I don't know. When the 7/7 bombers struck, the press was full of stories to the effect that
a) the bombs were home-made TATP explosive, as used in home-made Hamas rockets.
b) there was a fully-equipped cookshop in Hyde Park, Leeds, where the TATP was brewed.
c) said cookshop belonged to an Egyptian chemist who happened to have gone to Egypt. The chemist was named as Magdi Mahmoud al-Nashar, of Leeds University. Apparently he was detained in Cairo by Egyptian police and interviewed by a couple of Special Branch officers - after which it was quietly announced that no charges would be brought. He doesn't seem to be at Leeds any more.
Then we were told 'organic-peroxide' bombs. I assumed flour and peroxide as that seemed to be the choice of the 21/7 bombers. In fact the 21/7 wiki, for what that's worth, said "The explosives used by the bombers consisted of Chapatti flour powder mixed with liquid hydrogen peroxide, detonated by a booster charge. This was the same explosive mixture used by the bombers in the 7/7 bombings two weeks earlier."
Yet the 7/7 inquests are now apparently telling us the bombs were peroxide and black pepper. Sounds like a flavour of crisp.
"The bombs used in the attacks on three tube trains and a double-decker bus contained about 10kg of explosives made from a mixture of concentrated hydrogen peroxide and pepper and detonated by a 9-volt battery. Forensic explosives expert Clifford Todd said using this combination of materials for a bomb was thought to be "entirely unique" both in the UK and worldwide."
The Telegraph has most detail :
"The July 7 bombs and July 21 bombs were not identical, but they were, a source at DSTL said, "similar in principle". Both were a mixture of hydrogen peroxide, made by boiling down hair products, and an organic material - in the first case black pepper, in the second chapatti flour. The detonators were also similar - in the case of July 7, made from hexamethylene triperoxide diamine (HMTD), made by combining hydrogen peroxide, hexamine tablets from camping stoves, and an acid such as citric acid or dilute sulphuric acid. In the case of July 21 the detonator was triacetone triperoxide (TATP), made by combining hydrogen peroxide, acetone from nail varnish and sulphuric acid. The differences amount to just two main ingredients - nail varnish rather than camping stove tablets and flour rather than pepper - and leave the possibility that both bomb-makers were taught by the same man. The cost of the devices was also relatively low - just £500 was spent on hydrogen peroxide."
It must be a pain boiling down all that hairdresser grade peroxide (17 vol?) when a chemical supplier can do you 100 vol.
Now call me daft, but why pepper ? Indeed, why flour ?
I guess you want something flammable (high heat of combustion per gram), fine particles, can be soaked in concentrated peroxide. I've forgotten my chemistry - wouldn't charcoal be better, or doesn't it burn fast enough - as I guess the speed at which the mixture burns affects the power of the bang?
I suppose the driver is that the fuel must be able to mix intimately with the H2O2 - which is a watery oxidiser. Can't mix that with, say, diesel, as oil and water proverbially don't mix.
Is there someone in the Kurram Valley or Mohmand grinding up various foodstuffs and evaluating them as explosives? Seems that there must be.