Trying to prevent more deterioration by pollution of the late 15th-century painting, Italian officials limited viewing to groups of 20 people for 15 minutes at a time, television reports said. By next year, a reservation system should be in place to shorten waiting times, the reports said.
The painting, on the wall of a dining hall in a monastery adjacent to Santa Maria della Grazie church, has undergone 19 known restorations in the last four centuries.
The latest one, begun in 1978, aimed to restore badly faded colours and undo some of the damage inflicted by previous restorations.
In 1995, visitors began passing through two separate glass cabins. Between the cabins, filters remove impurities brought in from the outside, such as dust, and pump in clean air.
Leonardo's use of tempera - a technique less durable than true fresco painting, which is done on wet plaster - is partly responsible for the work's severe deterioration.Reuse content