On Wed, 21 Aug 2013 20:18:52 -0500, "Lloyd E. Sponenburgh"
lloydspinsidemindspring.com wrote:

mike fired this volley in news:kv3o58$bem$1@dont-
email.me:

I tried starting fires with most of them. g The smallest ones that
worked at all were around 6" diameter, and for that I needed a little
ball of red cedar inner bark, like you'd use with flint-and-steel fire
starters.

A 3" mirror with a proper aluminum coating will collect and focus about
4.8 Watts of energy (including losses in reflection) from noon-day sun.

The sun's disk's substended angle of about 0.5 degrees will focus to a
dot of less than 0.1" diameter. That's an area of about 0.008 square
inches. Just to be real generous, let's say 0.01 square inches (for poor
focus).

That's a heat concentration of 480 Watts per square inch. That's not
only enough, but FAR more than enough to ignite any tinder capable of
absorbing the heat.

What'd you try to light, aluminum foil?

Mostly little balls of rubbed red cedar inner bark -- the same stuff
you use to start a fire with flint and steel, or with a fire bow. Wild
grape vine inner bark works well, too.

Have you actually tried this with such a small reflector, Lloyd? I've
been a magnifying-glass fire bug since I was 7 or 8, and here's my
experience with it:

You need not only a temperature high enough to make the fuzz ball, or
dry leaf, or paper glow and char; you also need to be able to de-focus
it enough to heat a large enough area that it will produce enough
combustable gas to will sustain a flame. You can take a 3" dime-store
magnifying glass and char leaves with it all day long, but only rarely
will the leaf actually catch fire. Generally, it glows and chars but
won't sustain a flame. Once in a while a serendipitous amount of
gentle breeze will fan the spark and a leaf, or bark fuzz ball, will
flame. But it may take a lot of tries. The first time someone tries
flint and steel they usually wind up chasing a little spark right
across your piece of charred rope or whatever, until they get the hang
of blowing on it just right to start a flame in the rubbed bark.

OTOH, take an 8" x 10" plastic fresenel lens, de-focus it on paper or
a leaf to a half-inch spot, and it will burst into flame in two or
three seconds.

--
Ed Huntress


Lloyd