[Wavytone]

Ok...

Let's assume paper density is 100g per square metre.
Image area = 0.000177 sq.m
Mass of paper in the solar image 0.0177g, ie 0.0000177 kg
Specific heat of paper 1336 J/kg/C
Temperature rise required dT = 226 degrees
Joules required therefore J = 5.3 Joules
Heat flux over the solar image H = 17.7 W

A Joule is a watt-second so the time to reach ignition temp = J/H = 0.3 seconds, which I suspect is optimistic (ie too quick) from what I saw with 4" and 8" mirrors producing much smaller images.

Beyond what Al suggested there are more issues -
- paper isn’t opaque - a fair bit of the energy will escape out the back;
- atmospheric losses (there is huge difference between solar noon on a clear midsummer day, vs the crap sky we have at the moment - which is struggling to warm a black metal surface),
- moisture content of the paper (it takes a lot of heat to drive off water), and also
- the transition from white paper through charring to ignition. Once that change begins it becomes carbon and absorbs the energy quickly.

I have a magnifying glass, a 70mm f/6.7 refractor, 6" f/15 Maksutov, and 10" f/12 Mak and Mental has a 10"f/5 newtonian... these could make an interesting experiment over the Xmas hols next week - assuming we get some sun (still overcast with smoke here). Assuming they are sufficient for ignition, it would then be neat to stop them down so the ignition is slow enough to time with a watch.

Might even try a couple of hand grenade eyepieces.