A chemistry experiment that had been sitting in a cupboard at Horowhenua College for years turned into something the Defence Force had to blow up on the school’s back field, and about 130 students were moved out of the science block on Tuesday morning while a bomb disposal robot went in to fetch it.
Emergency services were called to the Levin school about 10.20am. Principal Grant Congdon told RNZ that a solution made up for a senior chemistry experiment had changed inside its flask over time, leaving behind a residue that could go off if it was knocked, rubbed or warmed.
“It had been there for such a long time that there’s been a transformation within the flask to the point where a precipitate had formed and that any sort of movement, friction or heat can potentially make it explode,” Congdon said.
“On the side of the flask, you could see it there as a black powder, that was the part of what was in the flask which was potentially dangerous.”
A staff member spotted it and raised the alarm. Police, Fire and Emergency New Zealand, an ambulance and two members of the Defence Force bomb squad in protective suits attended with a bomb disposal robot. The robot lifted the flask out and explosive ordnance disposal staff destroyed it on the back field. A police spokesperson said the science block was evacuated and there were no reports of injuries.
“A big bang and the chemical is gone,” Congdon said.
The school did not have time to warn the neighbours first. “It was a big enough bang for someone several hundred metres away to feel the blast impact. So it was a fairly decent noise,” Congdon said. He described the day as a first in his years as an educator, and said he could not put an exact figure on how long the flask had been in the cupboard, only that it amounted to years.
The solution was Tollens’ reagent, a mixture of silver ions and ammonia that generations of senior chemistry students have met as the silver mirror test. Add it to an aldehyde and the silver plates out on the inside of a test tube in a thin bright film.
The catch is what happens when the leftovers are put away rather than tipped out. Silver dissolved in strong ammonia slowly throws down silver nitride, a black solid once known as fulminating silver. It stores energy as it forms and gives it all back at once when it breaks apart into silver metal and nitrogen gas. It explodes when heated to about 165 degrees, and the chemical literature describes it as something often made by accident during work with silver compounds and ammonia, leading to surprise detonations. The black powder on the side of the Horowhenua flask is the textbook picture of it.
The laboratory answer is short and long established. Make the reagent fresh, use it, and kill the leftovers with a little dilute acid before they get a chance to sit. Vogel’s Qualitative Inorganic Analysis, the reference work that trained most of the people who teach this, says aged reagent can be destroyed with dilute acid to prevent silver nitride forming. What no reference recommends is a stoppered flask and a shelf.
Which is exactly how these things happen. Nobody at Horowhenua College decided to store an explosive. Somebody made up a solution for a class, the bell went, the flask went into the cupboard behind the prep room door, and the person who knew what it was moved on. The chemistry then did its work quietly for years while the label faded and the school changed around it.
New Zealand schools have had a run of this. In May the Defence Force explosive ordnance disposal team drove up from Wellington to New Plymouth Boys’ High School after a teacher found a chemical had crystallised in the science lab. That school was partially evacuated from around 9am on Tuesday 19 May, closed at midday, and a stretch of State Highway 3 was shut while the response ran. Headmaster Sam Moore said at the time that “everyone at school is safe, and there is no danger to any person on site”. The chemical was taken out and detonated on the school’s top field.
It is not only schools. In March, police cordoned off part of Sheffield Crescent in Burnside for about four and a half hours after a report of an explosive substance in Christchurch. The Defence Force team found a jar of crystallised picric acid, a yellow substance once used in dyes and medicine which dries out and crystallises into a sensitive explosive when it gets old or is stored badly. Different chemical, same story. A substance that is perfectly manageable while it is in use becomes a callout once it is forgotten.
Schools are meant to have a system for exactly this. Under the Health and Safety at Work (Hazardous Substances) Regulations 2017 a school must appoint a laboratory manager, and the Ministry of Education puts the duty to have policies and procedures for identifying and preventing risks on school boards. The joint Ministry and science educators’ guidance for school laboratories, Safety and Science, tells schools to keep an inventory of hazardous substances in each laboratory and storage area, and is blunt about leftovers. As soon as a hazardous substance is no longer required, it says, it must either be disposed of or labelled and its container treated as a storage container.
Whether every school can actually run that system has been argued over for years. A 2019 submission to the Tomorrow’s Schools review by Ian de Stigter, a science technician at Mt Albert Grammar School, published by the Science Technicians’ Association of New Zealand, said most state and integrated secondary schools were not meeting their legal obligations on hazardous substances, largely because unfunded duties were landing on schools while costs rose faster than operating grants. He counted laboratory managers at 72 per cent of the 235 schools surveyed in 2017, with 62 per cent of the teachers holding the role paid nothing for it. The picture has moved since, but the underlying point has not. Somebody has to physically open every cupboard.
Congdon said the college would review its storage and safety procedures. “And obviously it raises questions for us to just check and see how is our storage procedures for chemicals and gases and all those sorts of things around the college, just to check that we’ve got procedures in place that are fit for purpose,” he said.
He was full of praise for how the day was handled. “The oversight from FENZ, the bomb disposal guys were just so professional and the communication to everybody was on point,” he said. “Staff were amazing. Everyone just worked very well together.”
For the 130 students walked out of the science block it will be the most memorable chemistry lesson of the year, and the only one with no assessment attached. The flask is gone, nobody was hurt, and every science technician in the country now has a reason to go and look behind the prep room door.
Have you found something odd at the back of a school or workplace cupboard, or were you near Horowhenua College when the bang went off? Tell us in the comments below.
This article was written by AI, briefed to report the facts, hopefully without some of the bias people bring to the job 🙂
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