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Tools for putting mug back together

Lockdown Challenge: Microwave steam and microwave glue

Image credit: Neil Downie

For this week’s Lockdown Challenge, Neil Downie encourages the use of the microwave to mend broken pottery.

Possibly taking familial goodwill to its limit, Neil Downie’s experiment this week involves using the kitchen microwave for non-culinary purposes. However, as you are the adult and the microwave (and kitchen) is yours, it is up to you to what extent you encourage your children to use and abuse it in the name of inspiring science. Firstly, Downie demonstrates the very hot water (i.e. steam) ballon and then moves on to a method of repairing crockery.

The IET also has a host of resources, which adults can use to engage children with the world of STEM. 

Lockdown Challenge #20 – Microwave steam and glue

Way back in 1940's Birmingham, John Randall and Harry Boot developed the magnetron, the high-power microwave generator. Five years later, someone – it had to be an American: Percy Spencer – tried cooking popcorn with a magnetron. It worked! And a new kitchen gadget was born. Now we all have one – or 93 per cent of UK households do, anyway.

First, a warm-up experiment. Put an ice cube inside a round balloon. Tie the balloon neck up and put it into the microwave oven. Stick it on for 60 seconds and drum your fingers while those microwaves bounce around inside the oven. Now, take it out. All melted? Well, that wasn’t a surprise. Microwaves are absorbed by the water molecules of the ice. That’s how microwaves cook things in general, by heating the water inside the food.

But that’s not all. Now put it back in for another minute or so. Drum your fingers a bit longer. Look inside. What’s happening? That balloon is getting bigger. Why? Steam, of course! Solids mostly turn to liquids when you heat them, and liquids turn to gases – steam, in this case. So that’s a great little lesson in the physics of solids, liquids and gases, and a great introduction to the fascinating world of microwaves. (Safety note: steam is HOT. Use oven gloves and hold the balloon away from your face when you take it out.)

Here is a video of a microwave steam balloon:

Ever broken a cup or other crockery and wanted to glue it back together again? Well, here is one way to do it: microwave glue. For this you will need a hot-melt glue gun (or at least the glue for one).

Take your busted mug and put a thin layer of hot-melt glue on one side of the broken surface. (Don’t get burnt by the glue gun; keep a little bowl of cold water handy.) You probably can’t do a good job... you just can’t get a thin layer of glue into the crack fast enough.

But don’t give up yet. Elastic band the mug together, half-glued as it is, and shove it in the microwave. The mug will heat up. I found that a newish (‘microwave-proof ceramic’) mug went 50/77/86°C after 1/2/3 minutes in the microwave. A similar old mug – made from old-fashioned pottery – heated up more, going 70/89/102°C. If it had got a bit hotter – something like 150°C or more – then the glue would have softened and the elastic bands would have closed up the crack to mend the mug. Maybe your broken mug got hotter and the fix worked. (Handle the hot mug with oven gloves.)

Mug being glued back together; elastic bands used to keep mug in place

Image credit: Neil Downie

If that doesn’t work, there is a solution... microwave glue! Find some iron filings (near the grindstone in your garage?), or make some with a piece of iron and a file. Hot-melt glue doesn’t absorb microwaves and doesn’t get hot. But the iron filings will absorb microwaves and get hot, heating the glue. Once you have made a reasonable amount, a few grams, you then need to put it into molten hot-melt glue. Put glue – more than the filings by weight – with the filings into a small, non-stick frying pan or saucepan and heat carefully. Once it is molten, mix it around with a stick, then take some and smear it on the broken surface, pushing the mug parts together and holding it with an elastic band once again.

Alternatively, leave the mixing stick in, wait until everything has cooled, and peel the microwave glue out of the pan using the mixing stick to help lever it out. Cut thin slivers from the sheet and jam them between the parts, holding the mug together with elastic bands.

Now back to the microwave. Fire it up for a couple of minutes, watching closely to see how things are going. With luck, the mug will heat up, the microwave glue will melt, and the elastic bands will close up the crack in the pottery, pushing a grey bead of glue out.

The challenges

  • Can you get the balloon to float out of the microwave into the air? (Steam is 40 per cent lighter than air.)
  • Can you mend a mug well enough that it holds water?
  • Can you mend something else with microwave glue?
Mug glued back together

Image credit: Neil Downie

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