For brothers Oreste and Vittorio - the seventh generation of the dynasty - the chance to come back is a welcome one.
"What we remember of Glasgow audiences is all good," smiles Oreste.
"We are curious to see whether they are just the same. Audiences have changed - people enjoy a different kind of comedy now.
"But more and more, they tell us they want to see traditional clowns - and that is what the Rastellis have always been."
Created in 1930, the Rastellis are probably the longest running clown act in history, held in high esteem by circuses everywhere.
Oreste joined the troupe in 1965, his younger brother Vittorio following in 1969. They won't say how old they are - "a clown never reveals his age," Oreste admonishes, frowning.
"It's for the young, single ones," he explains, with a twinkle in his eye.
"They don't want women to know how old they are in case they don't get taken. I always say, pick an old clown. He won't be around for long..."
In Zippo's Big Top, newly arrived in Glasgow for the circus's popular summer tour around Scotland, Oreste and Vittorio are preparing for the evening's performance.
In the ring they are larger than life, loud and funny, with traditional white faces and red noses, oversize clothes and bowler hats.
Out of it, they are quietly-spoken and polite - Vittorio is a man of few words, and his brother does most of the talking.
"Every gag, every routine is about music," explains Oreste.
"We don't do slapstick. We are clever clowns. And we don't cater just for children - adults enjoy our shows too.
"In fact, adults are easier to entertain - children are a difficult audience, they do not laugh easily.
"They are natural clowns themselves of course, which makes it harder for us to make them laugh. For them, we're just acting naturally."
The family name is legendary - their clout is such that in London recently, a Catholic bishop christened Vittorio's baby daughter Alessia in the ring - but with none of the brothers' own children so far following in their footsteps, it looks like Oreste and Vittorio could be the last generation of Rastelli clowns to grace the Big Top.
"It feels sad in a way, but everyone makes their own choices," says Vittorio, with a smile. "We made ours, back when we were young boys. One of my sons works in a bank, the other is a cameraman - maybe my new daughter will follow me into clowning, but I don't think so."
Oreste adds: "I think the circus is in your blood - it is part of the DNA and you cannot fight it.
"I don't become a clown when I put on my red nose - it's in me, all the time - the fire is always burning.
"So although my daughter went to university, away from my world - who knows what will happen in the future?
"As Rastellis, we have always felt the weight of tradition, which is why our act is the way it is.
WE feel the handbrake is on, in a way - that we dare not step too far away from what we have always done.
"It's not up to us to change things."
The Rastellis' fans - and there are legions of them, worldwide - will be glad to hear it.
l Zippo's Circus, featuring the Rastellis, is in Victoria Park until Sunday when it will leave to tour venues around Scotland including Greenock and Ayr.