increasing fresnel lens light concentration?

Recently acquired a fresnel lens from a projection tv. It is about 36"x36".
My original purpose was going to be to use it for melting metals, etc but
the focal point appears diffused. It ignites paper and boils water after a
time, but won't melt metals very well. Since this is a lens, is there any
way I can add an additional fresnel (or even glass) lens at the focal point
to decrease it even further so I get higher melting points?

Thanks in advance,
JR


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