The bricks produced at the Crow Refractory in Wiri are designed to withstand the blistering heat of New Zealand's steel production plants.
That meant they were the only things left unscathed when the building around them erupted into flames late last Thursday night.
Twelve fire engines and up to 60 firefighters battled the blaze through the early hours of Friday.
They cut large holes in the side of the building to get to the core of the blaze. One fire crew remained until nearly midday on Friday to dampen down hotspots.
The plant has been in Wiri for 20 years and has become New Zealand's leading provider of refractory services.
Specialist trades contractor TBS Group took control of the company in 2001.
Managing director Charles Cook says no-one was hurt in the blaze but extensive damage was done to the inside of the building.
The main storage area may have to be demolished, he says.
"Ironically, all the product we have got stored in there is not going to be damaged," he says.
"There is a whole lot of other stuff in there that has been written off but any product like that that's in there will be fine."
The fire-resistant stones are designed to withstand heat of more than 1500 degrees Celsius.
The average house fire burns at 740C and can reach temperatures of 1200C.
Arrangements had already started on Friday to ensure the factory's 20 staff would not be affected by the fire, Mr Cook says.
"The computer systems have not been damaged so we will move our network across to a new building and temporarily set up offices over the weekend.
"It will be a while before we can use these premises again."
Fire Service fire risk management officer Phil Faidley says the service is investigating the cause of the fire.
"There is no reason at this time to believe the fire was suspicious."
The presence of kiln stones did not contribute extra heat to the blaze - in fact the stones would have absorbed heat, he says.
- © Fairfax NZ News