Eleven towering piles of ivory rise above the savannah grasslands of Nairobi National Park, ready to be burned Saturday in a symbolic grand gesture against the trade threatening elephants with extinction.

It will be the largest ever burn of ivory, with the 105 tonnes, representing thousands of dead elephants, seven times larger than any destroyed before.

This is no simple bonfire -- but there is one fundamental problem.

"Ivory doesn't burn," said Robin Hollister, the pyrotechnic expert responsible for the fires. "If you try to burn it with a match or by throwing it into a fire, it won't ignite."

A short distance away, thousands of litres of a mixture of diesel and kerosene lie in a tank, waiting to be injected with pressurised air though steel pipes buried in the ground leading into the heart of the pyramids.

Hollister has helped organise all the cremations staged by Kenya since the first burn in 1989.

A former engineer who then worked creating special effects for films, he was recruited by the famous palaeoanthropologist Richard Leakey, head of the Kenya Wildlife Service (KWS).

- Firestarter -

Chucking the ivory onto a normal wood fire simply won't work, he said.

"The exterior will be charred, but the inside will remain intact," Hollister noted, adding that the same applies to the rhino horns, which form a twelfth pyramid of 1.35 tonnes from over 340 rhinos.

"If you wish to incinerate it, you have to take it to extreme temperatures."

It is a grand statement. On the black market, that quantity of ivory could sell for over $100 million (88 million euros), and the rhino horn could raise as much as $80 million (70 million euros).

Rhino horn can fetch as much as $60,000 per kilogram (2.2 pounds), more than gold or cocaine.

Incineration would be easier in an oven, but that wouldn't make the same impact visually, and the whole point is to send a message to stop the illegal trade in tusks.

Instead, organisers have had to calculate how to burn ivory piled high in the open air in a national park -- surrounded by dignitaries -- and ready to be filmed and photographed by the media.

"It's a show after all, the burning has to be symbolic," said Hollister. "There'll be ivory towers, nice flames, it will be very visual."

In total, 16,000 tusks have been piled vertically on metal pyramid frames some three metres high (10 feet) to hold the ivory in place.

When Kenyan President Uhuru Kenyatta approaches the largest of the pyramids, he will insert a flaming torch into the pyre.

Once the president walks safely away, the fuel will be injected under pressure in the heart of the pile through a perforated steel tube, and the flames will take hold.

The same process will be repeated for the other pyramids, due to be lit by heads of state and other guests.

Ashfaq Mughal, another engineer preparing the burn site, said they will be able to control the fire by regulating the fuel to feed it.

- 'Even if it snows' -

"We can control the pressure, so we will be able to adapt the heat if necessary," Mughal said. "During the tests, the metal structure bent."

To help the process, tens of tonnes of illegally cut precious sandalwood seized from smugglers have also been placed at the base of the pyramids.

"It is clear that if we wanted to be more efficient, we would do it differently," Hollister added.

"For example, we would not put the tusks vertically as you see them there, we would pile them on top of each other."

Even with all the fuel and planning the fire is expected to burn for days.

"I have no idea how long exactly the burning will take, but I think it will take a few days," he said.

Recent days have seen Nairobi swamped by torrential rainstorms, but Hollister smiled when asked if he feared it could put out the flames.

"It may be a hassle for guests, and will perhaps slow down the process, but I can assure you it will burn and it will be very hot," said he. "The ivory doesn't need to be dry because it doesn't ignite, and it doesn't absorb the water either."

Richard Leakey, passing by the pyres to oversee preparations, added his support, joking: "It will burn, even if it snows!"