Read somewhere recently that pieces of slate were used as plates. In 1853 a triangular clayslate slab symbol stone from Firth in Orkney was given to the national museum (NMAS 1892 inventory IB24) by Petrie in 1853
Petrie was actually born in 1853 and died in 1942 ...he was probably the only man that would have been able to give me the info i seek lol ...a true genius.