An audience of more than 200 was attracted to the Flett Theatre at the Natural History Museum, London, for this year\'s 26th annual Donovan Purcell Memorial Lecture.

The presentation followed last year\'s successful formula of having more than one speaker. This time there were three: John Fidler from English Heritage, Peter Buchanan from Historic Scotland and John Burton from Purcell Miller Tritton, the architectural practice which sponsors the lecture in the name of their late senior partner.

In fact, there were four speakers, because before the subject of castles was addressed, Prof Stephen Donovan, keeper of palaeontology at the Natural History Museum, took the opportunity of welcoming his guests to point out that the national stone collection is housed at the Museum and is available for inspection by appointment.

He also said a public exhibition of building stones was currently under construction and that an exhibition had already opened showing how the various rocks of the earth\'s crust were formed.

John Fidler opened the lectures on castles. His subject was Wigmore Castle, although he used Wigmore and other castles more to discuss the philosophy of conservation than the particular building.

He spoke of changing attitudes and how today English Heritage tried to preserve ruins, particularly, in their picturesque, ruinous state.

This involved microsurgery, he said, using techniques such as cathodic protection after investigations using magnetometers and endoscopes.

Lately, he said, he had been working with Heather Biles on determining the benefits of soft wall coverings - ie planting the tops of walls. Little is known about soft wall cappings and their effects on ancient structures, he said, but they appear to have some benefits.

Peter Buchanan from Historic Scotland presented the case of The Great Hall at Stirling Castle. This was a case of returning the building to its original medieval condition following 200 years of occupation and \'alteration\' by the British Army.

The army had turned the Great Hall into a three storey barracks, knocking out the main window and destroying a roof walkway and parapets in order to create more space inside.

In 1991 the work began restoring the Great Hall to its original condition. There was some conflict about what its original condition actually was because early drawings and paintings did not agree. Fortunately, however, the army had re-used many of the original materials on-site, giving more clues about the construction.

The restoration involved 1,000 tonnes of quarry blocks, most of it Catcastle from Bishops Aukland. Saddles and troughs were made from Dunhouse Blue, although the blue in it is iron which would, said Buchanan, turn brown over time.

Once the repairs were completed the Great Hall was coated in a lime wash because traces of such a wash were discovered on protected areas of masonry, indicating this was the original medieval finish.

John Burton concluded with a review of the work carried on the Norman Colchester Castle, the history of which, he said, actually stretched back 2000 years to the Romans.

After Colchester was burnt out by the British, headed by Queen Bodicea, in 60AD, the Romans built a wall using a material called Septaria, which is mixed with lime and a crushed brick pozzolan to form a concrete.

They built a temple to Claudius out of the same material. By the time the Normans came along the temple was ruins, but they used its Septaria walls as the foundations for their Barnack stone castle.

Over the years various alterations and changes had been made to the castle, which needed some interpretation, as many historical buildings do. What we needed to do, said Burton, as Donovan Purcell had always encouraged us to do, was to study the building.

The restoration included the erection of a glass roof. It had never received planning permission because it was installed as a temporary structure while the restoration was being carried out. However, it was erected 10 years ago and there were no plans to remove it.