THE fire service has revealed what caused the dramatic plume of smoke which turned the sky black following the blaze at the Assembly Rooms.
They had already said that the fire on top of the building's car park was believed to have been caused by over-heating machinery in an air-conditioning unit.
Now they have said that the smoke came from "polystyrene insulation" going up in flames.
Foamed polystyrene plastic materials have been ignited in huge fires in the past - for example, at Düsseldorf International Airport in 1996, and Browns Ferry Nuclear Power Plant, in 1975.
The Assembly Rooms fire destroyed the site's plant room from which its utilities are run.
A fire service spokeswoman said: "The majority of the plant room ducting and equipment had a substantial amount of polystyrene insulation covering all the ducting and services which would have been responsible for the initial thick black cloud.
"This was then coupled with the roof covering of insulation and pitch that soon became involved in fire too.
"The rapid generation of large volumes of smoke subsided within a few hours due to the fact that the fuel supply to the fire had been consumed."
The blaze started at about 5.20pm on Friday, March 14, and was brought under control by 7.30pm.
At its height, 75 firefighters and 16 engines were at the scene as flames 40ft high leapt into the skies.
Derby City Council is now considering whether to refurbish or replace the current building.
Asked what was happening to the staff at the Assembly Rooms and how getting an insurance pay-out for the damage was progressing, Claire Davenport, Director of Leisure and Culture at the authority, referred the Derby Telegraph to the authority's previous statement.
In that statement, chief executive Adam Wilkinson said: "The council is committed to a significant cultural facility in the city centre but due to the on-going assessment of damage after the fire, it has become clear we cannot re-open the building as quickly as we would like.
"We are continuing to work through the various options open to us, based on either refurbishing or replacing the current building."
Mrs Davenport said: "The council will issue further press statements when progress warrants it."
The old Assembly Rooms in the Market Place opened in 1755 but was destroyed in a blaze in 1963.
It was replaced with a new building in 1977 when it was officially opened by the Queen Mother.
