Summer STEM Challenge: The Art of Heat

William Fox Talbot was one of the first people to take photographs, ‘painting with light’, although in the 1830s hours of full sunshine were needed. Thermal paper also needs a lot of energy to get a pic. But you won’t need to wait hours. 

The simplest way to get a picture out of thermal paper is to rub it while it lies – sensitive side up – against an interesting, engraved surface. It’s that simple! The ‘magic’ is that the paper coating turns from clear to coloured when it gets to 100°C. The friction, of course, yields heat, and heat produces colour. It’s a bit like brass rubbing, where copies are made from artistic engravings by rubbing a stick of black wax over paper on the brass, but without the wax.

Most thermal paper is 40 or 60mm wide for receipt printers. Better is the A4-width paper for printers and faxes: £5-10 buys you a 30m roll of A4 width. Most turn black with heat, but there are other colours. You will also need a pad of the right size and firmness: something like a 10cm-square polishing pad.

Try out making copies from engraved wood first. Or try boot prints (shown above), handy for your part-time job with police scene of crime officers (SOCOs). You can test the system for your career in botany too. Rubbed pictures of the flat part of plants – leaves and flowers, for example – can show a surprising level of detail on thermal paper. And the picture will stay around while the leaves shrivel and disintegrate. Brass works less well because it conducts heat too well: raised patterns on pottery are better. Embroidery works nicely too – the pic (below) shows a map of the world from a cushion.

You can apply more obvious heat to achieve a picture. You can use a glue gun, even a low-temperature (130°C) one, to write or draw on thermal paper. Hot resistors work too: you can even make a DIY seven-segment printer this way! 

But why not dust off your artistic genes and make a thermal paper collage? A hairdryer can supply heat. Arrange the objects you desire on the paper and plant a hairdryer over it until it begins to turn grey. It will turn darker as it cools into a negative image of the collage. You can make a positive collage image with your oven. Put a sheet of thermal paper on a tea towel or similar. Put things into the oven that will stand the heat at a little over 100°C. Use oven gloves to place them or stamp them gently down onto the paper. Multiple copies? No problem. Repeat the stamping, or use multiple layers of thermal paper.

Alternatively, you can heat an arrangement of heat-proof objects on the grill pan. Heat under the grill for a minute or two, then take the pan out and put a piece of thermal paper across it, followed by a folded tea cloth, and press the tea cloth gently down.

The basis of thermal papers is the combination of a leuco dye with a developer, an organic acid like bisphenol A. Leuco dye molecules have two forms: one colourless, the other coloured, and are switched between them by the developer when heated. There is concern over human exposure to BPA, albeit at levels much higher than casual contact gives, but wash your hands after handling thermal paper a lot anyway. (This is why some makers switched developers.) You can use chemistry to do spray painting without spray paint on thermal paper.  Acetone-based nail varnish remover will work, although limonene label remover is better. With your shadow original composed, spray to get a negative image.