How does heat sensitive paint work?

08 August 2013

Question

Evening Chris,

I have a large mug. Not a massive face, or a much put-upon size 20 wife, but a receptacle from which to consume large amounts of tea.

I have a good understanding of the world about me, and know how most things work, but not this.

On the side of my mug, is written the words 'I love my dad' with a big red heart. (That there's also picture of 'Grumpy' on the other side is by the way).

When hot water is poured into the mug, the heart comes up a glowing red. When cold, it disappears altogether. How does that heat sensitive stuff work?

Regards

Tad

Answer

Dominic - I think the mugs that Tad is thinking of, they tend to be black at one temperature and then underneath that black ink, they've got a picture which is revealed at certain temperatures when that black ink becomes transparent. The way that black ink works, it's a black dye which is mixed with crystals of a salt which is either very acidic or very alkaline, and that paint is actually pH sensitive. It's sensitive to whether the environment is acid or alkaline. As the temperature changes, that material melts and it mixes with the dye. That actually changes the optical properties of that dye and changes it from being very opaque black paint to being completely transparent. So, it's all governed by the melting temperature of these crystals that you have mixed in with that dye that change the pH of the environment very radically to a particularly temperature. They mean that when the mug is hot, the dye is black, you can't see the picture and then as you drink your tea, you get down the mug, the mug cools down and the picture is revealed from the top as you get down your mug of tea. Hannah - That explains exactly how the mug that I have at home works like that and it's been working for years. So obviously, this particular paint keeps on going with this melting temperature pH change thing.

Dominic - Yes, absolutely. You've got these crystals in there and they can change between the liquid state and the solid state, mix in with that dye many, many times and that mug will carry on changing as many times as you want it to.

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