Recently the apologists for artefact hunting have begun claiming that they "rescue" objects in the ploughsoil from destruction by farm machinery and agricultural chemicals. A typical example is here. Discussing this in detail is beyond the scope of this blog, while there is no doubt that the ploughsoil is a hostile environment for archaeological artefacts, after having looked at the evidence for this very carefully (it is a whole chapter in the forthcomng book I wrote with Nigel Swift - Britain's Portable Antiquity Heritage) I do not accept that the phenomenon is as widespread or as severe in its effects as claimed by the supporters of artefact hunting in Britain.
There is a difference between corrosion and damage which makes a find "illegible" and that which merely makes it less collectable (the difference between corroded crud normal on archaeological artefacts from certain soils and what collectors consider a "patina").
Vignette: patina on a Chinese bronze