I鈥檝e been making notes to revise for my exams, and as I write, the pages curl inwards first from the top corners of the paper, and then the bottom corners. Why is this? I鈥檓 using a black ballpoint pen on white A4 printer paper.
鈥� I suspect your note-writing is quite small and closely spaced, and that you apply more pressure than normal with the pen. This compresses the fibres of the paper and causes them to expand sideways, thus effectively stretching the paper below the pen. Because you don鈥檛 write to the top, bottom and edges of the sheet, these parts remain unaffected and so curl up.
A classic example of this effect, but for a different reason, is a sandwich left out in the sun. Most people will notice that the corners and edges soon start to curl, and it will have become unpalatable. This time, the phenomenon is caused by the sides and upper surface of the bread drying out and shrinking.
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鈥淎 sandwich left out in the sun will soon start to curl at the corners and edges, and become unpalatable鈥�
The stretching effect caused by pressure is used by manufacturers to produce curved skin panels for custom-built cars and aircraft.
A machine called an English wheel is used for this purpose. This consists of a rigid 鈥淕鈥�-shaped frame supporting a large, flat wheel. Below this is a smaller wheel with a curved edge. A sheet of metal is fed between the wheels and worked back and forth.
In the hands of a skilled operator, quite complex shapes can be produced. There are many videos online that show the machine in action. Before the invention of the English wheel, the stretch-shaping of metal was achieved with a hammer.
Bob Halahan, Godalming, Surrey, UK
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This article appeared in print under the headline 鈥淓verything but the curl鈥