Continuing our series on Sites of Special Sentimental Interest, Martin Hesp has been hearing about an apparently insignificant crater in a Cornish moor which led to much, much bigger things

THERE'S a hole in the ground on a windswept moor in Cornwall where ravens go to be lonely and a few stonechats chirrup, and not a lot else happens at all. Thickets have long since choked the mis-shapen crater, so you can't climb into it – you can't even really see where it begins and where it ends.

A strange kind of place, then, to be listed in our Sites of Special Sentimental Interest (SSSI) series. Strange, unless you know something of the hole's history – then you begin to realise why someone should declare it to be a very special place indeed.

This crater in the lofty, airy bosom of Cornwall led to the employment of hundreds of thousands of men. Its story bequeathed the county countless millions of pounds. Its white powdery progeny is to be found everywhere – a large percentage of the man-made things you touch will contain it.

It is, for example, likely that you will come into contact with more than 20 objects related to this unremarkable hole by the time you reach work this morning.

For this is the place where one William Cookworthy discovered what we now call china clay, 262 years ago.

Back in 1746 the locals knew the stuff as "moorstone" and used it to patch up mine furnaces, but the young chemist from Plymouth realised the magical clay had far more potential than that.

It's a story Ivor Bowditch knows well. In fact, the community public relations manager for Imerys – the giant company that mines china clay in the region – probably knows more about the stuff than anyone. Ivor enthuses over china clay, he lectures on it, has a thousand anecdotes about it and knows a million kaolin-related facts.

Like the fact that the name kaolin stems from a place called "Gaoling" or "Kao-Ling" (meaning "High Hill") in Jingdezhen, Jiangxi province, China – which is where the ancient Chinese mined the stuff for porcelain in days of yore.

"Tregonning Hill holds a special place in my heart because the china clay industry has been my life's work," said Ivor as we strolled up the lonesome moor, high above the village of Ashton.

"I joined the industry 42 years ago – back when I was talking to people who'd been horse-wagoners and steam engine drivers.

"And here we are at the very site where William Cookworthy first discovered clay back in 1746 – and really, on the back of the discovery, began an industry that has grown worldwide.

"Imerys operates on all continents and in more than 50 countries," says Ivor with pride. "And really it all started here – clay was first used for porcelain, but today it is used in a whole host of manufacturing activities including the manufacture of paper, paint, rubber, plastics, sealants and adhesives. Everyday we probably use or touch something containing china clay."

And it is true to say you will have already benefited from the results of Cookworthy's work this morning. In fact, you are touching it now – it is used in the manufacture of the paper these words are written on. There would have been some china clay in the light switch you turned on earlier. There would have been quite a lot of it in the washbasin and the toilet bowl. It was even in your toothpaste.

Not that young William Cookworthy had any idea of the full potential of china clay when he took home a saddlebag full of it to Plymouth after stopping with a fellow Quaker friend. But he did know enough to suspect it might be the type of clay the Chinese used in the making of fine porcelain.

"Cookworthy had read the writings of Jesuit missionaries who visited China and described the process of making porcelain and the raw materials being used," says Ivor. "And it was with this knowledge that he set himself the task of trying to find these raw materials – though it was almost by mistake he found them here.

"Cookworthy was a Quaker chemist and he was visiting a fellow Quaker who was a mining captain at the Wheal Vor mine at the bottom of the hill here where the miners were using the clay to make firebricks for the early steam engines. In fact, the first ever steam engine was used below the hill here."

William Cookworthy was born on April 12 1705, the son of a Quaker from Kingsbridge. The Quakers tended to look after one another and in 1719, at the age of 14, young Cookworthy was taken on as an apprentice by Silvanus Bevan, a Quaker chemist and druggist in London.

The Devon boy had to make the 200-mile journey to the capital on foot as he had no money for the coach fare. But the trip was worth it. He was obviously a bright lad and, in addition to his training in dispensary, William also learned Latin, Greek and French, as well as metallurgy.

So bright was he, in fact, that after seven years Bevan offered him a position in a new wholesale pharmacy business he was setting up in Plymouth.

