Use clear lake, pond ice or try boiling water for 3 minutes let the water cool; then boil for 3 minutes again and let cool to remove gas. Create a foil container 2" deep and freeze slowly. Or buy a clear block of ice. To make fire, the ice must be clear.
Look at the bottom of http://primitiveways.com/fire_from_ice.html
to see a Big ice lens.
Step 1: Shape the Ice Lens for Fire Making.
Shape the ice lens for fire making. First shave with a knife to get the rough shape of a lens. You can also grind your ice fire lens on cement or a stone.
Step 2: Finish Shaping Your Ice Fire Lens.
Finish shaping your ice fire lens by using the heat of your hand to melt the ice lens smooth.
Step 3: The Edge of Your Lens Should Look Something Like This.
The larger the lens the better: it will collect more sun. You will want the lens to be about 2" thick and more that 6" in diameter. The lens should be smothly curved on both sides
Step 4: Focus the Brightest Spot of the Suns Light Onto Your Tinder.
Locate tinder that is completely dry and finely divided. With the sun at its highest in the sky hold your lens perpendicular to the sun and move it up and down to focus the brightest spot of light onto your tinder. Be careful to avoid dripping water onto your tinder. The tinder will first smoke and then ignite. Carefully add more tinder and very small sticks until a good fire is established.