How to

17 February 2017

Heat-Changing Mugs - How Heat-Sensitive Paint Works


Heat-changing mugs are one of those little gadgets that you’re likely to receive as a miscellaneous gift at some point in your adult-life. Perhaps it’s an office raffle prize, or something a family member brings back for you from their trip away.

They’re not an essential, but they’re a pretty cool twist on your bog-standard coffee mug all the same. If you’re unaware of these ‘magic mugs’, they’ll usually have a dull exterior when at room temperature and empty, only to reveal a mystery image once you pour hot water over your tea bag. Put simply; a photo, phrase or pattern will appear when heated, and I for one find them really cool. But how does this magic work?

Firstly, if you’ve never seen the magic in action before, have a watch of this video, made by a guy who’s equally as fascinated and easily-amused as me by the piece of crockery:


Pretty cool, right? So here’s how the exterior or the mug changes before your very eyes; it’s down to a process called thermochromism. It sounds complicated, but its definition is simply ‘the property of substances to change colour due to a change in temperature.’

The heat-changing mugs are coated in heat-sensitive, or thermochromic, paint. This type of paint used on the mugs contains leuco dyes and various other chemicals. The leuco dyes contain salt crystals, and these crystals have either a high acidity (low pH) or a neutral acidity (high pH), and where the pH falls on the scale depends on whether the paint is opaque or transparent.

When the temperature on the mug changes, a chemical reaction between the crystals and the leuco dye occurs, making the dye transparent. To explain the video above, the dye is opaque to start with, (a black colour), and turns transparent to reveal the full-colour background behind it, when hot water is added.

It can work the other way around too, the heat may make the paint opaque, and cool to a transparent state.

As reported by Science ABC, thermochromism is just one type of chromism, which is the umbrella term for the process of (usually reversible) colour changes in certain materials. Thermochromism is also how mood rings or jewellery work, they change colour depending on your body temperature.


So there’s the science behind the magic of heat-changing mugs. Only thing to note, to keep them working successfully avoid the dishwasher at high temperatures, and the microwave may also do a bit of damage.  Other than that, enjoy showing off your magic mug to your colleagues, I can guarantee they’ll be jealous as they compare it to their plain slogan mug, or similar.


Laura Sewell

An aspiring journalist, Laura is our content writer intern.  Pop-punk gig-goer and drag queen enthusiast, Laura is working her way into the industry, with an English A -Level and love of writing about anything and everything in tow.