CONSERVATOR Stephanie de Roemer stands with scalpel poised, the concentration etched on her face.

We're in her laboratory deep within the Glasgow Museums Resource Centre and she is demonstrating the technique used to painstakingly restore a piece not seen in public for decades.

Propped up on crates is a section of wall, salvaged from Miss Cranston's Ingram Street Tea Rooms shortly before the building was gutted and repurposed as hotel in 1971.

At first glance it looks unremarkable, but underneath a layer of paint – carefully removed by de Roemer and her scalpel – is a hidden gem: a stencilled design by Charles Rennie Mackintosh.

The pink and green rose motif is classic Mackintosh. The wall will go on show as part of an exhibition, Charles Rennie Mackintosh Making the Glasgow Style, opening at Kelvingrove Art Gallery and Museum on Friday.

Celebrating the 150th anniversary of the birth of the architect, designer and artist, the exhibition will feature more than 250 objects from the Glasgow Museums collection and Mitchell Library archives, alongside key loans from The Hunterian, Glasgow School of Art, the V&A and private lenders.

They include stained glass, ceramics, mosaics, metalwork, furniture, textiles, stencilling, needlework and embroidery, posters, books and architectural drawings.

Some of the works have never been on display and the majority – like the wall from Miss Cranston's Ingram Street Tea Rooms – have not been shown for 30 years or more.

That the designs from the tea rooms were saved at all was down to the quick thinking of a member of the conservation team at the Hunterian Art Gallery in 1971. Its unnamed rescuer knew that there had been stencil work on some of the walls.

With the building interiors mere hours from being ripped out as part of renovations, he made a hurried telephone call to a conservator in Edinburgh who talked him through the process of removing two sections of wall.

These were packed into crates and have remained largely untouched – save for the occasional visual inspection – for almost half a century.

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When de Roemer began work on them last summer, she admits to being unsure what secrets would be revealed. The lathe and plaster wall sections had been covered with plywood and tissue paper for protection. Even removing those was a delicate operation.

"It was exciting because it is like an archaeological excavation," says de Roemer. "I took one layer off after another. We weren't sure what we were going to see underneath. The last layer was tissue paper. I removed that to find the paint layer.

"There were some scratches where someone had obviously had a look, but you could see pink and green from the stencil work, so I knew something was there. It was exciting when I saw that."

The wall itself needed some TLC. "Think of it as being like a crumbling cookie," she says. "On top of your cookie you have a caramel layer and then a chocolate layer. What we are trying to do is bring it all back together."

Uncovering the stencilling itself was meticulous and time-consuming work, done largely by what de Roemer describes as "mechanical cleaning" using a scalpel to gently lift off the top layer of paint.

My heart is in my mouth watching her, the technique requiring pinpoint precision. "It is a carefully thought out process," she says. "Once you have cleaned it, you can't put it back on. We use some of the cleaned off paint to do analysis. Even in the dirt layer, you can have interesting information.

"That evidence can lead to lots of other stories. The object for conservation is a bit like a witness from the past and it contains all these narratives."

One interesting nugget was a small scribble of Biro pen beneath the top layer of paint on one of the walls. This helped place the redecoration of the tea rooms to after the invention of the Biro in 1938.

"It is little things like that which gives us context," says de Roemer. "This is all very scientific, about historical facts and looking for evidence, but what I always find really touching, what really gets you, is the human story behind it.

"It is like a puzzle. You have a spaghetti ball and then try to untangle the strands. There are parts we are unravelling that might be scientific and others that will be fascinating for an anthropologist, art historian or archaeologist."

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In its heyday, Miss Cranston's Ingram Street Tea Rooms would have been one of the most publicly accessible ways for many people to view Mackintosh's work.

"At that time, Glasgow was an industrial city and it would have been grey, but inside the tea room everything was very light and about nature," she says. "It would have been a real oasis coming from the dark, rainy streets."

These days it is hard to fathom someone painting over Mackintosh's designs, but at the time, says de Roemer, redecoration would simply have been about changing fashions.

"Interiors are transitionary spaces," she says. "It might be an original design, but the moment that person moves out and someone else comes in they might have completely different aesthetics or taste.

"It is also what makes this so exciting. That it has survived is quite something. Other parts of the tea rooms were things that you could physically take out, such as the light fittings, furniture or even wooden panels. But this comes from the building structure; it is part of that membrane."

The exhibition has been four years in the pipeline. Alison Brown, curator of Charles Rennie Mackintosh Making the Glasgow Style, has spent countless hours poring over collection catalogues and sifting through the archives.

These include delicate watercolours not shown in a generation – Pinks, Grey Iris and Part Seen, Imagined Part – and pieces of furniture, including a high-backed chair from Miss Cranston's Ingram Street Tea Rooms.

"The story is his life and there is a chronology that runs through the exhibition about what Mackintosh is up to, but also what is going on in the city and what his contemporaries are doing to put everything into context," explains Brown.

