with remains of the stew.
People were very poor in Scandinavia and Finland, so it makes sense.
I am sure something similar was done in UK.