Thank you most of the Staffordshire hoard was cleaned with a thorn i can't claim credit for the idea :-(, most likely liver of sulfer.
Thank you, nearly right type 1 Rodbert London, ref EMC/SCBI NUMBER:2022.0414 in this link only 13 recorded including this coin. https://emc.fitzmuseum.cam.ac.uk/advanced-results?rulername[]=Stephen&mint[]=London&MoneyerLemma[]=Rodbert&Find-spot=&emcnumb=&Preservation=&startdate=&enddate=&emc-scbi=