By Trevor Timpson
A conservator at the British Museum cleans a coin with a porcupine quill
Porcupine quills and cactus spines are among the tools used to prepare a £1m hoard of Viking treasure to go on public show in York on 17 September.
We will never know if the Vikings who buried a princely hoard of silver in a gilded cup near Harrogate in the 10th century expected to see it again.
They surely did not anticipate that the treasure would emerge from the earth after 1,100 years, to be set upon by highly-skilled conservators wielding items such as quills and ultrasound vibrators.
But that is what has happened.
After a "quick turn-round" in the conservation department of the British Museum (BM) the hoard - or some of it; only some 100 of 617 coins have yet been cleaned - is ready to go on show at the Yorkshire Museum.
The "muddy ball" (inset) turned out to be highly ornate and inlaid with niello
Much more work remains to be done, say the conservators - but already they are enthusiastic about the quality of the find.
"We had a very tight turn-round time and very few people to deliver it. We were trying to get this to look as good as possible in the least time possible... We only had two or three weeks," says Fleur Shearman, a metals conservator at the BM.
But, she adds: "apart from soil encrustations and the corrosion related to the lead and a little bit related to the copper in the cup, it's in superb, really outstanding condition."
The lead casing and the encrustation have kept the silver pristine - it has emerged shining when the coatings were removed.
"Under the surface you can see all sorts of finishing and manufacturing marks - even little nicks the Vikings had made to test the quality of the silver," says Miss Shearman.
THE VALE OF YORK HOARD
Metal detection enthusiast David Whelan and his son Andrew discovered the hoard in a field in 2007.
David later told the Northern Echo: "We got some lead out, and this round ball covered in mud."
The muddy ball was a ninth-century silver-gilt vessel made on the Continent in the Empire of Charlemagne.
Packed in and around it were 617 coins and 67 other objects.
Some of the silver came from the Middle East and Afghanistan.
The treasure has been bought jointly by the York Museums Trust and the British Museum.
More than half the money came from the National Heritage Memorial Fund.
The independent Art Fund charity granted £250,000; £200,000 was raised by public subscription.
David and Andrew Whelan share half the treasure's value; the rest goes to the (unnamed) landowner.
The hoard came to the BM first, soon after its discovery, for preliminary examination so it could be valued. Then, when the money had been raised, it came back for more thorough cleaning in August.
All pieces have to be treated with extreme care, the conservators stress - so as to avoid any damage to the items, and to preserve historical evidence. So keen are they to preserve the history of the cup, for instance, that they will not clean away the marks left by the coins on its inside.
Some of the more robust pieces can be treated by immersion in water or in mild chemical reagents, or by ultrasound vibration which loosens the coating which the centuries have left on them.
But others - such as the delicate decoration on the cup, and some of the coins which are extremely thin - must be treated with even more care.
In these cases "if you're using chemicals everything's applied on a very small scale, and any residues are washed off afterwards," says Miss Shearman.
"So if you have to put a chemical on - which we've had to do in some cases - you would use something like a cocktail stick and a little swab of cotton wool and just moisten it with the chemical, allow it to do its work and then wash off the residues."
A lot of the work consists of holding the pieces under a binocular microscope and working the coating loose with ultra-sensitive hand tools. This is where the porcupine quills come in. Cactus spines are also sometimes used.
The hoard contains jewellery, coins, ingots and silver cut into pieces
As she and her colleagues worked on the silver-gilt cup, Miss Shearman says, more and more of its secrets were revealed: "It just looked grey, featureless - in fact it's highly decorated".
Beautifully engraved with animals and foliage, it also turned out to be inlaid with niello - a black metallic compound.
"Although many aspects of the job are very repetitive and routine it also does have this extraordinary special quality of discovery about it," she points out.
"Discovering the truth about something is the most important thing, not producing something that looks beautiful; so if you can get something that looks beautiful as well then you've really won; you've won on all counts."