This is a very general effect in compound substances such as paper, wood, biological tissues, and ground or fibre glass, where there is a mix of materials that refract light differently. The light gets scattered so much as to ruin any clear image. The air in paper has a lower refractive index than the fibres, but if you can replace it with something that matches the refractive index of the fibres, the light can pass straight through, making the whole mass transparent or at least translucent, if the match is goodish, but imperfect. We call such a process impregnation.  Suitable oils work well, as long as the paper is nicely homogeneous, and you can remove nearly all the air.

In fact, years ago, when glass was very expensive, oiled paper or parchment was sometimes used for window panes. A primary school teacher of mine who had visited Japan, told us that paper walls were widely used in some areas, at that time, and that the foreigners were great objects of curiosity, but the Japanese did not wish to stare openly. They would go indoors, and wet paper walls in spots to provide peepholes. The dark spots were visible from outside however, so that the party was treated to a sense of being watched wherever they went in such an area.  

In some ways wood behaves similarly. A piece of dry teak or mahogany has a dull and whitish cast, even if carefully sanded, but if you treat it with a good transparent oil that soaks in to replace the air in the wood fibres, the light can go in deep, not getting out again before it has passed through a lot of the wood's pigment, so that it comes out a nice, rich brown, and shows the handsome patterns of the different colours in the wood. Far less white light now returns from the outer layers and what does, gets swamped by the deep brown.  

What is more, you see still more deeply when looking down the grain, than looking across it, so that well-planed, oiled wood changes its colour beautifully as you rotate it in the light.  You get similar effects with the semi-precious stones called tiger's eye and hawk's eye. They are formed when certain kinds of asbestos fibres are impregnated with silica, much as the wood gets impregnated with oil. A handsomely  coloured rug with a well-oriented pile behaves in a similar way in good light.

Similarly, biological tissues preserved in alcohol or other preservatives, are opaque because the substance of the tissue has a higher refractive index than the liquid.  Replace the liquid with one of a suitable refractive index, such as glycerol or methyl salicylate, and you often can make it transparent, showing bones, blood vessels and the like, possibly suitably stained. This is an example of what they call "clearing". It is a sort of impregnation, and a lot trickier than oiling paper, but it can provide some very spectacular specimens.  Incidentally, you often see that specimens in amber have gone transparent, and very useful the fact is to palaeontologists too. What has happened is that the resin diffused into the creatures' bodies and cleared them, often very beautifully, before it set into amber.

Now to clear my mind...

Cheers,

Jon