A prisoner scarred another imate for life at Low Moss Prison in Bishopbriggs in a row over a cream cake.
Christopher Kane, 38, slashed William Douglas’s face as he served food at the prison in March 2015, a court heard.
Kane was seen on CCTV carrying out the attack but claimed a “mystery man” hidden from the camera had done it.
However, a jury at the High Court in Glasgow convicted him of assaulting Mr Douglas to his severe injury and permanent disfigurement.
The court heard that Mr Douglas’s teeth could be seen through the wound.
Prison manager Diane O’Hara, who went to the victim’s aid, said she heard Kane say: “That’s the last time you don’t give me a cream cake.”
Cream cakes are offered to inmates at the prison as a treat every three weeks.
Judge Lord Clark told Kane it was a “brutal and vicious attack” involving a razor.
Kane, who represented himself, told the jury he was friends with Mr Douglas.
“That’s the truth and friends don’t do that to each other,” he said.
In evidence Mr Douglas, claimed he had not seen his assailant.
When he was shown the CCTV footage of the attacker he said: “I can’t make his face out.”
A police officer told the jury there was a culture of not “grassing” in prisons.
Lord Clark deferred sentence on Kane until next month.