CCTV footage of a fire at the Waikato Museum which burned carvings and part of a 200-year-old waka showed it started when rags of linseed oil spontaneously combusted.
There had been three fires in the Waikato started by the same cause in the past year and the region's area commander says not many people are aware of the dangers of not storing the rags properly.
The fire started at 8.22pm on Sunday near the 30m-long waka Te Winika, and the flames were extinguished within four minutes of the smoke alarm being activated.
"A couple of minutes' difference would have been the difference between that and absolute major loss," said Waikato area fire commander Roy Breeze.
The museum's sprinkler system had also stopped the fire from spreading, he said.
An investigation which included watching CCTV footage from the museum was completed yesterday and confirmed the most likely cause was that a rag with linseed oil had combusted.
The rags had been used to stain the waka and wooden carvings and had been lying near a wooden workbench temporarily set up for the restoration work. The fire destroyed the bench.
Linseed oil evaporates rapidly and can cause a chemical reaction when in contact with material, becoming hot enough to cause it to self combust. The chance of this happening is more likely if oil-soaked rags are stored in a restricted space where any heat produced cannot dissipate.
Mr Breeze said the cause was relatively uncommon.
"There was work carried out [at the museum] and [the fire] was totally accidental because people just don't understand the danger of the rags."
Hamilton City Council general manager of community Lance Vervoort said the damage was minor. Only a small piece from the waka which had been removed as part of the restoration work was burned, along with three carvings on a nearby wall.
Mr Vervoort said Tainui carvers were working out how the carvings could be repaired and the council's insurance company would assess the value of the damage tomorrow, which he did not expect to be significant.
Only one of the museum's nine galleries, the tangata whenua room, had been closed due to the fire and this was expected to be completely reopened by the end of the month.
Linseed oil rags caused a fire which destroyed a Te Kauwhata house in January and another at a rural property in Otorohanga last year.
How to store linseed oil rags
*Keep in a metal airtight container to stop the oxygen supply.
*Store dried flat in a single layer to avoid them heating up.
*Store away from other combustibles.
*Keep out of direct sunlight.
*Dispose of carefully after use.