A metal detector enthusiast who found one of the largest hoards of Roman coins in Britain was hoping for a substantial reward after his find was declared treasure at an inquest.

Dave Crisp found around 52,000 coins dating from the 3rd century AD buried in a field near Frome in Somerset in April.

The coins, made from debased silver or bronze, were in a ceramic pot just over a foot below the surface.

The find also included coins issued during the reign of Carausius from 286 to 293. He was the first Roman emperor to strike coins in Britain.

Mr Crisp said: "I have been metal detecting since 1988 and this hoard is the most exciting and important find that I have ever made."

The find was declared a treasure and property of the Crown at an inquest at East Somerset Coroner's Court in Frome.

Under the Treasure Act, Somerset County Council Heritage Service can buy the hoard for its new Museum of Somerset when it reopens in 2011.

The amount to be split between Mr Crisp and the landowner will be set by the Treasure Valuation Committee in the coming weeks.

Mr Crisp, from Devizes, Wiltshire, joined staff from the British Museum at a public event in Frome library after the inquest.

Since the discovery in late April - which weighed around 160kg - experts from the Portable Antiquities Scheme (PAS) at the British Museum have been sifting through the coins.

They believe the stash was probably intended as some sort of religious offering, rather than storage for later use. A selection from the find is on display at the British Museum until mid-August.

Anna Booth, PAS finds liaison officer for Somerset, said the money would be split equally between Mr Crisp and the landowner.

"It will be fairly substantial but how substantial, we don't know," she said.

"If they find lots of rare coins, the price could go up. It won't be millions, but beyond that it is hard to speculate."

Now that coroner Tony Williams has declared it a treasure, it will go before the Treasure Valuation Committee to assess the value of the hoard.

Ms Booth added: "Once it has been evaluated, the British Museum will be given a chance to acquire it. If not, the local museum will have it.

"The British Museum has already said it hopes the find will be acquired by the Museum of Somerset.

"We are now going to be on a fundraising drive to get the money."

Roger Bland, head of portable antiquities at the British Museum, said it was an "extraordinary" discovery.

"It's the largest hoard of coins that has even been found in a single pot," he said.

"In 1978, there was another find that was a little bit bigger but that was in two pots.

"We are at the beginning of understanding it properly. We have been able to wash and count all the coins and do a preliminary sort."

He said most were bronze with about five silver coins.

"It's the biggest ever group of Emperor Carausius, 766 that we have identified and we know there are more among the unidentified coins," he said.

"He is not well known this man, he was a Roman commander who set himself up as emperor in Britain and ruled for seven years.

"To find such a big group of his coins will give us a lot of information about this episode in our nation's history which is not well understood."

Carausius was one of the first to use quotes from classical authors on his coins, one in the find bears a line from Latin poet Virgil.

The method of burial has also given experts a revealing insight into British history.

Mr Crisp said: "Leaving it in the ground for the archaeologists to excavate was a very hard decision to take, but as it had been there for 1,800 years, I thought a few days more would not hurt. My family thought I was mad to walk away and leave it."

But Mr Bland said: "Because Dave Crisp didn't dig it up and allowed archaeologists to excavate it, we have information about how it was buried.

"The pot must have been put in the ground and the coins added later.

"We also think it's very unlikely whoever put them there intended to come back and collect them later.

"It looks like they were put permanently in the ground maybe as an offering for a good harvest."

Mr Crisp found a lesser important hoard of 60 silver coins, buried about 100 years later, in the same field before he discovered the pot of coins. This was also declared treasure by the coroner.

The landowner, who is only being identified as Geoff to avoid treasure hunters descending on his property, said: "I have always loved history but I never expected anything as important, exciting or old as this to be found on my land."

Two professional coin dealers will sort through the Somerset hoard and offer a commercial valuation which the committee will discuss in October before recommending a valuation.

The British Museum will then have four months to raise the money.

Mr Crisp said: "The most important thing to me is the historic find. The money will be very nice when it comes along, whatever it is, but this find will be on show for everybody to see for generations to come.

"At the end of the day that gives me as much pleasure as the money. It feels great, it really does. I have found something for the country and for Somerset in particular which will go on show and forever I will be recorded as the finder."

The 63-year-old chef said he would carry on working and metal detecting, which he described as "the best hobby in the world".

Adults and children were queuing at the library to get a look at some of the coins.

"I'm over the moon with today and with the actual find of the hoard," he said. "To see the amazement on the children's faces is fantastic."