If you take an ordinary dry scatter rug and put it on the floor, preferably in sunshine, you probably will find that its colours deepen considerably when you turn it so that you are looking down the fibres. If you then rotate the rug till you are looking over the backs of the fibres, the colours seem paler. Similarly, if you look at a piece of planed wood in similar orientations, you will find that the grain is far more obvious in one direction than in the opposite direction. Both of these phenomena have a similar cause. When light penetrates deeply into the structure of material before it is scattered or reflected out again, a large part of it is absorbed by the pigments in the material. As a result less light gets out again and most of what does escape is of the colour dictated by the pigments. In other words the colour deepens and darkens.
That is a very general principle.
Now consider a slightly different case. It applies very broadly, particularly too porous materials like fabrics, but let us again take wood as a convenient example, a nice, planed piece of brownish, dry wood. It will look modestly brown, with a whitish, slightly powdery, cast. Rotating it in the light will have some of the effect I described, but nothing dramatic. We can change this by painting it with wood stain, but that would hardly be interesting, because it would be a patent example of adding the colour.
But now suppose instead of a stain, we paint it with a highly liquid mix of thinners and transparent, colourless varnish. We are reasonably expect this to have no effect on the colour of the wood, merely to make it shinier, right? Well, as you know, not a right at all! Suddenly the wood becomes a lot browner, and the more porous the wood, and the more deeply the varnish soaks in, the more deeply coloured the wood.
We still find that rotating the wood affects the depth of the colour, but the whitish, powdery cast simply vanishes, and the overall appearance is far browner than that of the dry wood.
Light penetrating the dry wood was scattered, much as light gets scattered by say, cotton wool. And in fact most of it gets scattered back from very near the surface of the wood, because the wood is full of hollow cells containing nothing but air. The air has a far lower index of refraction than the material of the cell walls. Much as light will reflect from the surface of glass or water in air, because of the difference in refractive index, just so the light can bounce around in the air contained in dry, hollow cells of the wood, much of it bouncing back towards your eye. It therefore returns before having gone through much wood. The varnish however fills up those little dry hollows, and it is fairly close to the refractive index of the dry material of the wood cells. That stops the light from scattering so much, so it penetrates far more deeply into the wood and has to pass through far more pigment, which means that less of it gets scattered back, and what does get scattered back shows the pigment colour far more strongly.
This is a clue to how you can get a far handsomer colour from any wood that you varnish, whether with pale or dark varnish. Apply the first coat, preferably an extra coat, diluted half and half with thinners to make it very liquid. Apply it slowly and very generously so that it can soak in well, filling wood cells with varnish as deeply as possible. Then apply the normal coats of varnish over that first penetrating coat. Especially for high quality decorative woods, the effect is quite dramatic.
You also will see a similar darkening, and deepening of colours when you put clothing into the washing water. The principle is much the same.
Cheers
Jon
