There are razor-sharp samurai swords, lethal poison darts and dauntingly huge 17th-century ceilings but a particular challenge for the V&A as it moves to new storage facilities comes from a surprising area – women’s hats.

“People think that textiles and fashion don’t have a lot of hazards in them, it’s all just frilly dresses,” said Ruby Hodgson, the collections move team manager. “It actually has so many. Hats in particular are a bit of a nightmare.”

The Guardian was given a behind-the-scenes tour of the V&A stores as the collection prepares to leave Blythe House in west London and move to a modern, purpose-built facility in east London.

It is the biggest move of the museum’s collections since the second world war, involving 250,000 objects, 350,000 books and more than 1,000 archives. The gargantuan task of auditing, barcoding, photographing and packing is well under way and regularly throws up interesting challenges.

Hats can be hazardous because until the 1930s, many with feathers were treated with arsenic and those featuring fur were felted using mercury, a chemical that can lead to paranoia, shaking and hallucinations – hence the term “mad as a hatter”.

The V&A has spectacular examples of hats from the Edwardian era, when there was a craze of putting whole birds on them. Most of them would have been treated with arsenic.

Ruby Hodgson holds a wedding bonnet from 1845 with leaves that might have been dyed with arsenic. Photograph: Linda Nylind/The Guardian

If a hat has fur or feathers, or even dried leaves and flowers, “pretty much it is assume the worst – wear gloves”, said Hodgson. “We don’t have time to test everything, unfortunately.”

The dangers would have been to the people making the hats, rather than anyone today, but no chances are being taken. For the move to east London, Hodgson said the challenge was making special mounts and trays so nobody needed to touch them.

In another section of Blythe House, specialist training has been given to technicians involved in the auditing and photographing of ceremonial Japanese samurai swords. More than 200 are in the collection, many given to British ambassadors by the Japanese government in the 18th and 19th centuries. They are fantastically sharp and need to be unsheathed to be photographed. About 10-15 can be done in a day. No fingers have been lost – so far.

“Don’t even joke about it,” said Philippa MacKenzie, the head of the collections move programme. “The paperwork.”

Inevitably, staff are coming across things they were not expecting. A bomb in the theatre department stores, for example. “I got a text last week saying we’ve found a bombshell,” said Hodgson. “My immediate thought was: ‘Oh God, are there explosives still in it?’”

Fortunately, there were not, and the only hazard was traces of lead. The bombshell was dropped on the Prince’s theatre – now the Shaftesbury – in the West End of London and still has an old label reading: “Part of rocket shell dropped on roof of theatre Friday night March 24th 1944.”

Also in the stores and being prepared for the move are the earliest examples of Christmas crackers, the brainchild of a Victorian London confectioner and baker called Tom Smith who sold wedding cakes and sweets from his shop on Goswell Road in Clerkenwell.

Essential to his idea was the bang produced when the crackers were being pulled. For this, Smith used silver fulminate, which is an explosive, and meant factory workers ended up covered in scars and burns.

Crackers are less of a headache than hats because they had their snaps removed when they were given to the V&A.

Blythe House is a vast 1903 building near Olympia that was originally the Post Office Savings Bank headquarters. Since 1984, it has been used as stores by the V&A, the British Museum and the Science Museum. All three are now moving out.

Mackenzie estimates four lorries, every working day for a year, will be required to make the journey to the new stores in Stratford, designed by the architect Liz Diller and her company Diller Scofidio + Renfro. It is scheduled to open in 2023.