This is because the detergents and other substances used to clear up the unsightly and - in this age of environmental concern - embarrassing mess dissolve in the water and spread. And while the mess appears to be cleared up - because seabirds and seals are no longer coated in oil - it has actually sunk below the surface, where it can cause long-lasting damage by spreading through the food chain.
A new solvent made from oranges and developed by a small British company aims to solve that problem. Pronatur not only separates into oil and water so that, in the words of the developer Richard Monbiot, the mess 'can be put in plastic bags and carried away', it is also safer.
Tests have shown it is not harmful to wildlife or to the humans using it and that it meets the criteria of the Montreal international protocol that requires the phasing out of CFCs by 1996. It has won the approval of such government bodies as the Ministry of Agriculture, Foods and Fisheries and the Department of Trade and Industry. Although the substance was not used in the Shetlands clean-up operation because the oil there could be dispersed by traditional methods, oil companies such as BP and Conoco have taken supplies of it. Mr Monbiot said it was particularly suitable for clearing up heavy crudes, such as those spilled in the Exxon Valdez accident.
The solvent - which is made from derivatives of orange oils and a synthesised mineral oil that catches fire only at high temperatures - is based on something that Mr Monbiot saw being sold in the United States to householders as a means of removing domestic oil and grease stains.
After many years of selling chemicals to industry, he spotted an opportunity for safe solvents. He developed the brew to improve it for industrial use and has been marketing it for about 18 months.
The product - which sells at about pounds 2 a litre (comparable with existing chlorinated and paraffin-based solvents) - is still suitable for domestic use but can also shift heavy oils and grease without harming paintwork or most materials, except silk, leather and rubber.
'I'm not a research chemist. I see myself as a cook putting recipes together,' said Mr Monbiot, who is launching a marketing drive for the product because he thinks companies are just becoming aware that time is running out for their existing cleaners.
His company, which has a range of other solvents and lubricants ('We sell stuff to make the mess and something to clean it up with'), has only existed in its present form for six months. Before a December 1992 management buyout, he and the commercial director, Paul Richardson, were part of an organisation called Glenmarkie.
Looking around for venture capital finance with which to fund the move, they stumbled across Edward Billington & Sons, a Liverpool-based private company that looks out for investment opportunities.
Billington essentially set up Pronatur, with John Hassett, its chief executive, acting as chairman. Although the company still has only a handful of staff, the sales force is expanding, and Mr Monbiot expects to do about pounds 750,000 of business this year.
'We're a very small operation. Our secret is what we do with the materials,' he said.
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