Rebbeca Pierce, of Ilam school, asks :-

How do they make paper you can see through?

Graeme Robertson, a chemical engineer and Chief Executive Officer of the Cawthron Institute, Nelson, responded.

Paper is made from tiny fibres, called cellulose. If you looked at one under a microscope, it would look like a miniature drinking straw. They are about 2-5 mm long and so narrow that 50 of them placed side by side would only measure 2 mm in total.

A sheet of paper is just a collection of these, all laid one on top of the other. When the fibres are very stiff, you can imagine that they form a great tangle, full of gaps and air spaces between them. A bit like a pile of pick-up sticks. When light falls on a sheet of paper made from these stiff fibres, it gets reflected off all of these surfaces so that not much can pass through.

It is very like new snow. If you can imagine freshly-fallen snow on an asphalt road, the tiny snow crystals are full of these intricate shapes, creating lots of gaps and spaces for the light to be reflected. So it appears very white, and covers up the dark road very well.

If you tread on the snow, however, you destroy the little snowflakes and along with them, the gaps and air spaces. The light doesn't get reflected back, and can pass right through. You can see the dark road underneath.

That's the secret for making transparent paper. If the tiny fibres are treated so that they are soft and squishy, they look more like a pile of spaghetti, not pick-up sticks. There are fewer gaps and air spaces. We can even squeeze the sheet of paper hard, especially when it is wet, and that makes the fibres get even closer together. Wherever they touch, light can pass right through.

In New Zealand, transparent paper is only made at one paper mill: Caxton paper Mills in Kawerau. Most of the other paper mills try hard to do just the opposite (imagine an envelope made from paper which was transparent, or a newspaper where you could read both sides at once).