  We review the existing mathematical models which describe physicochemical
mechanisms capable of producing a symmetry-breaking transition to a state in
which one chirality dominates the other. A new model is proposed, with the aim
of elucidating the fundamental processes at work in the crystal grinding
systems of Viedma [Phys Rev Lett 94, 065504, (2005)] and Noorduin [J Am Chem
Soc 130, 1158, (2008)]. We simplify the model as far as possible to uncover the
fundamental competitive process which causes the symmetry-breaking, and analyse
other simplifications which might be expected to show symmetry-breaking.
