How could you turn an opague material to translucent material ?

Answer 1

Frankly, the question is too general. Opaqueness can result from a wide variety of factors, hence the ways of removing it (resulting in transparency) can differ. For example, grey ice (which has a lot of air dissolved, forming many tiny bubbles) can be melted into water, which is perfectly transparent. Typical wax in the candle is almost opaque but becomes pretty transparent when molten (then the individual molecules become loose).

In many cases, evaporation will work, too. Almost everything can be evaporated and thus made at least partially transparent. Solid sodium looks like a shiny metal, but in vapor form it will transmit most of the spectrum (not all, though, there are two infamous lines of absorbtion).

A more interesting example concerns liquid crystals (watches, various LCD displayes and even HDTVs), where the transparency depends on how well the adjacent long molecules are aligned. When the electric field is properly applied, alignment leads to transparency, or vice versa... And thus you can have a cool flat TV!

Answer 2

Gold (an opaque material) can actually be made translucent by hammering it extremely thin. This may be true of other metals as well.