A prison officer was covered in a mixture of boiling hot water and sugar, it has been reported.
The brutal attack, known as napalming, was carried out at Forest Bank prison in Salford, Greater Manchester.
The victim had to be rushed to hospital for treatment for burns, Manchester Evening News reported.
One source said: "It is known as napalming amongst inmates, and very painful."
The incident took place earlier this month at Forest Bank prison.
Greater Manchester Police have launched an investigation.
A spokesman for Sodexo, which runs the privately operated jail, said: “An assault on a Prison custody officer is currently under a police investigation so it would be inappropriate to comment further.”
Earlier this month masked men on motorbikes torched a car outside the main entrance of Forest Bank.
Footage from the scene shows three men stood around a vehicle which seems to have been pushed up to the gates of the Salford prison.
One of the group starts a small fire just outside the car before leaning in and setting the interior of the vehicle alight.
Two of the men then climb onto a bike while the other runs towards the person filming.
GMP confirmed offenders set fire to a car outside the prison before leaving on two motorcycles.
No arrests have been made and enquiries are ongoing.
The Category B prison, run by Sodexo Justice Services, has capacity for 1,460 inmates.
