To lubricate clocks effectively is one of the horologist's greatest problems. In the past, mixtures of animal and vegetable oils were used. Great care is necessary in refining oils, to prevent their decomposition into a corrosive varnish-like substance after a few months of running. Only the best-quality oil, which must be of the correct grade for the size of the parts to be lubricated, should be used. The mainspring and escapement require different grades. Many beginners make the mistake of over-oiling clocks, even to the extent of lubricating the wheel and pinion teeth, which are designed to run dry. Only sufficient oil to form a small meniscus in the bottom of the oil sink should be applied to the pivot holes. Frictional rest and recoil escapements should have the pallet faces lightly oiled, although some clock repairers maintain that clocks fitted with the verge escapement suffer less wear if the pallets run dry. The chronometer escapement should never be oiled, except in the pivots of the balance and escape wheel. The mainspring should be lubricated with a suitable heavy oil or grease, and the levers of the striking work should be lightly oiled where they come into contact, as should the crutch's contact points with the pendulum.