by Malcolm Prior BBC News Online, Oxfordshire

It is hoped the Neolithic circle can be restored to full health

The ancient Rollright Stones on the Oxfordshire/Warwickshire border have been off-colour ever since vandals daubed them in bright yellow paint.

Hi-tech ultrasound equipment used by vets to clean plaque from animals' teeth is now to be used in an attempt to restore the stones.

It is hoped the device will leave rare lichen on the stones untouched.

Damage was caused to about 70 stones at the Neolithic circle when it was vandalised on 31 March.

The device is used by vets to take the plaque off animals' teeth

Druids held their own cleansing ritual at the site, near Chipping Norton, soon after the attack, and have since held a number of ceremonies aimed at helping the police find the culprits.

Meanwhile, conservationists have been struggling to find a way of restoring the stones without damaging the important examples of lichen said to date back to 1100 AD.

Various chemicals have been tested but ruled out for fear of damaging the lichen.

Site manager Dohn Prout has himself tried to clean up a small area of stone using tweezers and dental picks - a task that took about three hours.

But a trip to the dentist made by one of the site trustees' wives proved a revelation.

"She suggested we try ultrasound as they do when removing plaque.

More than 70 stones were daubed with paint by the vandals

"Thinking sideways, it was decided that vets worked on smaller teeth and areas of plaque and their equipment was more portable," said Mr Prout.

On Wednesday, vet Peter Aylmer, from the Chipping Norton Veterinary Hospital, will test out his ultrasound descaling equipment on the circle, which could date back to as early as 2500 BC.

Mr Aylmer told BBC News Online: "It is used for descaling dogs' and cats' teeth, to get the plaque off.

"The idea is that the ultrasound probe does not damage the fabric - you get the plaque off without damaging the enamel.

"We are hopeful we can get the paint off the stone without damaging what's underneath."

Mr Prout said: "It looks like it might be hopeful because when the manufacturers get the equipment back for repair they test it on dried paint on a tin can.

"So we know it works on paint but we do not know the effect it will have on the stones."

Mr Prout said he hoped English Heritage, which is managing the restoration, would purchase a descaler if the tests proved successful.