I waited till near sunset here tonight to test some of the fesnel lens tinder ideas suggested above; this test was by no means scientific but I figured that this would be when the sun's rays would be least strong (worst case scenario). All materials used were very dry as I had them stored inside, the outside temperture was app. 20*C.

Blast was right, dark tinders do seem to develop a coal much faster than light coloured tinders.

Dense objects like dry wood (poplar/pine), developed a coal much slower (if at all) than lighter objects like bark (poplar).

I tryed to light white birch bark again, but the best I could do was just burn a hole through it without developing a coal or igniting it.

Even typing paper, cardboard and dark newsprint were reluctant to light at sunset.

The real winner was again, True Tinder Fungus. I had some in my garage drying since last year and after concentrating the fresnel lens light beam on the surface it lit within 1 to 2 seconds. I tried it 10 times so I could determine the rate but it often lit while I was focusing the beam, before I even started timing. This stuff is amazing, no wonder it was found with "Otzi, the Iceman", it was the "lighter fluid" of our ancestors.

Take care,

Mike