Fashion: Mr Conran's fin de siecle: Severity, thy name was Jasper: Now he who cleaved to chaste lines and plain black has taken to more exotic stuff. Marion Hume applauds

JASPER Conran - he of the spare, pure line, the severe leather jacket and the perennial, unadorned, black dress - is now promoting the pleasure principle. The British designer who, along with Jean Muir, best typified low-key, high-quality dressing, is now obsessed with adornment.

He has become delighted by pattern - not the explosions of exotic flora that are blooming all through the current Seventies revival - but the subtle, tactile surface pattern techniques of the decade the Sevenites imitated, the Thirties. Conran has included devore, a fiendishly difficult process of acid-burning semi-sheer patterns into cloth, in his collections since 1989. But he admits that he has only now acquired the confidence he needs to bring a tricky technique invented in Lyons at the turn of the century forward.

His slender black evening column, that staple for the Jasper Conran customer by night, remains. But an alternative is a sinuous evening skirt which wraps its bias-cut panels around the body. These are alternatively of velvet and devore transparency, where Art Deco-inspired abstracted leaves have been set in relief by burning the surrounding pile away. The shape is reminiscent of the after-dark dressing of sultry silver screen starlets. But the colour combination of midnight blue, deep green, mustard or coral looks right for languid evenings of the approaching fin desiecle.

Not that the current re-spin of the Seventies is passing him by. Tie-dye, that home colour process essential for sunblasting colour on to one's seersucker kaftan, has also caught his eye. But his silk crepe dip-dyed shifts took 30 tries to get right.' I wanted the colours to bleed from one to the other, which we couldn't achieve by screen printing. We worked out we had to apply the dye to wet fabric, only to see the effect ruined if the cloth was just a bit too damp,' he says.

It is now, after 15 years in business and still only 33, that Conran feels he has time to learn the skills that he fears are disappearing from fashion. Since he licenced his name to the manufacturers Marchpole, he has been able, for the first time since he left Parsons Design School in New York in 1977, to be 'just a designer who lets someone else deal with the production headaches, the financial headaches'. So he is making up for lost opportunities. Not only is he indulging in more delightful surface detail in his work, but he is also learning - hands-on - about the couture techniques of the finest clothes in order to reapply this knowledge to modern, manufactured garments.

'When I was at Parsons, Charles James (the great American couturier) was still alive. I met one of his apprentices who showed me a slither of a white satin dress and it was the most exciting thing I had ever seen,' remembers Conran. 'So I got myself an apprenticeship too and was all set to learn bias cutting from the master, when he dropped dead.' He is now finding time to add to the principles of stress points and grain lines he learned at Parsons by sitting in with a cutter who has long worked on the Jasper Conran collection. 'So for once a week, I'm a student again.'

Conran admits that building up a fashion business, which as well as his collection includes a lower-priced line called J, menswear, tights and eyewear as well as shops in Japan and his own shop in London, has made him 'dour'. The antidote to that has been the theatre, which, he says, has liberated his thinking and is really responsible for his embracing an increasing number of exquisite, expensive and complicated pieces in his own collections.

He has already won a Laurence Olivier award for his 18th century-style costumes for The Rehearsal. He not only earned acclaim for his answer to the challenge of topping Cecil Beaton's work on My Fair Lady - for which Beaton had dollars 8m to spend for the film - he was able to perfect his craft. For a theatre production of the musical that toured Britian and has just closed, Eliza Doolittle and the street vendors 'wore a massive quantity of devore. So I learnt how it worked, and how it didn't, without having to think about whether it would sell,' he says.

In his collections, it does sell - in spite of the price tags. 'At first I didn't even dare price it.' (When he did, an acid-treated shirt worked out retailing at pounds 625, a devore panelled evening skirt at pounds 572.) 'Then the buyers, when they had got their basics, started asking to see 'the lovely stuff' - and they sell it.' says Conran. 'We do,' say Amanda Verdan, the director of buying for Harvey Nichols and Josephine Turner, who co-owns and runs A La Mode. Tonight sees the revelation of Conran's latest theatrical challenge and a further point on his learning curve. He has designed sets and costumes for a new ballet which opens this evening at The Royal Opera House. Tombeaux, an abstract piece performed by Viviana Durante and Bruce Sansom, is presumably a far from ordinary work. Certainly it is performed in a far from ordinary set with Ms Durante and the corps de ballet wearing acid-dipped indigo tutus that were baked in the oven and burned into devore patterns.

The same devore alchemy, also known as broderie chimique or chemical embroidery, transforms the 40ft by 25ft curtain, on which the pattern of sweeping ferns is burnt on to the backcloth. Conran, who believes it the biggest piece of devore in existence, admits he needed to find a very large oven to finish the process.

His most expensive skirts and bustiers are also oven-baked. 'While people want basics - which is what my new J line is about - I do think there's a desire for clothes with luxury and romance that appear to be in touch with craft and history,' he says.

So, as the J line is manufactured to be in stores for autumn; as Harvey Nichols prepares to give Jasper Conran clothes their own shop-within-the-shop alongside Donna Karan and Calvin Klein; and as the designer himself begins to prepare for a catwalk show next autumn alongside those purveyors of spare, simple shape at the New York shows; his own little shop in Knightsbridge will become more ornate. 'That is where the really delicious experimental clothes will go. The J line won't be sold there, just the really special pieces. I want it to be more like a salon than a shop. I want it to be a place of pleasure.'

(Photographs omitted)