A prisoner threatened staff at HMP Parc with a shank in a 15-minute standoff as he refused to return to his cell after an exercise break.
John Mylan, 34, was serving a 10-year sentence for robbery and possession of a knife at the Bridgend prison when the offence took place on April 8.
A sentencing hearing at Cardiff Crown Court on Thursday heard Mylan became agitated and angry when he was told to go back to his cell.
Prosecutor Steven Donoghue said the defendant was approached by a prison officer but he moved further away and placed his right hand in the pocket of his shorts.
When asked what he was doing Mylan protested he was unhappy at having been put in a 'high risk to staff' category after he was abusive to a member of staff the day before. He also complained someone had been interfering with his food.
He was asked again what was in his pocket and he said: "Never mind what's in my pocket. I'll show you what's in my f****** pocket – that's what's in my pocket".
The defendant then produced a toothbrush which had a razor blade melted into it and said: "The first one of you who puts your hand on me I'm going to open you up."
Mylan was told by the prison officers to let go of the blade but he refused, which gained the attention of other prisoners who began shouting at him and winding him up.
He asked to speak to the prison manager who attended and told him to calm down and release the weapon and the other prisoners told him he had "made his point".
After 15 minutes he let go of the weapon and was immediately taken to his cell. He later pleaded guilty to possession of a bladed article in prison.
The court heard Mylan had a number of previous convictions including for possessing a bladed article.
In mitigation it was said the defendant has been facing "exceptionally" difficult circumstances in prison due to the coronavirus pandemic which meant he was locked in his cell for close to 23 hours a day.
At the time of the offence Mylan said he was not taking prescribed medication for PTSD and schizophrenia which left him feeling anxious and paranoid.
Judge Rhys Rowlands said: "The weapon was made to inflict injury and capable of inflicting quite serious injuries.
"You issued threats towards staff and your behaviour took place in front of other prisoners and raised the risk of disorder."
Mylan, of HMP Parc, was sentenced to 10 months imprisonment to run consecutively with his current sentence.
