Sam Kelly admitted there had been some nights of broken sleep as he led a two-year project to restore one of Salisbury Cathedral’s treasures, a stained-glass window by Edward Burne-Jones and William Morris.

“It would be wrong to say I didn’t lie awake sometimes,” said the head glazier. “You are working on something that is very precious and if it fell on the floor, it would be very bad.”

Happily, the 6.5-metre (21ft) tall “Angels” window did not hit the floor. The restoration has been hailed a huge success and this week the artwork will be celebrated and rededicated at an evensong service.

The window was designed in the late 19th century by the pre-Raphaelite artist Burne-Jones and Morris, the textile artist and poet.

In the century and a half since, water ingress, condensation and the deterioration of much of the painted detail led to the need for restoration.

Kelly, who started as an apprentice at the cathedral 40 years ago and now oversees the care of all its stained-glass windows, said the first task in February 2024 was removing the big, heavy window safely from its position above the south quire aisle. “It wasn’t very stable and you have to be a bit careful that you’re not going to do any more damage,” he said.

Once the window had made it safely to the on-site workshop, it was carefully documented, partly to plan the restoration programme but also as a record so that future custodians of the cathedral’s glass would know exactly what work had been carried out.

The window depicts “angeli ministrantes” and “angeli laudantes” – ministering and praising angels. Burne-Jones designed the angels, Morris the decorative leaf design.

Kelly and his colleagues Vicky Pearce, Kate Kersey and Alfie Durrant carefully cleaned the glass to help restore the original colours. “That made it brighter,” said Kelly.

Referring to Burne-Jones’s chalk design, they restored lost details on to 2mm thick “backing glass” rather than adding anything to the original surface. “Nothing has been done to the original glass other than light cleaning,” said Kelly. “All of the additional detail is on the backing glass.”

Protective glazing and oak frames have been added to try to prevent any further deterioration.

Kelly said he was happy with the results. “We have learned a lot about the history and execution of this window and it’s pleasing to know the work we have carried out will allow it to be enjoyed by generations to come.”

The dean of Salisbury, the Very Rev Nicholas Papadopulos, praised the restoration project. “The team has restored the contours, clarity and colour of this 19th-century masterpiece through their painstaking work.

“We are proud that the artistry has been preserved and enhanced with contemporary skill. Sunlight will illuminate Burne-Jones’s angels once more, and their beauty will lift our spirits and rekindle our hope for many more years,” he said.

The window can be seen in the south quire aisle and will be rededicated at evensong on 7 May at 5.30pm.