The girls – sent there for supposed “short-term respite” from their homes – were force fed, assaulted, humiliated and left in tears.
One survivor told jurors: “I just wanted to curl up like a wee baby. I still do not understand how an adult could be like that.”
Robertson – now aged 77 – had denied the accusations during a trial at the High Court in Glasgow.
She accepted having a “strong voice” when sometimes dealing with the girls, but insisted she was not violent or bullied them.
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The pensioner was convicted of a total of 18 charges of cruel and unnatural treatment of the youngsters.
The charges spanned between January 1969 and August 1984.
A large group of women affected by the abuse at Fornethy were in court for the verdict.
There were shouts of “Yes” by the women in attendance as the guilty verdicts were read out.
They also hugged and cheered as they left the court building together.
Fornethy had been run by the former Glasgow Corporation and latterly Strathclyde Regional Council.
It was said to offer “convalescent care” for girls aged between five and 12 for up to eight weeks at a time.
But, the horrors of what went on there were revealed during harrowing accounts at the three-week trial.
One now woman recalled a “Miss Robertson”, who she believed was a teacher.
She then told how she was force-fed semolina, which she had been struggling to swallow.
The witness: “I remember Robertson was over at the other table and she came over to me shouting and bawling.
“She was saying: ‘You need to eat. It is good for you’.
“I was like: ‘I do not like it’. The next minute, I got my hair pulled back and the spoon was put down my throat.
“She just would not let go.”
The then girl claimed she was made to stand in a corner of the room facing away from others.
She remembers this sort of incident would “happen to everyone”.
Asked how she felt, the witness said: “Alone, embarrassed. I just wanted to go home.
“What did I do wrong?”
She claimed this sort of thing happened more than once, also when she refused to eat blancmange.
The witness: “It is sick. You just do not do that.”
The woman alleged she was also hit during a latter stay at Fornethy while helping scrub floors.
She said Robertson commented she had “missed a bit” and the then girl “tutted”.
The woman said her clothing was grabbed tight and she struggled to breathe. She was pulled back and forward “like a rag doll” before ending up on the floor on her knees.
The woman recalled: “I just wanted my mum.” She said she got beaten for “trying to do a good job”.
The witness also made to go on a long walk despite stating at one stage she was injured. The woman stated she was spoken to “as if she was nothing”.
Another survivor described the atmosphere at Fornethy as being “scary, cold, unloved”.
She added: “You would be able to write letters (for family), but only what they dictated and you had to copy that.”
Prosecutor Eilidh Robertson put to her during her pre-recorded testimony: “If we were to hear evidence from other teachers and staff that children were well looked after and happy, does that reflect your experience there?”
The woman replied: “No.”
She said she was “scared” to report any incidents at the time.
Many other girls suffered similar ordeals.
Robertson targeted one youngster was slapped, punched and had a blackboard duster hurled at her.
One child was put over Robertson’s knee and slapped on the bare buttocks.
Robertson, now of Essex, gave evidence during the trial.
Mark Stewart KC, defending, put to her at one stage was she ever violent or “make derogatory comments and use unpleasant nicknames” against the girls.
Robertson denied the claims.
Sentence was deferred on the first offender until December at the High Court in Glasgow.
Judge Lord Colbeck granted her bail meantime.