Repairs are constantly being carried out on Oxford's great classical buildings, writes CHRIS KOENIG

"Beware of Falling Angels" read a famous notice in a decaying Venetian church some years ago. But what is less well known is that many of the great classical buildings of Oxford were in a similarly dangerous state of picturesque decay during the 1940s, 50s and even 60s.

In 1965 for instance, a notice in Broad Street telling passers-by to Beware of Falling Heads would not have been altogether out of place, for in that year the university authorities pronounced the 13 so-called Emperors' Heads outside the Sheldonian Theatre to be so decayed and dangerous as to need replacing altogether, rather than restoring.

The heads have only been known as Emperors since Max Beerbohm described them as Caesars in his book Zuleka Dobson, published in 1911. In Wren's accounts for the Sheldonian they are described as termains, which may derive from the Roman god Terminus, protector of boundaries and boundary stones.

Sir Christopher Wren commissioned William Byrd, a stonecutter with a yard in nearby Holywell Street, to carve the original 14 heads, completed in 1669, but sadly, by 1868 they had become so decayed that new ones were made of poor-quality Headington freestone or Milton-under-Wychwood stone.

These, however, were almost immediately daubed with paint by undergraduates during commemoration week, and the process of removing the paint started off such a rapid degeneration that less than 100 years later the second generation of heads were in worse condition than some of their prececessors still surviving in Oxford gardens.

By the middle of the 20th century John Betjeman was comparing the heads with "illustrations in a medical textbook on skin diseases".

However, in 1968 the Royal Fine Art Commission took the view that a last attempt at restoring the heads should be made before commissioning a third generation. Three heads were consequently taken away for experiments - which unfortunately failed.

Oxford sculptor Michael Black was therefore commissioned to carve a head to see whether substitutes could be made to preserve the spirit of the old ones.

He tracked down all the original, pre-1868 heads and also studied closely the 17th-century prints of the Sheldonian Theatre by the university engraver David Loggan before producing his prototype.

His head won approval and in 1970 he started work on the remaining 12 using Clipsham stone. The wall, piers and railings were restored at the same time and the new heads were installed in October, 1972 at a total cost of just under £27,000.

Much of the cost of the new heads was met by the Oxford Historic Buildings Fund, set up in 1957 as a result of an urgent appeal to stop the decay of the city's best edifices, including the Sheldonian, the Bodleian, the Old Ashmolean and many of the best college buildings. Already in Wren's time Duke Humfrey's Library had been declared dangerous - he designed buttresses to hold it up - but by 1957 it was perilous, as was Selden End, the room above Convocation House, thanks partly to shrinkage caused by the installation of central heating.

The Oxford buildings had been haphazardly kept standing for centuries (American author Nathaniel Hawthorne wrote in the 19th century: "If you strike one of the old walls with a stick a portion of it comes powdering down. The effect of the decay is very picturesque") but the total neglect during the Second World War meant a solution had to be found.