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Most "invisible inks" turn brown when heated and they take a while to transform. I would like to make (or obtain) an "ink" that responds to the heat of an iron and permanently turns black or a similarly dark color - instantly. Thermal receipt paper does this where the paper is white until it is heated with almost instant response times AND the "ink" has a permanence of sorts (e.g. sticking a receipt in a freezer won't revert the reaction). What chemical combinations in what amounts will do an equivalent transformation?

Bonus question: What chemical combinations would create similar effects for standard palette colors? That is, transparent/hidden until heat from an iron is applied, whereupon it instantly appears.

My goal is to use such an ink with paintbrushes and fountain pens on canvas. Black is probably sufficient, but knowing how to make standard colors would dramatically expand my options. The ink ideally shouldn't be toxic to touch after drying nor emit toxic fumes when heating with an iron at a "Cotton" setting. It can be semi-toxic while wet but nothing that will melt nitrile gloves, fingers, or human lungs. I will, of course, wear sufficient protection at all times as I don't really need any chemicals leeching through my skin. Having a drawing/painting appear instantly in front of an audience from a seemingly blank canvas would be very dramatic and painting with invisible ink will be an interesting adventure.

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    $\begingroup$ Most paper used for receipts now are not thermal paper. If you scratch hard enough (even with a fingernail) you can leave a line. $\endgroup$ – LDC3 Sep 3 '14 at 0:34
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Have a look at wikipedia article on Thermal Paper. The whole trick is in the three components, which are mixed as solid substances and applied as coating to the paper. When you heat the paper, the solid substances melt and the coloring reaction occurs.

For this you need fine pulverized chemicals, stirred in a form of suspension in some volatile solvent and apply them on the canvas. It should not be a big problem, see e.g. patent on Thermal Paper.

If all this effort is for an artistic purpose, I would suggest as a first try to approach the companies manufacturing the thermal paper (there should be lot of them) and ask them for a bottle of the prepared suspension, telling them your plans. The success rate would not be high, but hopefully non-zero and it saves you 99% of work.

Additionally, the painting itself could be quite fun, I would expect the components to be UV-active, so the black light could help you.

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