A huge fire which destroyed an acid bath building at a Sheffield surgical instruments factory started accidentally.
The blaze started at Symmetry Medical, on Beulah Road, Owlerton, at 11pm on Saturday, and was tackled by 50 firefighters over four hours.
The operation was made difficult because of the number of acids in the burning building.
The chemicals are used to sterilise instruments made there.
The acid bath building has been destroyed and the roof has collapsed.
A further building was damaged but firefighters managed to save the rest of the site by spraying jets of water on surrounding areas.
South Yorkshire Fire and Rescue Service said the blaze was reported to them by security staff.
Watch manager Steve Jones, of Rivelin fire station, said: “This was a difficult operation. “We liaised with on-site security on arrival and needed extra cover so that at full-strength we had 10 pumps and one aerial ladder platform in action.
“The incident was also logged with police as critical.
“Inside the acid bath building were 4,000 litres of various substances including hydochloric and hydroflouric acids which are very hazardous.
“We initially fought the fire from the outside of the property.
“The area was secured and gas and electricity shut down.
“We got the fire under control and were able to prevent it spreading. “We were not able to enter the acid bath building until we were happy with the control measures.”
Mr Jones said the acid bath building was destroyed and there was minor damage to a neighbouring building.”
A South Yorkshire Fire and Rescue Service spokesman said: “We are satisfied that the cause of the fire was accidental.
“The acid bath is heated by gas and it is believed that a flame must some-how have ignited it.”
Symmetry Medical was unavailable for comment at the weekend.