‘There you go, then. Cement and plaster, with a couple of layers of dust. It was almost as if they’d built a wall inside the detector. I’m sorry for the chap, and all that. But facts are facts.’

‘What about where the fire started?’

‘Now, that’s interesting,’ said Downie. ‘The point of origin would normally be near the area of greatest damage. But there are three items of furniture in this room with differing upholstery. The nature of the upholstery makes a big difference.’

‘Do you mean polyurethane foam?’

‘Well, all three items contain polyurethane foam padding. It’s the covering that matters. Anyway, it appears to me that the initial fire was started by applying flame to a quantity of papers adjacent to this chair here, right among these toys.’

‘But the other chair seems to have suffered most damage. That, and the settee.’

‘It’s rather deceptive at first glance, isn’t it? You see, the chair this side of the room is upholstered with a thick cotton weave. It was probably a nice piece of furniture.’ Downie moved across the room. ‘The settee, on the other hand, was padded with polyurethane foam and covered in a partly synthetic fabric mixture, probably poly-cotton. Now the third item of furniture. This, I’m afraid, is a cheaply upholstered armchair, with a wholly thermoplastic cover over polyurethane foam, without any inter-lining.’

‘So the quality of the furniture varied. Perhaps the Mullens should have gone to Ikea and bought a complete suite. But they probably couldn’t afford it.’

Downie didn’t seem to hear her. ‘You see, despite the fire having been lit directly adjacent to it, the cotton-weave chair sustained less damage than the other two items. The natural fibre cover charred and pyrolized, but the weave didn’t fall away, so it provided some retardation of heat release. However, the cheaply constructed armchair was completely consumed, and the synthetically upholstered settee was also severely damaged. Both would have been ignited by radiated heat. The thermoplastic material melts and falls away to expose the underlying foam to the fire. Not so good.’

‘I think I see,’ said Fry, surprised to realize that she actually did.

‘Excellent,’ said Downie. ‘Well, that’s my theory for now. I’ll examine remaining fabric from the three items and test their burning characteristics. But the carpet is probably going to be most helpful to us. Carpet absorbs accelerant well, and retains residue longer.’

Petty had moved into the sitting room and was concentrating her attention on a heavily damaged area of carpet.

‘Is this where the accelerant was used?’

‘We think so. It’s one of the sites identified by the dog.’

Petty was photographing the burn pattern before she began to cut into the carpet. She rolled up a sample with the foam backing on the inside and eased it vertically into a container.