Craig slick tyres are banned in F1, and have been for about 2 years ::)
However the answer to your question is this. Worn tyres will loose grip gradually, and the effects you are getting are accurate. Its to do with the way the tyres are made in layers, and heat affects the rubber of the layer below as the new layer is added, or with injection mould tyres, the speed of cooling from inner to outer layer.
Now a warm tyre will grip better than a cold one (which is why all the cars have tyre warmers). They then do the warm-up lap to scrub off any bits of excess from the mould and to get the tyres up to temperature before the start.
All F1 tyres are X-rayed for imperfections, but as I said, the layer thing has yet to be cracked.... Once someone invents a cold-casting process (which physics says is impossible, but I never believe anything a physicist tells me, not even one of my oldest friends who is a high-energy nuclear physicist), then the problem will disappear and the tyre will be consistent throughout its use.
And I do know a lot about tyres as I spent a while working for Michelin in Watford (their head office in the UK), and spent a lot of time with the motorbike boss, and blagging trips to race tracks as part of the team (okay so I had to do a lot of menial physical labour, but I was in the pits)!!!
Ozzy
Ps. I never got the British GP

All the managers would work that weekend