By 1735 they were partners and his influence in the business grew further when he took on his late wife's brother to form "Cookworthy and Company". This gave William more time to spend on his real interests and he began to experiment in chemistry and metallurgy.

He also found time to read – and it was a chance perusal of a journal written by a missionary which first set him on the trail of china clay.

His quest was further inspired when three Americans visited him with samples of Virginian clay and porcelain in 1745.

The American visitors were involved in a business importing clay through the port of Bristol, but at that time English potters were only able to produce types of "earthenware" – real porcelain was only to be found in, and imported from, China.

In 1746, Cookworthy was invited down to Cornwall to visit his friend who worked in the mine near Tregonning Hill, about six miles west of Helston.

As he looked around he was fascinated to observe that miners were repairing furnaces with a local clay, which they called "moorstone" or "growan clay", noticing that it seemed to survive the high temperatures without cracking. He borrowed a spade and took some samples back to Plymouth where he conducted various experiments.

It soon became apparent that the material was capable of making an excellent porcelain and William lost no time in leasing several of the clay pits in the Tregonning Hill area. But the clay contained large quantities of mica and soon deposits of a superior quality were found on land owned by Thomas Pitt (who became Lord Camelford in 1784) in the parish of St Stephen's, near St Austell – and this area is still the main site of extraction today.

Pitt put money into Cookworthy's venture and, in December 1766, they set up a small factory called the Plymouth China Works where William continued experimenting to find the best ways of processing, glazing and firing his beloved clay. On March 17, 1768, Cookworthy obtained a patent for "Making porcelain from Moorstone, Growan and Growan Clay". It gave him the exclusive right to use china clay for porcelain manufacture.

It wasn't all smooth progress. Josiah Wedgwood and other Staffordshire potters raised objections to the patent and eventually the legal powers of the day decided that use of china clay could be released to enable the manufacture of other ceramic products provided that Cookworthy's patent formula was not infringed.

However, the Plymouth works continued production of the UK's first home-manufactured hard paste porcelain. Jugs, vases and decorated tea services were made, but the venture wasn't exactly earning big bucks so Cookworthy amalgamated it with a pottery in Bristol.

His cousin, Richard Champion, became manager of "William Cookworthy and Company" – and the rest is history. A history that changed the face of Cornwall and Devon.

More and more potteries started using porcelain and demand grew. Many of the potteries acquired rights to mine their own clay and mid-Cornwall went through something akin to a kaolin Klondike.

The craze for clay grew even further in the mid-19th century when it was discovered that the material could also be used in the manufacture of paper.

It was all pretty cut-throat out there in the Cornish "Alps" – as the area north of St Austell became known thanks to its white conical heaps of unwanted granite sand.

By the early 1900s more than 70 separate companies were digging away at the hills. There was huge price competition, but there were no set standards on the quality of the material mined. It was a somewhat anarchic market, to say the least.

Over-production was rife and there was little capital investment being made into the industry. The working conditions were said to be pretty poor and wages were low.

However, it was undoubtedly a strongly positioned industry – the Westcountry china clay pits held a virtual world monopoly over production. And it was good for the British economy – more than three-quarters of the output went abroad to places like the USA and Europe.

The industry desperately needed regulating and, not long after the First World War was over, three of the leading producers amalgamated to form English China Clays (ECC) Limited – a company which oversaw more than half the entire Westcountry capacity.

The multinational minerals company Imerys bought ECC in 1999 and is now the world's largest producer of china clay.

Times have changed of late, production has been cut and there have been job losses, but Ivor Bowditch believes there's still a big future for the china clay industry in the region, with Imerys investing millions of pounds in its gargantuan pits.

Back at Tregonning Hill, Ivor recalls the fate of his beloved hole in the ground: "Working on the clay deposit here continued through the 18th and into the 19th century – and indeed into the early 20th century. The last workings of any sort were abandoned here in the 1920s."

Today you can see no hint or clue that the old clay miners were ever busy here, save for the thicket-filled hole.

No buildings remain – no rusting machinery – just a plaque to commemorate the fact that this is where the Westcountry's most profitable industry ever was born.