The key, which has not been seen in public since the opening ceremony on December 20, 1899 will be offered at Lyon & Turnbull’s specialist auction of Decorative Arts: Design Since 1860 to be held on April 11 in Edinburgh.
It carries an estimate of £20,000-30,000 and following the sale a donation will be made by both the owner of the key and L&T to The Macktintosh Campus Appeal.
The sale coincides with the 150th anniversary of Mackintosh’s birth.
In 1899 the key was presented to Sir James King, the Lord Provost of Glasgow from 1886–9, by six year old Miss Mary Newbery, daughter of Fra Newbery, director of The Glasgow School of Art, who then performed the ceremony of unlocking the door.
John Mackie, director and specialist at L&T said “This is a remarkable object. As an architect and designer, Mackintosh is seen as a major transitional figure from the historicism of the 19th century to the abstraction of the 20th century. His masterwork, the new Glasgow School of Art of 1899 and 1910, became a building that has come to symbolise his achievement as an architect.”