A teenager was arrested on suspicion of arson after a huge fire broke out in Frogmore Paper Mill in Apsley on Saturday afternoon.
The fire service sent 13 appliances to the incident.
A spokesperson for Hertfordshire Fire and Rescue said: "One industrial unit approximately 50m x 10m well alight. Firefighters used breathing apparatus, jets and hose reels to tackle the blaze.
"Fire was under control at approximately 6.30pm but crews remained on the scene for dampening down etc.
"We are now working jointly with the police to determine the cause of the fire, which is currently listed as deliberate."
Police were called at around 3.35pm to assist at the scene of a fire in the Fourdrinier Way area of Apsley.
A spokesperson for Hertfordshire Police said: "It was reported that a fire had started in a skip, before spreading to a nearby building.
"Officers attended to put road closures in place while the fire service worked to extinguish the blaze. The road was reopened at around 5.50pm.
"It was deemed that the fire was started deliberately and investigations are continuing.
"A 13-year-old boy was arrested on suspicion of arson and has been released under investigation while enquiries continue."
Steven Dunn captured some images of the smoke from his home on Lawn Lane.
He said: "I was working from home and I was heading towards the kitchen when I saw some smoke out the window, at first I thought it was someone burning some leaves or something.
"But then I saw a huge mass of smoke and there was an orange tint to it and you could see flames at the bottom. The smoke covered the whole of Apsley.
"I was shocked by the amount of smoke that was there, and it was very thick.
"A number of emergency crews including fire, police, ambulance and a first response unit were dispatched to tackle the blaze which lasted over an hour.
"At one point you couldn't see the lights from the fire engines as the smoke was too thick.
"It was quite unsettling and fortunate that the wind was blowing the other way otherwise it could have affected a lot of homes.
"Even after it was all put out, when I opened my window there was a strange smell, almost like metallic.
"Fortunately no one was injured but I think there was some damage to the Frogmore Paper Mill, which is such a historic building, hopefully it's not too bad."
Councillor Adrian England, County Councillor for Hemel Town Division has pledged £1,000 from the annual 'Member's Locality Budget' to the fundraising for recovery and re-building.
County Councillors each have an annual 'Member's Locality Budget' of £10,000 to £15,000, which is made available to be allocated to organisations with various needs, for projects which will support people and places in their Divisions.
Councillor Jan Maddern (Nash Mills Division) has also pledged £1,000 from the 'Member's Locality Budget to the fundraising, she said: "This is a vital local community asset and is part of the hugely important history of the Apsley/Nash Mills paper making area. We must make sure it is saved.
"Of course, much of the reinstatement will be covered by insurance, but not all, so Adrian's and my donations will go towards the extra things that are not able to be reclaimed."