As part of the Edinburgh auction house’s April 11 sale of decorative arts, it sold for £40,000 (plus 25% buyer’s premium), above its £20,000-30,000 estimate.
The iron key, skilfully wrought with its totemic square terminal enclosing overlaid plant motifs, had not been seen in public since the opening ceremony on December 20, 1899.
It was presented to Sir James King, the Lord Provost of Glasgow from 1886-89, by six-year-old Mary Newbery, daughter of Fra Newbery, director of The Glasgow School of Art, who then performed the ceremony of unlocking the door.
Attached to the key is a planished silver commemorative plaque (hallmarked 1899), engraved using a trademark Glasgow School of Art typeface.
The current Sir James King was the vendor, and pledged to donate a percentage of the sale proceeds to the Mackintosh Campus Appeal launched in the wake of the 2014 fire in the building.
The buyer was Sir Ludo Shaw Stewart, owner of the Ardgowan Estate near Inverkip, Inverclyde.
He said: “I am delighted that the key will remain in Scotland and it will take pride of place on show at Ardgowan House. I have a particular interest in the key and The Glasgow School of Art as one of my forebears was a founding father of the first School of Art in Glasgow, started in 1753, a precursor and inspiration to the later Mackintosh School.”