"We begin with the Glasgow that Mackintosh grew up in and have amazing collections from this period because our museums service started at that same time. Mackintosh was born in 1868 and we started collecting in 1870."

This opening section contains a rich display of pieces that could have been seen at the old Kelvingrove House – a now demolished predecessor to the Kelvingrove Art Gallery and Museum – when Mackintosh was a young man.

It charts the ascent of his career as he worked on landmark buildings including Glasgow Art Club, the former Herald building at The Lighthouse, Martyrs' School, Queen's Cross Church, Glasgow School of Art and doing tea room interiors for Miss Kate Cranston.

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Brown, who is a curator of European Decorative Art from 1800 with Glasgow Museums, is keen to emphasise that the exhibition is not simply about Mackintosh as a design genius in isolation.

As its heart is examining the influence of his "kindred spirits" – The Four: Mackintosh, his wife Margaret Macdonald, her sister Frances and James Herbert McNair – and contemporaries such as architect James Salmon Junior and Talwin Morris, the art director for publisher Blackie and Son.

The contributions of Jessie Marion King, Ann Macbeth and Dorothy Carleton Smyth who, alongside the Macdonald sisters, were part of the pioneering group of women artists and designers dubbed "the Glasgow Girls", also feature.

"What I want is for people to go away from this exhibition realising there was such a dialogue going on between all these people," says Brown. "They weren't working in isolation. There was a dialogue and social events. This is what makes the Glasgow Style."

Among the highlights are four watercolours by Margaret and Frances Macdonald, titled The Seasons, which will be shown together for the first time since the early 1980s, together with a poster for the Glasgow Institute of Fine Arts designed by the sisters and James Herbert McNair.

Works never before seen include a standing clock by former Glasgow School of Art metalwork instructor Peter Wylie Davidson, an illustrated score cover for Wagner's Gotterdammerung by Dorothy Carleton Smyth and sketchbooks that belonged to Talwin Morris.

Tribute is paid to former Glasgow School of Art director, Francis Newbery, who is credited with spotting the talent of the Macdonald sisters, who attended day classes, and Mackintosh and McNair, who, by then in employment, attended evening classes.

"Newbury spotted like minds and introduced them," says Brown. "He was nicknamed 'The Ringmaster' and wore a top hat, but he also corralled people. He created the art club, social events and exhibition spaces so that pupils past and present could meet."

Mackintosh and his adventures in tea room design for Kate Cranston is another fascinating element. "He worked on all four of her tea rooms in the city centre," says Brown. "Buchanan Street opened in 1897. Mackintosh's first involvement was making stencilled mural schemes."

While fellow interior designer and architect George Walton did the main schemes, Mackintosh worked on the murals.

"The Part Seen work becomes one of the mural schemes in Buchanan Street," she adds. "Can you imagine being a diner and you have this frieze of statuesque ladies stretching up the wall and looming over you? It would be quite something."

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The pair also worked together on the Argyle Street tea room in 1898. "Walton was responsible for most of the interior decor and Mackintosh designed the furniture," says Brown. "This is when his big heavy furniture comes in and that iconic high-backed chair.

"Then, from 1900, Mackintosh becomes the sole designer for Miss Cranston. The Willow Tea Rooms on Sauchiehall Street opened in 1903. The Ingram Street Tea Rooms he works on between 1900 and 1912. His last tea room interior he did for her is the Dug-Out at the Willow Tea Rooms in 1917.

The working relationship between Mackintosh and Cranston was one that proved mutually beneficial. "Miss Cranston knew a good thing," says Brown. "She knew branding and was a very canny lady. She clearly loved Mackintosh's work.

"He gave her these amazing interiors which would be talking points and destination venues. Ideas in those tea rooms reappear in other projects. His tea room designs for her really capture the evolution of his ideas under one roof."

The exhibition is part of Mackintosh 150, a year-long celebration that includes events at The Lighthouse and Mackintosh Queen's Cross, as well as the re-opening of Mackintosh at the Willow, Miss Cranston's original tea rooms on Sauchiehall Street in June.

The Glasgow School of Art, The Hunterian, House for An Art Lover and the new V&A Dundee, due to open in September, will all host Mackintosh-dedicated programmes.

Brown hopes that visitors will come away from the Kelvingrove exhibition feeling inspired and curious to not only discover more about Mackintosh's work, but also his friends and contemporaries.

"There will be surprises," she says. "Some people think Mackintosh is just roses. He is not just roses. There are roses, but there are much more than roses.

"It is an exciting exhibition which will cover so many interests. I want people to go out and see the city in a new light and get a sense of the immense creativity that was going on in Glasgow."

Charles Rennie Mackintosh Making the Glasgow Style runs at Kelvingrove Art Gallery and Museum in Glasgow from Friday until August 14. Tickets cost £7 (adults), £5 (concessions) with entry free for under-16s. Visit www.glasgowmuseums.com